Thermal characterization of msw for purpose of its gasification and pyrolysis

SLOVENSKÁ TECHNICKÁ UNIVERZITA V BRATISLAVE FACULTY OF CHEMICAL AND FOOD TECHNOLOGY MASTER THESIS – CHEMICAL ENGINEERING THERMAL CHARACTERIZATION OF MSW FOR PURPOSE OF ITS GASIFICATION AND PYROLYSIS ABEL VALVERDE SALAMANCA UNIVERSITAT POLITÈCNICA DE CATALUNYA Director: Prof. Ing. Jozef Markoš, DrSc. 2014 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde Contents Contents ............................................................................................................................ 1 Summary ........................................................................................................................... 3 Acknowledgments ............................................................................................................ 4 1. Introduction .................................................................................................................. 5 1.1. Background ............................................................................................................ 5 1.2. Municipal Solid Waste (MSW) ............................................................................. 6 1.2.1. Composition and disposal of the MSW........................................................... 6 1.2.2. Biological Treatment ....................................................................................... 9 1.2.3. Landfilling ..................................................................................................... 11 1.2.4. Thermal Treatment ........................................................................................ 12 1.3. Pyrolysis and Gasification Technology ............................................................... 23 1.3.1. Fixed bed gasifiers (updraft and downdraft) ................................................. 24 1.3.2. Fluidized bed gasifiers .................................................................................. 25 1.3.3. Rotatory kiln gasifiers ................................................................................... 26 1.3.4. Entrained flow gasifiers ................................................................................ 27 1.3.5. Plasma gasifiers ............................................................................................. 27 1.4. Tar ........................................................................................................................ 27 1.5. Catalyst ................................................................................................................ 30 1.5.1. Dolomite and calcined dolomite ................................................................... 30 1.5.2. Nickel on alumina (Ni/Al2O3) ....................................................................... 31 1.5.3. AFRC ............................................................................................................ 32 2. Description of work .................................................................................................... 32 2.1. Thermal characterization of MSW equipment ..................................................... 32 2.1.1. Thermogravimetric analyzer (TGA) ............................................................. 32 2.1.2. Bomb calorimeter .......................................................................................... 35 2.1.3. Elemental analyzer (EA) ............................................................................... 36 2.2. Micro GC (Gas Chromatograph) ......................................................................... 37 2.3. Laboratory pyrolysis apparatus ............................................................................ 38 3. Results and discussion ................................................................................................ 40 3.1. Thermal characterization of MSW results. .......................................................... 40 11/06/2014 Page 1 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde 3.1.1. Elemental analysis and proximate analysis results ....................................... 40 3.1.2. Calorimeter results ........................................................................................ 43 3.2. Kinetics analysis .................................................................................................. 44 3.3. Thermal characterization of RDF and PPD results .............................................. 48 3.4. Products results .................................................................................................... 50 3.4.1. Products obtained .......................................................................................... 50 3.4.2. Micro GC (gas chromatographer) results ...................................................... 53 3.4.3. Thermal characterization of pyrolysis products ............................................ 55 3.5. Conclusions .......................................................................................................... 58 Appendices ..................................................................................................................... 60 Appendix A – Introduction ......................................................................................... 60 Appendix B – Experimental Data ............................................................................... 61 Appendix B.1 – TG curves of MSW components .................................................. 61 Appendix B.2 – Results of calorimetric analysis from MSW components ............ 67 Appendix B.3 – TG curves of RDF and PPD mixtures .......................................... 68 Appendix B.4 – TG curves of RDF and PPD char.................................................. 69 Appendix B.5 – Results of calorimetric analysis from RDF and PPD char ........... 70 References ...................................................................................................................... 72 11/06/2014 Page 2 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde Summary Due to the increase of municipal solid waste (MSW) as the population and their consume increase, new technologies of waste removal have to be developed, in order to find a MSW disposal method which do not fill lands and neither pollute the environment. Therefore, pyrolysis is raised as a way of MSW removal which at the same time can produce profitable products. This thesis is aimed at measuring the yields obtained from a pyrolysis process from different MSW mixtures, as well as the influence of temperature on the product yields is determined. For this purpose, a characterization of MSW components and the resulting products has been carried out by means of thermogravimetric, calorimetric and elemental analyses. After a quick introduction to the MSW production as well as the nowadays technologies of removal, the description of the equipment used during experimentation and discussion of results, the thesis arrives to final conclusions, where an overall view of the results is done. Keywords: municipal solid waste (MSW), pyrolysis, gasification, high temperature, degradation, tar, ashes, volatile fraction, thermogravimetric analysis, calorimetric analysis, elemental analysis, gas chromatography (GC). 11/06/2014 Page 3 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde Acknowledgments This thesis couldn’t have been possible without the help of some people, who have been supporting and contributing with their advices, knowledge and material support all throughout the thesis: Ing. Jozef Markoš, DrSc. – For his help and advice during the elaboration of the thesis and especially for the starting support which structured in the best way this thesis. Doc. Ing. Juma Haydary, PhD. – For his help in the experimentation and providing the aim of the thesis as far as advices for the research of it. Ing. Dalibor Susa – For his continuous help all along this thesis, providing documentation, experimental data, advice, constant supervision and availability in any moment, and for his final revision of the work. This thesis couldn’t have been completed without his help. And lastly I would like to thank fellow Slovak students Miroslav Mrva and Viktor Gelinger for their help in laboratory. 11/06/2014 Page 4 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde 1. Introduction 1.1. Background As years go way, the demand of energy increases, and becomes clearer the need to search for new sources, renewable and non-polluting. That is the reason why in these last years the development and research on renewable sources of energy is coming more and more significant. Lately many power plants have been built where the radiation of the sun is used to generate electricity. It can be solar thermal plants, where the heat from the sun is taken by panels that increase the temperature of the water that is circulating in pipes through the panels in order to achieve the amount of steam needed to drive a turbine. On the other hand, there are the photovoltaic solar plants, where the radiation of the sun is directly converted into electricity by means of a panel made of semiconductor. Nevertheless, this technology is only able to achieve the efficiency of 15% and a 20% (Schultz et al., 2007), and taking into account that the average of sun radiation that receives the panel is 1000 W/m2 a huge economical investment is needed in order to generate a considerable amount of energy. The wind is another way to generate electricity in a renewable and non-polluting way. It consists in large amounts of aeolian generators called wind farms, placed there after an exhaustive study of the strength of the wind and its flows, which are capable to convert the kinetic energy that the wind carries into a rotating movement of the blades, that is translated to the rotating movement of the axis which generate the electricity inside the generator. These kinds of generators have a quite good efficiency as it is between a 20% and a 45%, but it is very variable as it depends on the wind speed (SAEM Thales, 2001). Furthermore, the worst inconvenience is this variability of the wind, since it is not possible to control the amount of energy that is generated. Moreover, there are a lot of critics against the visual impact it has on the landscape, and the damage to some animal species like birds. Nowadays, one of the most used renewable energies used is the hydropower. It consists in taking profit of the energy of the water (either kinetic and pressure energy), in order to drive a turbine that transmits the mechanical energy to the axis, and finally electricity is obtained in the generator. It can be a different plant depending on the site of the plant: • Dam plant: These are the conventional plants where in a huge river dam is built in order to create enough head so the turbine can work. Depending on the head and the flow one kind of turbine is used (axial Kaplan for high flow and low head, radial Francis for quite high head and low flow, and Pelton for very high head and low flow). This kind of plants cannot store water once it passes through the turbine. 11/06/2014 Page 5 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde • Pumped-storage plants: These kind of plants are located between two reservoir of water (usually artificial), and the equipment used is a hybrid turbine and pump machine, or separate pump and a turbine, in order to work like a turbine when electricity is required, and work like a pump when there is excess of electricity in the network, so it can take profit of it pumping water to the upper reservoir and storing it. These plants are very useful, as they can store energy and give electricity when it is needed. • Run of the river plants: These are plants sited in the run of the river, where is made a small head, and the water is derived to a turbine, usually a horizontal Francis. • Marine plants: There are plants focused on taking profit of the marine water energy. The turbines usually are axial Kaplan or Bulbo, and the aim is to use either the strength of the marine water, the currents or the waves of the sea to generate electricity. These kinds of plants are not very used as the sea water is very corrosive and the turbines and the pipes need a lot of maintenance and there is not a huge amount of energy taken from it. The efficiency of hydropower plants is between 65% and 90% (Islam et al., 2011), so is one of the most efficient methods to generate electricity in a renewable and nonpolluting way. It needs a strong economical investment, due the need of a huge civil build, and an expensive apparatus and maintenance, but it is very profitable. 1.2. Municipal Solid Waste (MSW) Following the same target as the renewable energies that has been explained, it is possible to obtain energy from the items that everyday each person discards; those are the Municipal Solid Waste (MSW). This way, it is possible to use a renewable energy method that moreover reduces the amount of waste produced which is increasing every day. 1.2.1. Composition and disposal of the MSW The composition of MSW usually contains biodegradable waste, such as food waste, wood and paper (also recyclable); recyclable material, as paper, glass, cans, clothes, some plastics, etc.; Inert waste as sand, rocks, etc.; plastics not able to be recycled; hazardous and toxic waste, such as chemicals, paints, light bulbs, spray cans, herbicides, pesticides, etc.; and medical waste (EPA, 2012). Nevertheless, the exact composition depends on the region it comes from, so in a developed country recycling culture the main formers of the MSW are wastes not able to be recycled like plastics, but in a nonrecycling culture the waste can contain a lot of food waste, paper, etc. The MSW do not include industrial waste, neither agriculture waste, or radioactive waste, etc. 11/06/2014 Page 6 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde In the following figure it can be seen the MSW composition by region. Figure 1. MSW composition by region (Popraco, 2012, Source: The World Bank, 2012). In Figure 1 it can be seen how for example in Europe there is less organic waste (47%) than the others, maybe for its recycling culture, and more glass, metal and others (31%); and in Africa the waste consists of more organic items (57%) in comparison with the other non-recyclable wastes. The disposal of the MSW is different and also depends on the region it comes from as well as technological development, economic situation, type of waste, etc. 11/06/2014 Page 7 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde Figure 2. MSW disposal by region (Popraco, 2012, Source: The World Bank, 2012). In Figure 2 it is easy to see the difference between the disposal depending on the continent, as in Europe there is more recycling (11%) and incineration (14%) and less dumping (33%), in Asia there is more dumping (51%) and less recycling (8%). It is interesting too, how in North America the 91% of the waste is disposed for sanitary landfill, maybe because there is the enough space for it, and they have followed these politics since a long time. 11/06/2014 Page 8 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde to an efficiency between 20% and 25% for the thermal process, and between 25% and 35% for the power generation (Christensen et al., 2011). Figure 4. Energy balance of the furnace/boiler with typical values (Christensen et al., 2011). Figure 5 is a flow diagram about the energy balance on the furnace and boiler of the incineration plant. In it can be seen how an 86% of the waste energy is used for generating steam in order to generate electricity in the turbine, a 10% will be emitted as flue gas, 2% is lost by radiation and convection, and only a 2% will be ashes. The waste are reduced a 90% in volume, and between a 70% and 80% in mass. Incineration is an argued method in the society since in some countries it is seen like a way to eliminate waste and generating power, but in some others it is seen like another way of producing energy by polluting the atmosphere and endangering the public health. 1.2.4.2. Co-combustion Co-combustion or co-incineration is the name given to the use of combustible wastes in industrial combustion facilities that are not focused on the waste treatment of waste but they have to provide thermal energy or electricity to a process, and usually use fossil fuels for it. Those wastes used just for recovery energy in industry are often called refuse-derived fuel (RDF) or solid recovered fuel (SRF). The advantages of using this method are basically three: • Cost reduction: Some combustible wastes have similar heating value than common fossil fuels like oil or coal, but are cheaper. • CO2 reduction: Some wastes can be renewable. • Diversified fuel market: Using wastes an industry can reduce the dependence on a certain fuel and the prices it has in the market. Nevertheless, the use of combustible wastes in the industry must be controlled since there are some chemicals that need to be removed first from the waste, as Cd, Zn and Hg, which are dangerous for human either for the facilities. Moreover, its use can behave increased pollution without a good air pollution control, corrosion of the equipment because of Cl and S, damage for the catalysts, instable process, etc. 11/06/2014 Page 15 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde Nowadays cement kilns, blast furnaces, pulp and paper mills, drum mixer asphalt plants, ceramic manufacturers, lime works, coal-fired power plants, biomass-fired boilers, etc., are using this technology. Usually, between 10% and 20% of the energy input are substituted by waste, but it seems that this percentage can even reach the 70% (Christensen et al., 2011). 1.2.4.3. Pyrolysis and gasification: Those two processes are focused on converting solid materials by thermal treatment yielding three types of products like gas, char and liquid. Pyrolysis usually generates more products such as gas, tar and char, and gasification converts the materials containing carbon into mainly gas, this is the reason why gasification is often used after pyrolysis, using as inputs the outputs of pyrolysis in order to obtain more gas (useful for heating and other applications) and less ash, char, coke and tar. However, pyrolysis can be carried out without need of gasification. Therefore the output composition depends not just on the method, but also on the configuration of the experiment and the composition of the feed. Gasification has been done for a long time. In the 19th century gasification of coal was used in order to generate gas for applications as illumination. During the World War II, in Europe appeared wood-fueled gasifiers, known as gas generators, due to the run out of oil-based fuels. And between 1970s and 1980s, with the oil price crisis, the gasification technology evolved in order to find a cheaper substitute of oil-based products by means of gasification of coal. Nowadays, the most significant application of gasification is aimed to produce H2 and CO rich gas from coal, in order to use it as chemical feedstock or energy production; nevertheless, the use of biomass and MSW gasification is gaining interest so it can be found new technologies for sustainable energy, although it is still few significant in front of the other applications. The reason why pyrolysis and gasification of MSW is becoming more important is the difficulty for some countries to build incineration plants, which require a high economical invest, and the advantage to preserve better the energy of the waste (Christensen et al., 2011). 11/06/2014 Page 16 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde In Table 2 the benefits and drawbacks of pyrolysis and gasification with respect to incineration are explained. Benefits Drawbacks Recover of chemicals such as hydrogen and other chemical feedstock rather than converting the chemical energy of waste into hot flue gasses. The input fuels need some homogeneity, so if the feedstock doesn’t reach the required homogeneity, it must be pretreated and homogenized. Better energy efficiency. The control of the process is complicated and there can be troubles with slugging, tars and contaminants. Less corrosion. Less need to clean the flue gases since they have smaller volume and have better quality. The use for pyrolysis and gasification for MSW has only been demonstrated for small scale and for specific fuel types, so it is needed a careful review of the technology to use for a specific waste mix, facilities, etc. Better CO 2 capture. Lower emissions of dioxins. For high-temperature processes, solid residues have more quality. Nowadays, energy conversion efficiencies obtained with pyrolysis and gasification cannot compete with modern waste incinerators. As gasification units work with low load, it is possible to build small gasification plants producing less than 1 MW. Lower cost. Table 2. Benefits and drawbacks of pyrolysis and gasification with respect to incineration. (Juniper, 2001; Klein et al., 2002; Malkow, 2004). The process of pyrolysis and gasification include a wide range of operations with complex heterogeneous and homogeneous reactions. As has been said before, pyrolysis and gasification are different processes despite being very similar, as they have mainly difference in the gasification agent and the processing temperature. Nevertheless, they are often combined, carrying out first pyrolysis process connected to the following gasification process, as can be seen on Figure 6. Figure 6. Scheme of pyrolysis and gasification including an outline of the outputs (Christensen et al., 2011). 11/06/2014 Page 17 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde As it is drawn in Figure 6, pyrolysis and gasification do not have to be connected, although usually gasification includes heterogeneous reactions using the outputs from pyrolysis. • Pyrolysis During pyrolysis process the organic material undergo a thermal degradation in absence of oxidizing agents as oxygen, steam, and CO2. The typical temperatures are between 300 and 800 ºC (Christensen et al., 2011). Although it is an endothermic process, so in order to the process proceed energy is required. The energy that products contain and its composition depends on the MSW input characteristics, so they can vary significantly depending on the input. Table 3 summarizes the main products obtained: Product Composition Definition Gas Mainly hydrogen (H 2 ), methane (CH4), carbon monoxide (CO), carbon dioxide (CO2), and other volatile components of the MSW. Between 20% and 50% by weight of the input. Approximate heating value around 3-12 MJ/Nm3. Liquid Tar, oil, and water containing complex hydrocarbons such as organic acids, phenols, polycyclic aromatic hydrocarbons (PAHs), and alcohols. The aqueous phase can be a considerable part of the liquid. Between 30% and 50% by weight. Approximate heating value around 5-15 MJ/kg. Solid The remaining part of products are solids similar to char containing metals, sand, glass, etc. The char can be around 20% and 50% by weight, which can contain between 10% and 50% of ash. Approximate heating value around 10-35 MJ/kg. Table 3. Summary of products from pyrolysis process (Juniper, 2001; Williams, 2005). The values in Table 3 are just indicative values for general well-sorted refuse derived fuel, automobile waste, or biomass waste. If the waste input has more quality (well-sorted waste), higher ratios of oils and gasses will be obtained; but if the input waste is mixed, it can produce more char and solid residues (inorganic residues). The water present in the waste is also influential, particularly for the gas and liquid products. 11/06/2014 Page 18 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde The pyrolysis process is divided into different phases. In the first phase the moisture is released from the waste by drying it at 100-120ºC. After this phase, volatile compounds are released and the compounds with complex carbon strings are degraded to more carbon simple strings by a series of complex reactions. As the temperature increase from 200ºC to 800ºC, oxygen, hydrogen and nitrogen bonds are broken, so the gaseous outputs are formed; these are the primary reactions (Bilitewski et al., 1997). The secondary reactions take place after these primary reactions, where the tar can be converted into more gases and char, as well as there is an increase of CO2 and CH4 generation. Secondary reactions are also carried out in gasification process. The pyrolysis reactor is heated through the walls although compaction of the waste and friction also contribute to heating. Pyrolysis is said to be held in an inert atmosphere, but the reality is that it takes place into a pyrolysis gases atmosphere that generate some secondary conversion reactions. Temperature Range (ºC) Chemical Reactions 100-120 Thermal drying, dehydration. 250 Deoxidation, desulfurization, molecular splitting of water and carbon dioxide, splitting of hydrogen sulfide. 340 Breakage of bonds of aliphatic compounds, splitting of methane and other aliphatic compounds. 380 Carbonization 400 Breakage of carbon-oxygen and carbon-nitrogen. 400-600 Decomposition of bituminous compounds into lowtemperature oils and tars. 600 Cracking of bituminous compounds into heat resistant components (gaseous, shortchained hydrocarbons), formation of aromatic compounds (benzene and derived compounds). >600 Olefin (ethylene), reaction of ethylene to cyclohexane, thermal aromatization to benzene and higher-volatility aromatic compounds. Table 4. Pyrolysis reactions as a function of the temperature (Bilitewski et al., 1997). Reprinted with permission from Waste Management by B. Bilitewski, G. Hardtle, K. Marek et al., XV 9783540592105©(1997) Springer Science + Business Media. • Gasification During gasification process, the organic material undergo a thermal and chemical conversion into a mainly gaseous output by means of partial oxidation in presence of a gasification agent, that usually is air, steam or oxygen. When gasification is used after pyrolysis, the inputs of the process are the outputs of pyrolysis (gas, tar and char), so they can be upgraded by partial oxidation of the complex hydrocarbons contained in tar and char. The ranges of temperatures of 11/06/2014 Page 19 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde operation during the gasification process are between 800ºC and 1100ºC when air is used as oxidation agent and up to 1500ºC when it is oxygen (Christensen et al., 2011). Although gasification is an exothermal process, endothermic reactions are involved in it so they need heating that can be supplied for example from the steam when it is the gasification agent. In Table 5 the products of gasification process are explained. Product Composition Definition Gas Similar to the gas obtained in pyrolysis but with a higher content of carbon dioxide (CO2). Between 30% and 60% by weight of the input. Heating value depends on the gasification agent, but it is approximately around 3-12 MJ/Nm3, higher with oxygen as gasification agent. Liquid Smaller quantities of tar and oil. Between 10% and 20% by weight of the input. Approximate heating value around 5-15 MJ/kg. Solid Mainly ashes containing metals and other inorganic components. Ashes can be around 30% and 50% by weight of the input. Approximate heating value around 10-35 MJ/kg. Table 5. Summary of products from gasification process (Juniper, 2001; Belgiorno et al., 2003; Williams, 2005). The waste input, temperature of operation and the configuration of the process are very influential in the gasification products. The produced gas composition also like its heating value is highly dependent on the gasification agent used. The heating value of the gas output is affected by dilution from the gasification agent. For example, using air as gasification agent is cheaper than using oxygen, but the resulting gas can contain up to 60% of nitrogen (Juniper, 2001). So as gasification can be discriminated depending on the gasification agent: indirect gasification, when there is not an oxidizing agent, like steam (Hauserman et al., 1997; Staniewski, 1995), and direct gasification if the process takes place with an oxidizing agent like air or oxygen. This difference can be seen in Figure 7. 11/06/2014 Page 20 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde Figure 7. Types of gasification depending on the gasification agent (Belgiorno et al., 2003). In the indirect gasification there is a low gas production rate that reduces the cost of energy recovery and gas cleanup systems but it is a complex process and increases investment costs (Hauserman et al., 1997). Direct gasification with oxygen as the gasification agent has the same advantages as indirect gasification, but the use of pure oxygen is expensive as its cost can reach more than 20% of the overall electricity production (Della Rocca, 2001). On the other hand, direct gasification with the presence of nitrogen, taking air as gasification agent, reduce the volumetric efficiency and produce a gas with lower heating value (De Feo et al., 2000; Paisley, 1998). The main advantage of carrying out gasification process after pyrolysis is that by operating at higher temperatures and adding a gasification agent, pyrolysis products such as tar and char are further converted to CO, CO2, H2 and CH4 (Juniper, 2001). Reactions involved in pyrolysis and gasification process are several and complex. In Table 6 there are the most important reactions that take place during pyrolysis and gasification process. 11/06/2014 Page 21 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde Description Reaction energy (kJ/mol) Effect of temperature increase Effect of pressure increase Solid-gas reactions C + 1/2O 2 → CO Partial combustion 110.6 To right To left C + O2 → CO2 Combustion 393.8 - - C + 2H2 → CH4 Hydrogenation 79.9 To left To right C + H2O → CO + H2 Water -gas -131.4 To right To left C + CO2 → 2CO Boudouard -172.6 To right To left Gas-gas reactions CO + H2O → CO2 + H2 Shift 41.2 To left - CO + 3H2 → CH4 + H2O Reforming 201.9 To left To right Secondary solid-gas reactions Tar + H 2 O → CO + H 2 Steam reforming - - - Tar + H 2 → hydrocarbons + gas Hydro cracking - - - Tar + catalyst → char + gas Cracking - - - Table 6. Important pyrolysis and gasification reactions (partly based on Juniper, 2001). Adapted from Pyrolysis and Gasification of Waste: A Worldwide Technology and Business Review © (1997) Juniper Consultancy Services. Figure 8. Example of pyrolysis and gasification plant. This is a scheme of Thermoselect process for treatment of MSW, which was tested in a demonstration plant in Italy and after was build the first commercial plant in Karlsruhe (Germany) in 1999 with capacity 225000 t/year (Christensen et al., 2011). 11/06/2014 Page 22 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde Nowadays there are some companies in the world that use pyrolysis and gasification process to obtain energy in a commercial plant. Some of them listed in Table 7. Company Description Compact Power (UK) Pyrolysis and gasification with oxidizing agent is used in order to convert waste into fuel and coal. It can take an input of 8000 tonnes of waste a year. Shanks (UK) Is developing bio-drying with gasification, which uses bacteria to heat and dry waste. It has 50 MW installed on fluidized bed gasifiers. Kemestrie (Quebec) It has developed the technology called Biosyn, based on fluidized bed gasification. It treats 90% of organic waste. Organic Power (Norway) This Norwegian company has 8 projects to build in Scand inavia and South Korea small plants using combination of gasification and pyrolysis. Table 7. Example of some companies that are applying pyrolysis and gasification in waste treatment (Friends of the Earth, 2002). 1.3. Pyrolysis and Gasification Technology There are several different kinds of pyrolysis and gasification technologies worldwide as there are many manufactures with their own specific varieties. Nevertheless, those different technologies can be divided in two main groups whether the technology is focused on energy recovery, in order to produce gas with high content of H2 and CO which can be used in gas motor, gas turbine or combustion boiler; or if the technology is focused on material recovery, in order to produce a stable solid residue that can be used in construction works or produce H2 and CO rich gas used as chemical feedstock. Often material recovery occurs at the expense of energy recovery (Christensen et al., 2011). Otherwise, reactor system can be also divided into two different kinds depending on the stages involved. It can be a one-stage process, as it uses only pyrolysis or gasification, usually when there is an homogeneous feedstock such as biomass and industrial waste fractions with a small variation of either physical and chemical characteristics over time. Nevertheless, when the main objective is to achieve better gas characteristics (more H2 and CO content) or residue properties, or when the feedstock is a complicated mix as municipal solid waste, usually it is employed a two-stage process. It means a first stage of pyrolysis followed by a second stage of catalytic pyrolysis or gasification in a second reactor. A brief description of pyrolysis/gasification reactors (for pyrolysis the rectors are the same but no gasification agent like air, oxygen or steam is introduced) used for MSW is reported in the following. 11/06/2014 Page 23 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde 1.3.1. Fixed bed gasifiers (updraft and downdraft) A deep bed of waste is present in almost all the volume of the reactor in a fixed bed gasifier, and there are different zones inside the reactor depending on the direction of the MSW flow and the gasification medium flow. Depending on the direction of the flow the reactor can be updraft or downdraft. 1.3.1.1. Updraft reactors Here the waste moves counter-currently to the gases as the waste is fed in at the top of the gasifier and the gasification agent at the bottom. The waste passes through different zones while it circulates to the bottom (drying, pyrolysis, reduction and oxidation) until the outputs (ash and solid residues) are collected at the bottom. The gasifier is configured as cylinder shaped reactor mounted vertically. The off-gases (methane and tar-rich gas) are collected at the top (Christensen et al., 2011; Arena, 2011). The main benefits of this kind of reactors are the simplicity of the construction and the thermal efficiency, as gas flow heat the reactor while they are circulating upwards the reactor. Drawbacks are the high tar production, the carryover of dust with the gas, slagging, and the difficulty of thermal control in each different zone inside the reactor (Christensen et al., 2011). Figure 11. Schematic overview of updraft reactor (Christensen et al., 2011). There are several plants in Japan, Germany and Italy. 1.3.1.2. Downdraft reactors Downdraft reactors are similar to updraft reactors with the difference that the gasification agent is introduced into the reactor at the top or the sides, therefore waste and gases move co-currently towards the bottom of the gasifier. There are the same zones as for updraft gasifiers inside the reactor but in a different order. The ash is collected at the bottom of the gasifier, under the grate, and the gases leave at the base (Christensen et al., 2011; Arena, 2011). 11/06/2014 Page 24 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde Figure 17. Picture of dolomite from Navarre (Spain). The use of dolomite as catalyst for two-step pyrolysis favors the heavy hydrocarbon cracking and increase hydrogen production (Simell et al., 1997; Pérez et al., 1997; He et al., 2009). The catalyst causes an increase of steam reforming and water gas shift reactions, therefore tar, H2S and NH3 contents decrease, as well as the lower heating value (LHV) of the gaseous outputs (Karatas et al., 2012). Compared to other natural catalysts, such as olivine and limestone, dolomite is superior as it is more porous, it has higher internal surface area, more sulfur retention, lower cost and more availability. On the other hand, dolomite is less resistant to attrition, more fragile and has less mechanical strength (Corella et al., 1991). However, CH4 fraction in the products can decrease using dolomite as catalyst, maybe because of a decrease in hydrogasification and methanation reactions (Corella et al., 1991). Moreover, the use of dolomite produces carryover of fines, attrition and elutrition. Therefore, the level of dolomite used must be controlled. In order to solve some of the drawbacks of dolomite, one measure often taken is to calcine dolomite, as it gets more effective in the retention of H2S. The reason why may be the presence of CaO and MgO while dolomite contains CaCO3 and MgCO3 (Karatas et al., 2012). 1.5.2. Nickel on alumina (Ni/Al2O3) Nickel on alumina (Ni/Al2O3) is a nickel-based catalyst commonly used as catalyst in pyrolysis or gasification for tar removal. Figure 18. Picture of KUB-3 (Ni and NiO on Al2O3). This catalyst is very effective for tar removal and producing high quality gas (Aznar et al., 1998; Wang et al., 1998; Bangala et al., 1998). Nickel on alumina is able to produce a methane-rich gas, increasing the heating value of the gaseous products, at the same time that it removes tar (Baker et al., 1996). However, it is easy to deactivate due to 11/06/2014 Page 31 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde carbon deposition and nickel particle grow, producing a decrease in the active surface area of the catalyst particles. Nevertheless, in order to reduce cocking, a bed of dolomite can be introduced into the reactor (Aznar et al., 1998). This catalyst is widely used because of its commercial availability, effectiveness (in some experiments conversions have reached even 90-92% of the volatile fraction) and relatively cheapness (no more than dolomite) (Li et al., 1996). The catalyst can be prepared by two different methods. For low concentrations of nickel the method used is usually wet impregnation, while for higher nickel concentrations coprecipitation is used. 1.5.3. AFRC AFRC is another natural catalyst, such as dolomite, olivine, limestone, etc. It consists on an agglomeration of several kinds of minerals, some of them used as a catalyst for their own, like dolomite. AFRC is the catalyst used for the second pyrolysis reactor which the thesis is studying. The exact composition of AFRC is described in Table 12. Table 12. Composition of AFRC catalyst. 2. Description of work 2.1. Thermal characterization of MSW equipment Thermal characterization of MSW has been done by means of thermogravimetry (TG), bomb calorimeter and elemental analysis (EA). It is a reproducible, informative, rapid and relatively cheap method to characterize the quality, composition, kinetics, etc., of organic matter such as MSW, char, tar, etc. Moreover, it is able to work with very small samples of material (Plante et al., 2009). Thermogravimetric analyzer, bomb calorimeter and elemental analyzer used in order to carry out the characterization of the MSW input and the products of the experiments carried out during this thesis will be explained. 2.1.1. Thermogravimetric analyzer (TGA) Thermogravimetry (TG) is an analytical technique used to determine the different material’s fractions (moisture, volatile fraction, fixed carbon and ashes) and their stability when it is being heated, by monitoring its weight during the experiment. 11/06/2014 Page 32 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde The experiment is carried out in an inert atmosphere in argon and nitrogen, while the analyzer records the weight of the sample as a function of increasing temperature. For the final stage of the experiment, when combustion of fixed carbon occur, oxygen is introduced in order to oxidation succeed. Inside the analyzer, besides the main crucible with the sample, there is another empty reference crucible, so the analyzer also records the heat flow difference between the two crucibles. This technique is called differential scanning calorimetry (DSC). Therefore, the energy released or absorbed via chemical reactions during heating process is monitored. The thermogravimetric analyzer used for the thesis experiments is a vertical TGA, so the crucible is considered free from buoyancy effects, but it is necessary to calibrate the crucible before starting the experiment to compensate the differential thermal expansion of balance arms (Öner, 2007). Figure 19 is an example of a graph obtained after the analyzer software on the computer has analyzed the data and has generated the graph representing the evolution of the weight with the increase of the temperature. Figure 19. Example graph obtained by TG with the different sections identified (Wagner and Zemo, 2013). In Figure 19 it can be seen the different stages. In the first stage, quite linear, the moisture is evaporated. The second stage is where volatile fractions are emitted. In the third stage O2 starts to enter inside the TGA and there is a fast combustion which releases gases (mostly CO2). Finally in the fourth stage there are only the remaining ashes of the combustion. The experiment is divided in 4 steps: in the first step the sample is prepared and calibrated, writing down the mass of the empty crucible and the crucible filled with the sample (it will be needed after to calibrate the analyzer); a second step is needed to purge the chamber inside the analyzer where the crucibles are (with argon and nitrogen) during 45 minutes approximately; the third step is when the experiment is carried out 11/06/2014 Page 33 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde and it ends depending on the heating rate choose (5ºC/min, 10ºC/min or 20ºC/min); and the final step is for cooling the analyzer, in order it can be ready for a new experiment starts. In this case, all the experiments have been carried out with a heating rate of 5ºC/min, 10ºC/min and 20ºC/min (all but the char outputs, only made with 20ºC/min heating rate, and the RDF and PPD input samples, only made with 10ºC/min heating rate), until the temperature reaches 800ºC in an inert atmosphere (argon), then 20 minutes of isothermal regime with argon and after 20 minutes more of isothermal combustion with O2. The reason why with the MSW input samples the experiment has been done with each heating rate is because it is necessary in order to figure out the kinetics of the material. The thermogravimetric analyzer used to carry out the experiments in this thesis is the simultaneous thermal analyzer NETZSCH STA 409 PC Luxx. Figure 20 is a schematic picture of the thermogravimetric analyzer used. Figure 20. Thermogravimetric analyzer (simultaneous thermal analyzer NETZSCH STA 409 PC Luxx). The technical information is given in Table 13. Design Top-loading systém Heating SiC (1500˚C) Working temperature 20-1550˚C Heating rate, cooling 0-50 K.min-1 Holder TG-DSC, TG, DTA Measuring range of the instrument 18 g Weight of sample to 18 g Accuracy of the instrument 2 μg Atmosphere Inert, oxidative Possibility vacuum to 10-2 Pa Crucibles for TG-DSC Al2O3, Pt-Rh (35 μl) Table 13. Technical information of Thermogravimetric analyzer (simultaneous thermal analyzer NETZSCH STA 409 PC Luxx). 11/06/2014 Page 34 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde 2.1.2. Bomb calorimeter The main objective of a bomb calorimeter is to determine the combustion heat (or enthalpy of combustion, ∆Hc0). Combustion heat means the heat released when all carbon and hydrogen from hydrocarbons is combusted with oxygen to form carbon dioxide and water (Hope College, 2000). Reaction can be seen in Equation 1. 𝐶𝑋𝐻𝑌𝑂𝑍(𝑠)+�𝑋+ 𝑌 4− 𝑍 2�𝑂2(𝑔)→𝑋𝐶𝑂2(𝑔) + 𝑌𝐻2𝑂(𝑙) Equation 1. Combustion of hydrocarbons reaction (Hope College, 2000). The heat released by the oxidation of other components of the sample, such as sulfur, are also included in the combustion heat. In a bomb calorimeter the combustion heat is determined by a substitution procedure, comparing the heat released by the sample with the heat obtained from a standardized material with a known calorific value. The sample is burned inside a vessel (also called bomb) in a high-pressure oxygen atmosphere, and after the heat released is absorbed by an absorbing medium, the temperature change of this medium is analyzed. Then, this change in temperature is multiplied by a relation previously found with the energy released or heat capacity of a standardized material. Finally, some correction must been done before obtaining the final results, as is necessary to adjust the values to the heat transfer in calorimeter (Parr, 2007). Combustion heat is usually exothermic, therefore the value is usually negative (this is why combustion heat is often referred to -∆Hc0). Figure 21 shows a schematic representation of a bomb calorimeter. Figure 21. Scheme of a bomb calorimeter (Parr, 2007). The main parts of the bomb calorimeter are: the bomb or vessel where the combustion of the sample occurs; the water bath which surrounds the vessel agitated by a stirrer; the insulating jacket in order to cancel thermic perturbations from outside the calorimeter; 11/06/2014 Page 35 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde and the thermometer which measures the temperature changes inside the bucket (Parr, 2007). Calorimeter used to carry out the experiments in this thesis is an FTT isoperibolic bomb calorimeter from Fire Testing Technology Company Limited. The technical information is given in Table 14. Type Isoperibolic Water temperature 25˚C Temperature measurement Resolution 0,001 ˚C Operating pressure of oxygen (max) Prescribed in 30 bar, tested 200 bar Reproducibility [% RSD] By EN ISO 1716 0,2 % Operating temperature (max) 30 ˚C Permissible ambient temperature < 30 ˚C Permissible relative humidity < 55 % Standardization Benzoic acid Tools steel crucible, ignition wire, cotton Landfills samples to 1 g Table 14. Technical information of calorimeter (FTT isoperibolic bomb calorimeter from Fire Testing Technology Company Limited). 2.1.3. Elemental analyzer (EA) In order to get a reliable identification and verification of the element composition of the MSW, it is necessary to carry out experiments of organic elemental analysis. Elemental analyzer is mainly used to determine MSW composition of carbon, hydrogen, nitrogen and sulfur. Inside the elemental analyzer, organic substance undergoes an oxidative decomposition, combustion at high temperature in an oxygen atmosphere, and therefore a reduction of nitrogen and sulfur oxides, followed by thermal conductivity detection (Fadeeva et al., 2007). However, the oxygen content of the sample must be measured separately using pyrolysis (to measure it there must be absence of oxygen in the atmosphere of the sample) and reduction of the sample (Allison et al., 2007). As happens with the oxygen content, inorganic elements must been also analyzed separately. If they are not, they would make average with other species and the results would not be valid. This process produces the formation of carbon dioxide, water, nitrogen and sulfur dioxide. The analyzer is based on the ion signals intensities from the fragments, which are proportional to the mass concentration of the original species, and the analyzer is able to record them obtaining data (Allison et al., 2007). Inside the analyzer there is a combustion furnace where the sample inside a crucible is placed. The software used is able to measure the products of combustion during time, so it can finally figure out the fraction of carbon, hydrogen, sulfur and nitrogen originally present in the sample. When the combustion is over the crucible is removed from the furnace. 11/06/2014 Page 36 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde Elemental analyzer used to carry out the experiments in this thesis is an elemental analyzer Vario Macro Cube from ELEMENTAR. The technical information is given in Table 15. Gas requirements helium, oxygen Helium Gas purity ≥ 99,996 % Pressure in the intake to 2,5 bar Gas flow 600 ml/min Oxygen Gas purity ≥ 99,995 % Pressure in the intake to 2,5 bar Gas flow 600 ml/min Temperature module for CHNS Combustion tube 1150 ˚C Reducing tube 850 ˚C Landfills samples to 1,5 g Table 15. Technical information of elemental analyzer Vario Macro Cube from ELEMENTAR. 2.2. Micro GC (Gas Chromatograph) A gas chromatograph – mass spectrometer (GC-MS) can measure the content of H2, CO, CH4, N2, CO2, etc. of a gas flow. GC creates a time separation between measures by mixing in a tube the gas analyzed with a gas (in this case helium) which causes a separation of the components in time. The result of the analysis is shown in a chromatogram, where each peak represents one component of the original mixture. The time when the peak appears indicate which component it is, and the area of the peak measure the amount of the gas in the mixture. In Figure 22 an example of GC chromatogram is shown. 11/06/2014 Page 37 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde Figure 22. Chromatogram obtained from pyrolysis gases outputs. H2, N2, CO, CH4 and CO2 are the yields GC can detect. In Figure 22 it can be seen how depending on the time when a peak appears indicates a different yield, due to different elution times for each one. The area contained between the peak and the base in the horizontal line is used (by means of calibration equations) to find out the molar fraction of every yield. The GC used in the laboratory is Micro Box III (SLS MICRO TECHNOLOGY, Hamburg, Germany) gas chromatograph with a thermal conductivity detector TCD. The column inside the chromatograph (in which the time separation occurs) is a 65 cm long carbosphere packed column with a 1 cm2 footprint. It is able to detect components like H2, CO, CH4, N2, CO2, CHx and others. The starting temperature is 50ºC with a 5ºC step change until the end temperature of 240ºC. 2.3. Laboratory pyrolysis apparatus The apparatus employed in the pyrolysis process, consists of a pyrolysis tubular reactor set in series with another fix bed tubular reactor for a catalytic pyrolysis. Both reactors can be heated separately at different temperatures, although during the experiments both were heated at the same temperature by means of tube furnaces. The first reactor is fed with tire sample of MSW (with capacity for 20 g of MSW) which is pushed across the reactor by a rotating screw driven by an electrical motor which is possible to vary the rotation speed in order to set the residence time wanted. Moreover, nitrogen is supplied inside the reactor in order to provide a nitrogen atmosphere during pyrolysis. At the end of the reactor products are divided: char falls into a solid products deposit where it is stored while volatile products (permanent gases and condensable hydrocarbons) 50 100 150 200 250 050 -400 -200 0 200 400 600 800 1000 1200 temperature/°C 10-3.digits time/s CO H 2 N 2 CO 2 CH 4 11/06/2014 Page 38 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde circulate directly to the second pyrolysis reactor, with catalyst inside it, in order to undergo further degradation. This second decomposition is produced by high temperatures and the catalyst, while the first decomposition in the first pyrolysis reactor is produced only by the influence of temperature. After catalytic pyrolysis, product gases pass through a set of 6 impingers, 4 of them containing isopropanol and the 2 left empty, in order to condense tar and other condensable gases. This method for cooling condensable hydrocarbons is standardized. The rest of gases are directed to an ice bath as a precautionary measure for GC-MS (gas chromatographer – mass spectrometer), in order to cool the few possible condensable gases left before the flow is introduced into GC. After, in regular intervals GC takes a little sample of the gas flow which is analyzed, while the main gas flow is released to the atmosphere. Because of entire system takes place in presence of nitrogen, it is regulated through a flow-meter. Figure 23 represents a scheme of the pyrolysis apparatus used in laboratory during the experiments, including the electronic equipment used in order to analyze the products. Figure 23. Scheme of the pyrolysis apparatus employed on laboratory. For further acknowledgement of the tar collection method, the set of impringers is constructed according to the “European Tar Protocol” (Neeft, 2005). Like it can be seen in Figure 22, it consists of six impingers, five of them are filled with isopropanol (IPA) and one is empty. Impingers 1, 2 and 4 are inside a heated bath with a temperature of +35ºC, while impingers 3, 5 and 6 (6 is referred to the empty impinger) are inside a cold bath with a temperature of -20ºC. Therefore, following the order for impingers 1, 2, 3, 4, 5 and 6, the sequence of temperatures for each one is hot, hot, cold, hot, cold and cold. Impingers 2, 3 and 5 have glass-sinters so better gas dispersion will be obtained. The empty impinger is used as a droplet collector. All the set of impingers are placed in a styrox box which has a styro-foam wall to isolate warm from cold baths. When the tar (and other condensable gases) collection ends, the content inside impingers is collected 11/06/2014 Page 39 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde in order to analyze it, while tubing and glass parts are washed with isopropanol solution (Romar et al., 2010). Finally, for more detail of the first pyrolysis tubular reactor employed, Figure 24 shows the inside of this rector were the screw rotate in order to push down the sample through the reactor. Figure 24. Scheme of the first pyrolysis reactor with the rotating screw inside. 3. Results and discussion 3.1. Thermal characterization of MSW results. Previous to the pyrolysis experimentation, experiments with thermogravimetry analyzer, calorimeter and elemental analyzer have been carried out in order to characterize the MSW samples used. With these experiments, the exact composition of the different materials which can be found in a MSW sample has been found. With data obtained, it has been possible to complete an elemental analysis and a proximate analysis, followed by the determination of the combustion heat of each material former of MSW. 3.1.1. Elemental analysis and proximate analysis results First, a sample of MSW is divided into the different materials which compose it: polystyrene (PS), polyurethane (PU), textile, white paper, recycled paper, low density polyethylene (LDPE) and high density polyethylene (HDPE). With elemental analyzer the content of nitrogen (N), carbon (C), hydrogen (H) and sulfur (S) of each one is found, after calculating the average value of different experiments done with the same component. Then, with TG the approximate analysis of each component is carried out, also finding the average value between three experiments carried out with the same material but at different heating rate (5ºC/min, 10ºC/min and 20ºC/min). The results 11/06/2014 Page 40 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde These values have been found from calculating an average of the activation energy (E) and pre-exponential factor (A) dependence on the conversion extent curves. These curves are shown in Figure 29 and Figure 30 respectively. Figure 29. Activation energies (E) dependencies obtained for the thermal degradation of white paper, recycled paper, high density polyethylene (HDPE), low density polyethylene (LDPE), polystyrene (PS), polyurethane (PU) and textile studied in TGA experiments. The dotted curves are from the experimental values and the continuous lines are the regression lines of these dotted lines. E: Activation energy. α: Conversion extent. 11/06/2014 Page 47 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde Figure 30. Pre-exponential factors (A) dependencies obtained for the thermal degradation of white paper, recycled paper, high density polyethylene (HDPE), low density polyethylene (LDPE), polystyrene (PS), polyurethane (PU) and textile studied in TGA experiments. The dotted curves are from the experimental values and the continuous lines are the regression lines of these dotted lines. A: Pre-exponential factor. α: Conversion extent. 3.3. Thermal characterization of RDF and PPD results In order to elaborate a better study of the pyrolysis results, the content of the mixture of MSW introduced into the first reactor has been fixed. Two different mixtures have been considered: RDF and PPD. RDF (Refuse Derived Fuel) results from sorting (it has a specific content of each component, in this case a specific content of textile, paper, PU, PS, HDPE and LDPE) and drying MSW, and it is focused on producing a high calorific fuel taking from MSW its high calorific fractions (VADEB, 2012). Moreover, it can benefit recycling other lower calorific fractions of MSW which can be more easily recycled (EC, 2003). PPD stands for the heavy fraction of RDF. It means that it is a mixture similar to RDF, with the same purpose to get the higher calorific fractions of MSW, but in the mixture there is less content of paper and more PE, textile and PU. Moreover, this mixture includes some inorganics. 11/06/2014 Page 48 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde The composition of RDF and PPD samples can be seen in Table 20. Samples RDF (%) PPD (%) White paper 63,17 19,37 Recycled paper LDPE 15,78 37,30 HDPE 19,10 3,82 Textile 1,94 32,41 Polystyrene 0 0 Polyurethane 0 6,81 Table 20. Composition of RDF and PPD samples. Values are expressed in percentage. As Table 20 shows, the main differences between those two mixtures are the content of paper, as RDF has significantly more paper content than PPD (white paper and recycled paper are not distinguished because of the difficulty of sorting both kind of paper), the increase in the amount of LDPE and Textile content in PPD while there is a decrease in HDPE, and the presence of PU in PPD while in RDF there is no content of PU. For TG experiments, several experiments where made with each kind of mixture, in order to have more reliable results. In Table 21 the proximate analysis for RDF and PPD is shown. The TG curves used in order to carry out the proximate analysis are given in Appendix B.3. MATERIAL MOISTURE VOLATILE FRACTION FIXED CARBON ASHES RDF 1,33 77,35 0,000 21,32 PPD 1,15 88,14 3,74 6,97 Table 21. Proximate analysis for RDF and PPD. Values are expressed in percentage. Unusual numbers are painted in red. Results in Table 21 can be explained largely for the composition of each mixture. The increase of volatile fraction in PPD is caused by the increase of LDPE and PU content in this mixture (while there is only a few decrease in HDPE), which are mainly volatile fraction. The same way can be explained the high content of ashes in RDF, as it has higher amount of paper content which generate this amount of ashes. Finally, there is an unusual result in RDF fixed carbon content, as for RDF composition there should be some content (like fixed carbon content of PPD is 3,74%). This unusual value can be explained by an error of measurement in TG, caused by some remaining oxygen from previous experiments which remains inside the furnace, the nitrogen flow introduced in TG is not able to remove it, and sample undergoes a slow combustion during time at high temperatures that do not let the combustion step occurs. As PPD release a higher volatile fraction, during pyrolysis process more gases will be released between condensable and non-condensable gases. After the characterization of RDF and PPD samples, everything is ready to start the pyrolysis experiments, and analyze the results. 11/06/2014 Page 49 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde 3.4. Products results After all pyrolysis experiments in laboratory have been done, tar and condensable parts from impingers are collected, char from first pyrolysis reactor are collected and analyzed, and non-condensable gases from catalytic pyrolysis reactor are measured and analyzed too. Non-condensable gases have been analyzed by gas chromatography (GC) while for the analysis of char a thermal characterization has been carried out, with thermogravimetric analysis, elemental analysis and calorimetric analysis. During the performance of the experiments, different temperatures in the reactors have been considered in order to compare the influence of temperature in the process and determine the most favorable conditions. Pyrolysis process has been carried out at 600, 650, 700, 750 and 800ºC. Therefore, GC analyses, elemental analyses, thermogravimetric analyses and calorimetric analyses have been carried out for each temperature. Moreover, char, tar and gases collection and measurements have been also done for each temperature. 3.4.1. Products obtained The content of char generated after first pyrolysis, tar generated and non-condensable gases released after second reactor have been measured for each temperature and for each different MSW mixture (RDF and PPD). The content percentage of those outputs from the samples inputs is given in Table 22. SAMPLE RDF (weight %) PPD (weight %) T (°C) Char Tar Gases Char Tar Gases 600 40,10 0,40 59,50 46,14 0,27 53,59 650 39,00 0,39 60,61 53,22 0,21 46,56 700 36,10 0,33 63,57 43,24 0,19 56,56 750 37,10 0,34 62,56 52,16 0,10 47,74 800 36,80 0,30 62,90 57,75 0,12 42,13 Table 22. Char, tar and non-condensable gases content for each temperature set and for each different MSW mixture (RDF and PPD). Values are expressed in weight percentage. As it can be seen in Table 22, there is a clear trend to obtain less char, less tar and more gases as the temperature set in the reactors increases. This trend is less clear in PPD results as the char content decreases with temperature with the exception of 650ºC which has higher content than 600ºC and more important 800ºC, which has the most elevated value of char content. Moreover, there is a decrease on the gases production after 700ºC, reaching its lowest value at 800ºC. Lower temperatures have not been considered as for lower temperatures the composition of products has higher amount of tars. This occur because of at temperatures below 400ºC the reduction of tar is only produced by the condensation reactions at gas/vapor product temperatures, while for higher temperatures than 500ºC severe secondary cracking reactions start producing a higher conversion of the carbon in the waste and a decrease in tar content (Bridgwater et al., 1999). Moreover, tar increases 11/06/2014 Page 50 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde as a higher heating rate is set and char increases with slow heating rates with relatively low temperatures. To obtain more equal weight content distribution in products, moderate heating rates with moderate final temperatures must be set. For high heating rates up to high temperatures oils undergo further broken down and gas yield increases, whether if after the process a rapid quenching occur (flash pyrolysis) the products obtained can be mainly tars instead of gases. Finally, if the aim of the process is to get high gases fraction, the suitable conditions are slow heating rates up to high final temperatures (Christensen et al., 2011). Shorter residence times and char rapidly removal from the reactor can also increase tar content (Bridgwater et al., 1999). Figure 31 shows an idealized representation of the evolution of tar, char and gases content while the temperature increases. Figure 31. Idealized representation of the relation between tar, char and gases content with temperature. Note that in practical application other factors can induce to a significantly different representation (Christensen et al., 2011). 11/06/2014 Page 51 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde In contrast with idolized Figure 31, in Figure 32 the representation of the relation between tar, char and gases content and temperature for RDF and PPD is shown. Figure 32. Relation curves between char, tar and gases content and temperature for RDF and PPD. The reason why for PPD mixture there is not a lineal trend in char decreasing and gases increasing as the temperature increases (it is more similar to a parabolic trend, with an optimum at 700ºC) can be its composition. Like happen with some biomass wastes containing cotton, corn, rice, grass, straw, stover, etc. due to the chemical structure and composition at high temperatures tar are low but solid yield increases which produce a gases content reduction (Amutio et al., 2012). In this case PPD introduces PU, more textile and LDPE and less paper and HDPE, which for its singular chemical composition can lead to the same situation. Therefore, it can be seen that the optimal temperature condition for RDF tar reduction is 800ºC and for PPD 750ºC. However, the optimal temperature condition for gases production and char reduction for RDF and PPD is 700ºC. 0,25 0,27 0,29 0,31 0,33 0,35 0,37 0,39 0,41 30 35 40 45 50 55 60 65 70 600 650 700 750 800 weight (%) Temperature (ºC) Char, Tar and Gases content in RDF products Char Gases Tar 0,05 0,1 0,15 0,2 0,25 0,3 35 40 45 50 55 60 600 650 700 750 800 weight (%) Temperature (ºC) Char, Tar and Gases content in PPD products Char Gases Tar 11/06/2014 Page 52 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde Moreover, using RDF mixture for the pyrolysis process less tar and char, and more gases are obtained than using PPD. 3.4.2. Micro GC (gas chromatographer) results During pyrolysis process measures in GC have been periodically done obtaining different values for the gas content during all process. The data has been collected and analyzed, and after excluding N2, the maximum values of H2, CO, CH4, CO2 and residual non-condensable gases have been obtained. This residual gases are more complex non-condensable gaseous hydrocarbons with more carbon (CxHy), such as C2H4 (ethylene), C2H6 (ethane), C3H6 (propene), etc., and non-metal hydrides like H2S (Ta-Chuang et al., 2006). In Table 23 and Table 24, the maximum values of gas contents for RDF and PPD respectively are given. MAXIMUM VALUES RDF T(°C) H2 CO CH4 CO2 Residue 600 0,26 0,03 0,06 0,06 0,59 650 0,22 0,06 0,13 0,11 0,48 700 0,21 0,08 0,18 0,12 0,41 750 0,23 0,11 0,24 0,11 0,31 800 0,33 0,19 0,14 0,04 0,30 Table 23. Maximum values of H2, CO, CH4, CO2 and residue content from non-condensable gases released of RDF mixture. Values are given in weight fraction (x). MAXIMUM VALUES PPD T(°C) H2 CO CH4 CO2 Residue 600 0,23 0,08 0,08 0,12 0,49 650 0,25 0,10 0,09 0,15 0,41 700 0,23 0,10 0,10 0,16 0,41 750 0,27 0,14 0,10 0,10 0,39 800 0,29 0,18 0,09 0,09 0,35 Table 24. Maximum values of H2, CO, CH4, CO2 and residue content from non-condensable gases released of PPD mixture. Values are given in weight fraction (x). In Table 23 and Table 24 it can be seen as at lower temperatures there is a higher production of H2 and CO, and a lower production of residue for PPD than for RDF. Otherwise, at higher temperatures there is a higher production of H2 and CO, and a lower production of residue for RDF than for PPD. Overall, CH4 production is always higher for RDF and CO2 production is always higher for PPD. 11/06/2014 Page 53 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde For better appreciation of the values in Table 23 and Table 24 and their evolution with temperature, a graph has been elaborated for RDF and PPD which is represented in Figure 33. Figure 33. Relation between H2, CO, CH4, CO2 and residue contents of the gas phase products and temperature for RDF and PPD. X represents the weight fraction. As can be seen in Figure 33, there is clear trend in each component evolution with temperature, similar for both mixtures RDF and PPD. While some components like CO2, CH4 and H2 present a more or less parabolic trend, there is a clear linear trend for increasing CO content (due to the enhancement of decarboxylation and decarbonylation reactions) and decreasing residue content with increasing temperature (Amutio et al., 2012). The reason why CO2 content is higher at low temperatures is because it is produced by carboxyl release at those temperatures while CO and CH4 are produced instead of CO2 as the temperature increases due to secondary cracking of volatiles 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 600 650 700 750 800 x T (°C) Composition of the gas phase products RDF H2 CO CH4 CO2 Residue 0 0,1 0,2 0,3 0,4 0,5 0,6 600 650 700 750 800 x T (°C) Composition of the gas phase products PPD H2 CO CH4 CO2 Residue 11/06/2014 Page 54 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde (Coroson et al., 1989). Therefore, the optimal temperature in order to obtain less residue and CO2 content, and more H2, CO and relatively high CH4 content either for RDF as for PPD is 800ºC. Carrying out the process at this temperature a higher quality gas is produced (high H2 and CO content), a relatively high amount of CH4 can be useful in order to produce calorific gas, and the emissions are more respectful with environment as there is lower amount of CO2 and residue content. If the aim is the production of CH4 then the more favorable temperature of operation is 750ºC, where for either RDF as for PPD the maximum content of CH4 is produced. Finally, the overall less favorable temperature of operation is 700ºC, as there is the highest content of CO2, the lowest content of H2 and relatively low amount of CO content for RDF and PPD as well. The results obtained here are confirmed for several other experiments already carried out (Figueroa et al., 2013; Amutio et al., 2012; Jasminská et al., 2013; Fu et al., 2011). 3.4.3. Thermal characterization of pyrolysis products A thermal characterization of RDF and PPD char obtained from the first pyrolysis reactor has been done, in order to get more information which verifies the previously obtained results and furthermore to get new conclusions about pyrolysis process. The thermal characterization consists on thermogravimetric analyses in order to find the ash content of the char output and calorimetric analyses in order to determine its combustion heat. 3.3.3.1. Thermogravimetric results Thermogravietric analyses were set in order to find out the ash content of the char obtained from the first pyrolysis reactor. Experiments were set for RDF and PPD, for 600, 650, 700, 750 and 800ºC. In Table 25 results are shown, and the TG curves used to obtain the ashes results can be found in Appendix B.4. ASHES (%) TEMPERATURE RDF PPD 600 66,48 70,74 650 56,35 73,78 700 64,52 72,28 750 64,13 81,42 800 71,14 81,88 Table 25. Ash content of char from pyrolysis for RDF and PPD at 600, 650, 700, 750 and 800ºC. Values are expressed in percentage. For a better understanding of the data in Table 25 and its dependence on temperature, Figure 34 shows a graph where the ash content in char is represented for each temperature. 11/06/2014 Page 55 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde Figure 34. Graph representing ash content in RDF and PPD char depending on temperature. As can be seen in Figure 34, ash content in PPD char present a clear trend to increase as temperature increases. It is caused by the volatile matter that is forcibly released of the char particles as the temperature has increased in the reactor and therefore less volatile matters are left in the particle from char. Moreover, at higher temperatures result into higher fixed carbon content and therefore there is less fixed carbon in char. All this produce a higher content of ashes in PPD char at higher temperatures (Parthanom et al., 2012). Therefore, the higher ashes content in PPD char is obtained at 800ºC. However, behavior of ash content in RDF char depending on temperature is different than PPD. As it can be seen in Figure 34, there is an almost lineal trend for the RDF curve, which presents a maximum in ash content at 800ºC with 71,14%, and the lowest value (56,35%) at 650ºC. These results can be caused by the different composition between RDF and PPD. For RDF char fixed carbon and volatile fraction in char do not decrease at the same time like for PPD, as for RDF char volatile fraction increases while fixed carbon decreases as the temperature increases (Dou et al., 2007). Therefore the relation between the ash content on RDF char and the increasing temperature must not be a completely linear increase, as other researches, for example, have found that ash content decrease after 500ºC and increase again after 700ºC (Cozzani et al., 1995); for char from pyrolysis of used tires have been found that the maximum ash content on tar is at 550ºC and decreases after as the temperature increases (Popovic, 2000; Li et al., 2005). Due to this different results obtained by several investigations, it can be concluded that the waste used, its composition, the type of reactor used and the parameters set in it are very influential in RDF results. In this case, the temperature conditions where higher ash content from RDF char is obtained (which means that during pyrolysis process it releases higher volatile fraction) are 800ºC. 50 55 60 65 70 75 80 85 90 95 600 650 700 750 800 Ashes (%) T (ºC) Ash content for RDF and PPD char RDF PPD 11/06/2014 Page 56 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde Polystyrene TG curve Figure 40. TG curve of polystyrene (PS) carried out with a 20ºC/min heating rate. The blue line represents the decrease of mass percentage along with the time. The red line is the derivative curve. As it can be seen in Figure 40, PS is composed only for volatile fraction. LDPE TG curve Figure 41. TG curve of low density polyethylene (LDPE) carried out with a 20ºC/min heating rate. The blue line represents the decrease of mass percentage along with the time. The red line is the derivative curve. -3,0 -2,5 -2,0 -1,5 -1,0 -0,5 0,0 0,5 -20 0 20 40 60 80 100 120 020 40 60 80 Mass% Time (min) TG PS 20 ºC/min mass%-time mass%'(time) -3,0 -2,5 -2,0 -1,5 -1,0 -0,5 0,0 0,5 0 20 40 60 80 100 120 020 40 60 80 Mass% Time (min) TG LDPE 20 ºC/min mass%-time mass%'(time) 11/06/2014 Page 63 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde In Figure 41 it is shown how LDPE is composed only for volatile fraction and a few content of ashes. In Table 28 the values of the proximate analysis for this curve are shown. TOTAL mass% Volatile fraction 94,95 Ashes 5,05 Table 28. Proximate analysis of LDPE TG curve carried out with a 20ºC/min heating rate. White Paper TG curve Figure 42. TG curve of white paper carried out with a 20ºC/min heating rate. The blue line represents the decrease of mass percentage along with the time. The red line is the derivative curve. In Figure 42 the curve which represents the decrease of the mass percentage as the time goes by presents three different steps during volatile fraction is released. This is because of the three main components of paper: cellulose, hemicellulose and lignin. Those three components have different chemical structure and different degradation rate. Lignin is difficult to degrade and therefore it starts to degrade slowly until it reaches high temperatures when it is totally degraded. On the other hand, hemicellulose undergoes a fast degradation at low temperatures while cellulose degrades at temperatures around 325ºC and 375ºC (Blasi, 2008). Therefore, in the first step mainly hemicellulose is degraded while lignin is starting to degrade; in the second step mainly cellulose undergoes degradation along with a slow degradation of lignin; and in the third step lignin is totally degraded (Burhenne et al., 2013). -1,2 -1,0 -0,8 -0,6 -0,4 -0,2 0,0 0,2 0 20 40 60 80 100 120 020 40 60 80 Mass% Time (min) TG WHITE PAPER 20 ºC/min mass%-time mass%'(time) 11/06/2014 Page 64 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde In Table 29 the values of the proximate analysis for this curve are shown. TOTAL mass% Moisture 1,29 Volatile fraction 75,72 Fixed carbon 5,97 Ashes 17,02 Table 29. Proximate analysis of white paper TG curve carried out with a 20ºC/min heating rate. Recycled Paper TG curve Figure 43. TG curve of recycled paper carried out with a 20ºC/min heating rate. The blue line represents the decrease of mass percentage along with the time. The red line is the derivative curve. Figure 43 presents a different curve with respect to white paper, as there are not three clear steps when volatile fraction is released despite having similar chemical composition to white paper, containing hemicellulose, cellulose and lignin too. In this case, there is only one unique step for volatile fraction. In Table 30 the values of the proximate analysis for this curve are shown. TOTAL mass% Moisture 2,79 Volatile fraction 78,07 Fixed carbon 11,74 Ashes 7,40 Table 30. Proximate analysis of recycled paper TG curve carried out with a 20ºC/min heating rate. -1,2 -1,0 -0,8 -0,6 -0,4 -0,2 0,0 0,2 0,4 0 20 40 60 80 100 120 020 40 60 80 Mass% Time (min) TG RECYCLED PAPER 20 ºC/min mass%-time mass%'(time) 11/06/2014 Page 65 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde HDPE TG curve Figure 44. TG curve of high density polyethylene (HDPE) carried out with a 20ºC/min heating rate. The blue line represents the decrease of mass percentage along with the time. The red line is the derivative curve. As for LDPE thermogavrimetric curve, the composition of HDPE is mainly volatile fraction with a low content of ashes. In Table 31 the values of the proximate analysis for this curve are shown. TOTAL mass% Moisture 0 Volatile content 98,62 Fixed carbon 0 Ashes 1,38 Table 31. Proximate analysis of recycled paper TG curve carried out with a 20ºC/min heating rate. -1,6 -1,4 -1,2 -1,0 -0,8 -0,6 -0,4 -0,2 0,0 0,2 0 20 40 60 80 100 120 020 40 60 80 Mass% Time (min) TG HDPE 20 ºC/min mass%-time mass%'(time) 11/06/2014 Page 66 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde Polyurethane TG curve Figure 45. TG curve of polyurethane (PU) carried out with a 20ºC/min heating rate. The blue line represents the decrease of mass percentage along with the time. The red line is the derivative curve. In Figure 45 can be seen as when volatile fraction is released there is a small step at the beginning and a second larger step. This can be caused by a non-pure PU sample, which can contain little fractions of other plastics. In Table 32 the values of the proximate analysis for this curve are shown. TOTAL mass% Moisture 0 Volatile content 77,05 Fixed carbon 0 Ashes 22,95 Table 32. Proximate analysis of polyurethane (PU) TG curve carried out with a 20ºC/min heating rate. Appendix B.2 – Results of calorimetric analysis from MSW components In Table 33 the original results of the calorimetric analysis from MSW components are given. Two calorimetric experiments were carried out for each component, so for the thesis analysis an average of every two values was calculated. -1,2 -1,0 -0,8 -0,6 -0,4 -0,2 0,0 0,2 0,4 0 20 40 60 80 100 120 020 40 60 80 Mass% Time (min) TG PU 20 ºC/min mass%-time mass%'(time) 11/06/2014 Page 67 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde Combustion Heat (MJ/kg) MATERIAL Value 1 Value 2 PU 19,43 23,68 TEXTILE 21,39 19,77 PS 41,44 42,73 HDPE 33,57 35,83 White Paper 13,41 14,57 Recycled Paper 14,62 14,54 LDPE 44,22 43,86 Table 33. Original results obtained from the calorimetric analysis of MSW components. It can be seen as the results from both experiments are very similar, so the average between them can be calculated. Appendix B.3 – TG curves of RDF and PPD mixtures Thermogavrimetic curves of RDF and PPD mixtures where used in order to get the proximate analysis of each mixture. Those experiments with thermogravimetric analyzer were carried out only with one heating rate, the same for both mixtures: 10ºC/min. In Figure 46 and Figure 47 TG curves of RDF and PPD mixtures are represented. The lines represented in the graphs have the same meaning as for MSW components TG curves. Figure 46. TG curve of RDF mixture carried out with a 10ºC/min heating rate. The blue line represents the decrease of mass percentage along with the time. The red line is the derivative curve. -0,7 -0,6 -0,5 -0,4 -0,3 -0,2 -0,1 0,0 0,1 0 20 40 60 80 100 120 020 40 60 80 100 120 Mass% Time (min) TG MIX RDF 10 ºC/min mass%-time mass%'(time) 11/06/2014 Page 68 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde In Table 34 the values of the proximate analysis for this curve are shown. TOTAL mass% Moisture 1,33 Volatile content 77,66 Fixed carbon 0 Ashes 21,01 Table 34. Proximate analysis of RDF mixture TG curve carried out with a 10ºC/min heating rate. Figure 47. TG curve of PPD mixture carried out with a 10ºC/min heating rate. The blue line represents the decrease of mass percentage along with the time. The red line is the derivative curve. In Table 35 the values of the proximate analysis for this curve are shown. TOTAL mass% Moisture 1,15 Volatile content 88,13 Fixed carbon 3,74 Ashes 6,97 Table 35. Proximate analysis of PPD mixture TG curve carried out with a 10ºC/min heating rate. Appendix B.4 – TG curves of RDF and PPD char In order to find the final ash content of RDF and PPD char, thermogravimetric curves of each mixture char obtained from pyrolysis at 600, 650, 700, 750 and 800ºC were used. All the thermogravimetric experiments were carried out at 20ºC/min heating rate. As there are several curves, for better understanding of the difference between each TG curve with the temperature, they have been represented in two graphs, Figure 48 and Figure 49, representing all curves at different temperature of the same mixture chars in the same figure. -0,7 -0,6 -0,5 -0,4 -0,3 -0,2 -0,1 0,0 0,1 0 20 40 60 80 100 120 020 40 60 80 100 120 Mass% Time (min) TG MIX PPD 10 ºC/min mass%-time mass%'(time) 11/06/2014 Page 69 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde Figure 48. TG curves of RDF char from pyrolysis carried out at 20ºC/min heating rate. The blue line represents the decrease of mass percentage along with the time. The red line is the derivative curve. Figure 49. TG curves of PPD char from pyrolysis carried out at 20ºC/min heating rate. The blue line represents the decrease of mass percentage along with the time. The red line is the derivative curve. In Figure 48 and Figure 49 is easy to see like with the ashes analysis the evolution of the TG curve with the increasing temperature. As has been said previously on the ashes results discussion, PPD char TG curves follow the trend to undergo a lower mass percentage decrease as the temperature increases, while there is not a clear trend for RDF char TG curves. Appendix B.5 – Results of calorimetric analysis from RDF and PPD char In Table 36 the original results of the calorimetric analysis from RDF and PPD char are given. Two calorimetric experiments were carried out for each mixture char obtained from pyrolysis at 600, 650, 700, 750 and 800ºC, so for the thesis analysis an average of every two values was calculated. 50 60 70 80 90 100 110 010 20 30 40 50 60 70 80 Mass% Time (min) TG CHAR RDF 20 ºC/min 600ºC/min 650ºC/min 700ºC/min 750ºC/min 800ºC/min 70 75 80 85 90 95 100 105 010 20 30 40 50 60 70 80 Mass% Time (min) TG CHAR PPD 20 ºC/min 600ºC/min 650ºC/min 700ºC/min 750ºC/min 800ºC/min 11/06/2014 Page 70 Thermal characterization of MSW for purpose of its gasification and pyrolysis Abel Valverde COMBUSTION HEAT (MJ/kg) MATERIAL TEMPERATURE Value 1 Value 2 RDF 600 7,25 8,66 650 7,28 8,31 700 7,10 6,55 750 8,83 8,11 800 12,49 15,52 PPD 600 9,32 10,12 650 6,34 8,54 700 9,55 9,09 750 10,42 8,22 800 7,81 5,87 Table 36. Original results obtained from the calorimetric analysis of RDF and PPD char. 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