Techno-economic assessment of biogas plant upgrading by adsorption of hydrogen sulfide on treated sewage–sludge

Depósito de Investigación de la Universidad de Sevilla https://idus.us.es/ This is an Accepted Manuscript of an article published by Elsevier in Energy Conversion and Management, Vol. 126, on October 2016, available at: https://doi.org/10.1016/j.enconman.2016.08.005 © 2016 Elsevier. En idUS Licencia Creative Commons CC BY-NC-ND 1 Techno-economic assessment of biogas plant upgrading by 1 adsorption of hydrogen sulfide on treated sewage–sludge 2 3 P. G. Aguilera, F. J. Gutiérrez Ortiz* 4 Departamento de Ingeniería Química y Ambiental, Universidad de Sevilla 5 Camino de los Descubrimientos, s/n. 41092 Sevilla, Spain 6 Phone: + 34 95 448 72 68 7 * Corresponding author, “Francisco Javier Gutiérrez Ortiz” [email protected] 8 9 Abstract 10 Biogas plant upgrading by adsorption of hydrogen sulfide on treated sewage–sludge was 11 techno-economically assessed. Three different processes were included in the study: the 12 desulfurization of biogas by adsorption, the in-situ regeneration of the adsorbent and its 13 production from sewage-sludge. Biogas plant upgrading was performed for a flow rate of 14 1000 Nm3/h of biogas with a H2S concentration of 2000 ppmv and a breakthrough 15 concentration of 200 ppmv, which is the technical limit value for internal combustion engines. 16 The cost due to the steam required for the in-situ regeneration was evaluated in two different 17 scenarios: as a bought external utility and as an in-situ produced utility, installing an electric 18 or a biogas steam boiler. According to the cash flow analysis carried out, all the options 19 require a similar minimum selling price for the upgraded biogas (about 0.27-0.29 €/Nm3), 20 with a cost of the overall desulfurization process between 2.5 and 4.0 c€/Nm3. 21 Keywords 22 Desulfurization, biogas, sewage-sludge based adsorbent, hydrogen sulfide, techno-23 economic assessment. 24 2 1. Introduction 25 Anaerobic digestion is a biochemical degradation process that is widely performed from 26 organic matter present in municipal solid waste (MSW) deposited in a landfill. Previous 27 results of life cycle assessment have shown that anaerobic digestion is always favorable with 28 respect to the scenario in which organic wastes from MSW are aerobically processed [1]. 29 Four main stages are normally involved in the process, including hydrolysis, fermentation, 30 anaerobic oxidation and methanogenesis. Landfill gas (LFG) or biogas is produced by 31 anaerobic digestion process. It is mainly composed of methane, carbon dioxide, moisture, and 32 minor components such as H2S. By using biogas as energy resource, greenhouse gas 33 emissions are reduced by the substitution of fossil fuels [2, 3]. Nevertheless, biogas also 34 contains hydrogen sulfide, which is highly corrosive, so it has to be removed before entering 35 the biogas in an internal combustion engine. 36 For H2S biogas removal, one of the most utilized methods is adsorption on activated 37 carbon [4]. In recent years, the search for low-cost adsorbents has been promoted to reduce 38 the high operating costs. Locally available precursors such as natural materials, wastes from 39 agriculture, industry or domestic, as well as some of their by-products (slag, sludge, fly-ash, 40 bagasse or red mud) can be utilized as low-cost adsorbents. Activated carbon produced from 41 sewage-sludge by thermal treatment to remove hydrogen sulfide from biogas has been 42 previously tested, characterized and regenerated after being used, and the adsorption process 43 has been modeled [5-8]. In some cases, capacities of adsorption doubled that of the 44 commercial unimpregnated activated carbon tested and used as reference [5]. Moreover, the 45 design of the regeneration process was performed with the aim of minimizing the use of 46 resources such as water consumption, time and energy (temperature) demand while achieving 47 a high rate of regeneration. The selected in-situ regeneration consists of entering firstly steam 48 at 250 °C, and then air at ambient temperature, in a second step. At lab-scale, it could be 49 3 performed in a short period (20 min) and the adsorption capacity decreased only 2.6% on 50 average in each adsorption/regeneration cycle [6] (calculated for a breakthrough concentration 51 of 200 ppmv). 52 Scientific literature contains some studies on production of low-cost adsorbents from 53 different materials, where the treatment of activation and its applications are described [5, 9]. 54 In a recent review, the production cost of adsorbents from alternative precursors from wastes 55 was estimated to be less than 0.1 US$ per kg, and usually between 0.01 to 0.05 US $ per kg 56 [9]. However, to our knowledge, no other studies exist on techno-economic assessment of the 57 biogas upgrading process using adsorbents produced from sewage-sludge for H2S removal. 58 On the contrary, there are studies that compare upgrading technologies [10-12], as well as 59 techno-economic evaluations focused on the MSW energy recovery through gasification [13]. 60 Thus, this work is aimed at carrying out a techno-economic study of the production, 61 adsorption and regeneration systems once made the scale-up from the results of the previous 62 lab-scale investigation [5-8]. 63 2. Methodology 64 This section provides an overview of the collection process and cleaning treatments of 65 landfill gas for its use in internal combustion engines. Besides, the overall adsorption 66 treatment, its design and methodology to carry out the techno-economic assessment are 67 described in more detail. 68 2.1. Overall Process 69 Fig. 1 shows a general scheme of a biogas plant, from the MSW landfill to the 70 combustion engine. The biogas extraction from the MSW landfill is performed through a 71 series of vertical wells inserted into the waste. The composition of biogas is very variable, and 72 it is influenced by several factors, mainly the type of substrate, its content of organic load and 73 its biodegradability, as well as the operating conditions during anaerobic digestion. As a 74 4 result, composition is variable not only from one MSW landfill to another but also from one 75 well to another. Nevertheless, Table 1 shows average compositions of biogas from MWS 76 landfill [14]. The wellheads are connected to a collection header by a network of pipes, and 77 gas collection is carried out by applying small suction pressure using a blower or vacuum 78 induction system. When the biogas is extracted, it contains a trace amount of water vapor, 79 particulates and hydrogen sulfide, which have to be removed to prevent subsequent blockages 80 and corrosion. Suspended solids removal is done by a cyclone and/or a filter. In order to 81 remove water vapor physically after it cools down and may condense in pipes and equipment, 82 a condensate trap is installed in line. Besides, it is common to use gas cooling, and even 83 compression, to remove water vapor or humidity from the LFG [15]. Since small amounts of 84 H2S can cause serious corrosion, the concentration of H2S is usually controlled below 200–85 1000 ppmv, depending on the final use of biogas [10]. The location of the desulfurization 86 system is marked in Fig. 1 by a red dotted and dashed line. The partially cleaned biogas can 87 be directly used in a combined heat and power device, or be combusted for heat generation. A 88 flare is needed to control LFG emissions during the start-up and downtime of the energy 89 recovery system and to control gas that exceeds the capacity of the energy conversion 90 equipment [15]. 91 Table 1. Analysis of bulk gases withdrawn from the wells of a MSW landfill 92 5 Compound Concentration Methane 45 – 65 vol.% Carbon Dioxide 34 – 55 vol.% Nitrogen 0 – 20 vol.% Oxygen 0 – 5 vol.% Hydrogen 0 – 1 vol.% Hydrogen sulfide 0.01 – 1 vol.% Water vapor at saturation Carbonyl sulfide traces Ammonia traces Other organics traces 93 94 95 Fig. 1. General scheme of a biogas plant 96 97 6 2.2. Upgrading: Process design 98 Three different processes have been considered in this study: the desulfurization of 99 biogas by adsorption, the regeneration treatment of the adsorbent and its production from 100 sewage-sludge. They are supposed to be installed in an existing production and collection 101 biogas facility. Fig. 2 depicts the simplified process flow diagram of the fixed bed adsorbers 102 (six are active and the seventh is the reserve), including the steam production (or purchase) 103 for the regeneration treatment and the production of adsorbent. 104 7 105 Fig. 2. Flow diagram of fixed-bed adsorbers, steam-air regeneration and production of 106 adsorbent 107 108 As a design basis, a flow rate of 1000 Nm 3 /h of biogas and a H 2 S concentration of 2000 109 ppmv were chosen for the desulfurization process, which are typical values in a large-sized 110 landfill that collect at least over 100,000 t of municipal solid waste per year [11]. The 111 8 breakthrough concentration was fixed in 200 ppmv, which is the limit value to prevent 112 problems in internal combustion engines. The scale-up of the adsorption system from 113 experimental data [5, 6] was performed by Aspen Adsorption and Comsol Multiphysics 114 simulation based on previous work [7, 8]. To determine the number of beds needed in the 115 adsorption system, calculations were made assuming different numbers of beds (from 2 to 14) 116 and including an extra bed, which operates during regeneration to get a continuous operation, 117 with the objective of minimizing the equipment cost. Previously, from an estimated average 118 time of regeneration cycle and a superficial velocity equal to that used in the lab scale (0.026 119 m/s), the adsorbent bed was sized, thus leading to a bed height, which was kept constant while 120 bed diameter and number of beds varied as above mentioned. Once the diameter and height 121 bed were selected, the breakthrough time was simulated using Comsol Multiphysics, and the 122 main parameters were estimated. This way, the main equipment regarding H2S adsorption, 123 fixed-bed regeneration and sorbent production were sized. The results are shown and 124 discussed in section 3. 125 Once the breakthrough concentration is reached, the sewage-sludge based adsorbent has 126 to be regenerated (up to 14 times before replacing by fresh adsorbent). The selected in-situ 127 regeneration process is based on previous work [6] and consists of entering firstly steam at a 128 temperature of 250 °C, and then an air stream, in a second step, at ambient temperature. 129 Regarding the steam used at regeneration, two options were considered: to buy it (case 1), or 130 to produce it in an electric or a biogas boiler (cases 2a and 2b). In case of using an electric 131 boiler, the superheater would be also electric. The biogas steam boiler was simulated in Aspen 132 Plus in order to calculate the amount of purified biogas required to generate the steam for the 133 regeneration. In this way, steam would be produced using heat generated from biogas and not 134 from an external energy source. The simulation was performed using an approach of steam 135 boiler as a system built up of a combustion chamber plus a number of heat exchangers, where 136 15 service for the time that any of the other beds is under regeneration. For this design, it has 260 been verified that the adsorption time is at least six times as long as the desorption time [22]. 261 A scale-up to 1000 Nm3/h was performed with the same superficial velocity used in the lab 262 scale (0.026 m/s). Thus, it involves an internal diameter of each bed of 1.51 m and a bed 263 depth of 12.2 m, with an L/D ratio of 8.1. The pressure drop across the bed is very low (about 264 14 Pa, computed by using the Ergun equation), so the power consumption due to the 265 adsorbers is not significant. The outlet concentration of H2S reaches 200 ppmv after 225 266 hours of operation, according to the bed lifetime estimated through the Comsol model based 267 on previous studies [7, 8]. Knowing that the regeneration efficiency decreases 2.6% on 268 average in each regeneration cycle, and assuming 14 regeneration cycles before reloading the 269 bed with fresh adsorbent, the time of use of the adsorbent is estimated to be 117.7 days. The 270 amount of adsorbent needed is 621 kg/day and, for its production, 1036 kg/day of sewage 271 sludge must be fed to the rotary furnace since about 40% of the dry weight is lost during the 272 thermal treatment [5]. 273 Table 4. Economical parameters for different number of online beds 274 Beds adsorbing* 2 3 4 5 6 7 8 10 12 14 Flow rate (Nm3/h) 500 333 250 200 167 143 125 100 83 71 Bed diameter (m) 2.61 2.13 1.84 1.65 1.51 1.39 1.30 1.17 1.06 0.99 Activated carbon production (kg/day) 799 710 666 639 621 609 599 586 577 571 Raw sewage sludge (kg/day) 1332 1184 1110 1165 1036 1015 999 977 962 951 Case 1 (steam is bought) Total Investment Cost (k€) 929.2 924.0 937.0 956.0 812.5 828.4 845.0 877.3 908.6 938.5 Variable Operating Cost (k€/year) 145.4 98.3 74.7 60.6 51.1 44.4 39.4 32.3 27.6 24.2 Total Operating Cost (k€/year) 205.8 158.3 135.6 122.7 104.0 98.3 94.3 89.4 86.7 85.3 Case 2a (steam is produced in an electric boiler) Total Investment Cost (k€) 1182.3 1122.4 1104.0 1102.0 943.4 947.7 955.2 973.7 995.0 1017.2 Variable Operating Cost (k€/year) 421.6 283.8 214.9 173.6 147.4 126.4 111.6 91.0 72.2 67.3 Total Operating Cost (k€/year) 498.5 356.8 286.8 245.3 208.8 188.0 173.7 154.3 141.9 133.5 Case 2b (steam is produced in a biogas boiler) Total Investment Cost (k€) 2347.4 1882.5 1665.0 1545.3 1309.2 1258.8 1225.6 1188.1 1172.6 1169.1 16 Variable Operating Cost (k€/year) 16.5 14.7 13.8 13.3 12.9 12.7 12.5 12.2 12.0 11.9 Total Operating Cost (k€/year) 169.2 137.1 122.1 113.8 98.1 94.5 92.2 89.5 88.3 87.9 * One more bed is added as a reserve coming into service during regeneration of any of the other beds. 275 276 The results of regeneration scale-up are a period of steam regeneration of 13.9 h, using 277 422 kg/h of steam at 250 ºC, plus 4.6 h using 50.2 m3/h of air at ambient temperature. The 278 final steam-to-sorbent mass ratio is 0.56, which is somewhat lower than that obtained for the 279 lab-scale. This is a sufficiently good estimate for the purpose of the present techno-economic 280 study. 281 In order to obtain the steam that the regeneration process requires, a consumption of 282 biogas of 6.0% of treated biogas was estimated (case 2b). Moreover, the thermal power 283 demand of the process to get the saturated steam (at 4 bar) is 310 kW, and that exchanged in 284 the superheater is 25 kW. This energy would be electric if an electric boiler and superheater 285 were used. If the steam were generated at 7 bar (another normal pressure for commercial 286 boilers), the thermal power would be 314 and 20 kW for the boiler and superheater, 287 respectively. 288 In addition, the flow-rate of biogas needed to obtain the adsorbent in the furnace is 2.1% 289 of treated biogas, as a result of the calculation described in the section 2.2. 290 3.2. Economic analysis 291 Purchase equipment costs are shown in Table 5 along with the references used to 292 estimate them. Adsorbers cost was obtained considering them as a series of towers [28] of 293 stainless steel AISI 304 with a thickness of 3 mm and the other dimensions above specified. 294 Likewise, the rotary furnace for activation is made of carbon steel and its cost was calculated 295 by scaling to the new capacity (Eq. 1) [29]. In addition, the approach of biogas boiler cost was 296 used to calculate the purchase cost of the boiler using correlations from surveys of such 297 equipment vendors applied to natural gas [30], and the cost of (steam-gas) superheater was 298 17 obtained from a costing tool [28]. Similarly, the costs of the electric boiler and the electric 299 superheater were computed from surveys of vendors [31, 32]. It should be noted that the 300 investment costs associated with the boiler, two fans for biogas and air, as well as a water 301 pump were considered only for the option of generating steam for the adsorbent regeneration 302 on site instead of buying it and being considered a utility. In case 2a, only the pump was 303 necessary. For regeneration, an air fan was considered. Finally, a mobile bucket elevator (15 304 m height) to load the adsorbent into the columns, and a flat belt (17 m length) under the 305 adsorbers, to download the sorbent from columns as well as to take the adsorbent to the 306 bucket elevator, were accounted for [33]. Table 6 shows the estimated investment and 307 operating costs for all the cases. 308 Table 5. Purchase equipment cost of the most important equipment 309 Process Unit Cost and parameters Unit Ref. ADSORBERS 208.04 k€2015 [28] Height/diameter 12.2/1.5 m/m ROTARY FURNACE [29] C0 205.00 k€2003 CS 106.78 k€2015 n 0.6 qS 1036 kg/day q0 4000 kg/day ELECTRIC SUPERHEATER 9.95 k€2015 [32] STEAM-GAS SUPERHEATER 1.48 k€2015 [28] BUCKET ELEVATOR 15.57 k€2015 [28] FLAT BELT 17.64 k€2015 [33] Cost correlation (L in ft) 1.1ꞏL0.66 k$ 2003 ELECTRIC BOILER 49.74 k€2015 [31] BIOGAS BOILER 196.20 k€2015 [30] Steam mass flow 422 kg/h Power demand 310 kW Cost correlation (P in BHP) 600ꞏP – 10000 $2008 BLOWERS AND PUMP [28] Case 1 10.36 k€2015 Case 2a 19.97 k€2015 Case 2b 47.81 k€2015 TOTAL 18 Case 1 368.3 k€2015 Case 2a 427.7 k€2015 Case 2b 593.5 k€2015 310 Fig. 4 shows a breakdown of the total annual costs of biogas upgrading for all the case 311 studies, by annualizing the total capital costs at a 5% rate over the lifetime of the project (15 312 years). In all cases, the total annual costs are quite different, slightly lower in the case of 313 buying the steam (case 1). The latter presents a lower installed purchased equipment cost, 314 because the boiler and auxiliary equipment were not installed, but it has a much higher 315 variable operating cost than case 2b due to the cost of the steam, which must be bought. Case 316 2a shows high variable operating costs, especially due to energy consumption of the electric 317 boiler. 318 319 Table 6. Estimated investment and operating cost, assuming 8000 h per year 320 DIRECT COST (k€) %PEC Case 1 Case 2a Case 2b Purchase equipment cost (PEC) 100 368.3 427.7 593.5 Purchase equipment installation 40 147.3 171.1 237.4 Instrumentation and control 12 44.2 51.3 71.2 Piping 5 18.4 21.4 29.7 Electrical systems 10 36.8 42.8 59.4 Service 4 14.7 17.1 23.7 Land 0 0.0 0.0 0.0 Building (including services) 0 0.0 0.0 0.0 Yard improvements 0 0.0 0.0 0.0 Total Direct Costs (TDC) 171 629.8 731.3 1014.9 INDIRECT COST (k€) %TDC Engineering design and procurement 7 44.1 51.2 71.0 Construction expenses 8 50.4 58.5 81.2 Legal expenses 2 12.6 14.6 20.3 Contractor´s fee 3 18.9 21.9 30.4 Start-up assistance 4 25.2 29.3 40.6 EPC constructor contingency 5 31.5 36.6 50.7 Total indirect costs 29 182.7 212.1 294.3 TOTAL INVESTMENT COST (k€) 812.5 943.4 1309.2 FIXED OPERATING COSTS (k€/year) %TDC 19 Labor 1.56 9.8 11.4 15.8 Maintenance 1.50 9.4 11.0 15.2 General expenses 3.07 19.4 22.5 31.2 Management and operation services 0.44 2.8 3.2 4.5 Cost of marketing. logistic and others 1.32 8.3 9.7 13.4 Insurance 0.50 3.1 3.7 5.1 Total 8.39 52.8 61.4 85.1 VARIABLE OPERATING COSTS (k€/year) Precursor (€/kg) 0 0.0 0.0 0.0 Process water (€/t) 1.10 0.0 1.8 1.8 Electricity (€/kWh) 0.12 4.0 145.7 11.1 Steam (€/t) 29.10 47.1 0.0 0.0 Total 51.1 147.4 12.9 TOTAL OPERATING COST (k€/year) 104.0 208.8 98.1 321 Fig. 4. Breakdown of annual costs of biogas upgrading 322 According to the cash flow analysis performed, the cost of the overall desulfurization 323 process is 2.47 c€/Nm 3 . This way, by considering that production cost of biogas is 0.04 324 €/kWh [26] and that 0.16 Nm 3 of biogas (60 vol.% CH 4 ) corresponds to 1.0 kWh [26, 27], the 325 upgraded biogas would have a minimum selling price of 0.274 €/Nm 3 , if the steam were 326 computed as an external utility (case 1). Nevertheless, this option would not be possible if the 327 MSW landfill was not placed in an industrial zone and the steam availability was limited or 328 20 null. The biogas could be sold at 1.52 and 0.80 c€/Nm 3 more expensive, if the steam is 329 produced in-situ (case 2a and 2b, respectively), and so the corresponding break-even points 330 are somewhat higher. As shown in Fig. 5, since the initial investment is higher for the 331 installation of the boiler, the cash flow is lower at the beginning of the lifetime of the plant in 332 the cases 2a and 2b. On the contrary, in the end, the cash flows of the cases 2a and 2b are 333 higher, mainly in the latter, because of the fact that the projects have the same rate of return 334 (5%). The simple (non-discounted) payback period and discounted payback period are 9.4 and 335 14.2 years, respectively, for cases 1, 2a and 2b. 336 337 Fig. 5. Cumulative non-discounted and discounted cash flow diagrams over the lifetime 338 339 Apart from the base case explained above with 100% equity financing, studies assuming 340 a debt financing of 50% and 100 % with an interest of 7% were also performed, obtaining 341 that, for these latter, the price of biogas desulfurization increases to 2.61 c€/Nm 3 and 2.68 342 c€/Nm 3 for case 1, 4.16 c€/Nm 3 and 4.24 c€/Nm 3 for case 2a, and 3.54 c€/Nm 3 and 3.66 343 c€/Nm 3 for case 2b, respectively. 344 21 3.3. Sensitivity analysis 345 Estimates, based on the methodology used [21], have an expected accuracy of roughly 346 ±30%. Therefore, a sensitivity analysis was performed selecting total investment cost, cost of 347 electricity and raw material as sensitivity variables. To see their effect on the cost of biogas 348 desulfurization, a change in ±30% of the TIC (total investment cost) with respect to the base 349 case was carried out, and the costs of precursor and electricity were also changed to 0 ± 0.02 350 €/kg and 0.12 ± 0.06 €/kWh, respectively. Fig. 6 shows that the changes applied on sewage-351 sludge price from the base case barely affect the cost of desulfurization of biogas, and this 352 varies only ±0.08 c€. In addition, for the changes in energy cost, the desulfurization cost 353 varies ±0.03, 0.93 and 0.08 c€ for the case 1, 2a and 2b, respectively. In case 2a, there is a 354 greater variation because the power consumption is higher due to the use of an electric boiler. 355 On the other hand, by changing the TIC, the cost of desulfurization of biogas has a variation 356 of ±0.3 c€ for the case 1, ±0.4 c€ for the case 2a, and ±0.6 c€ for the case 2b. Therefore, by 357 considering all the case studies, the range of prices of H2S removal from biogas would be 358 between 2.02 and 5.41 c€/Nm3, which is quite competitive, as shown below. 359 360 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 -0,02 -0,015 -0,01 -0,005 0 0,005 0,01 0,015 0,02 H 2 S removal cost (c€/Nm 3 ) Sorbent purchase price (€/kg) Case 1 Case 2 a Case 2 b 22 361 362 Fig. 6. Sensitivity analysis of the cost of biogas desulfurization 363 3.4. Comparison with other technologies 364 There are numerous technologies available for H2S removal from biogas, which can be 365 divided into biological and physicochemical processes. These latter can be classified as 366 chemical absorption into a liquid (water or caustic solution), reactive (chemical oxidation) 367 and adsorption on a solid (iron oxide based materials, activated carbon or impregnated 368 activated carbon) [4, 12]. 369 Physical and chemical treatments are fast and efficient, but they are not economical due 370 to the frequent replacement of media that is required when dealing with high H2S loading 371 elimination. Chemical methods require the addition of chemicals (e.g., NaOH or iron salts), 372 which are costly and produce secondary wastes. Biological methods are utilized to convert 373 H2S into sulfate or sulfur by sulfide-oxidizing bacteria with addition of air/oxygen [34], which 374 reduces the quality of the biogas by increasing the N2 concentration [4]. Therefore, studies on 375 2,0 2,5 3,0 3,5 4,0 4,5 5,0 0,06 0,08 0,1 0,12 0,14 0,16 0,18 H 2 S removal cost (c€/Nm 3 ) Electricity selling price (€/kWh) Case 1 Case 2 a Case 2 b 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 -30 -20 -10 0 10 20 30 H2S removal cost (c€/Nm3) % Uncertainty in investment costs Case 1 Case 2 a Case 2 b 23 biological processes have focused on removing low H2S concentrations (10–50 ppmv) to 376 prevent rapid pH drop caused by sulfate accumulation, which has negative effects on 377 population of microorganisms [35]. Thus, each technology has advantages and disadvantages. 378 Among these technologies, virgin and impregnated activated carbons have been widely used 379 to remove H2S from gas stream. Table 7 shows the operating and investment costs, as well as 380 the total annualized cost of diverse technologies of H2S removal from literature [36, 37]. It 381 must be noted that the cost values showed are difficult to compare because these come from 382 systems with different gas capacities, H2S removal amounts, methodologies for cost 383 estimation and/or year of estimation. Some advantages and disadvantages regarding each 384 technology stated in Table 7 are included in Table 8. 385 From references found, the biological system BioSulfurex is the only well-documented 386 technology that could compete with that studied in the present paper. 387 388 Table 7. H2S removal costs from biogas using different technologies 389 24 Technology Capacity (Nm3/h) H2S before / after (ppmv) Operational cost (€/year) Investment cost (€) Total cost annualized (c€/Nm3) Activated carbon Siloxa a 500 447 / 0.6 35500 70300 1.08 Adsorption with iron oxides Sulfatreat b 4247 3500 / 200 2360000 2500000 7.40 Adsorption with iron oxides Sulfatreat b 4247 1000 / 200 755000 1450000 2.67 Biological scrubber Thiopaq b 4247 3500 / 200 720000 4900000 4.09 Biological scrubber Thiopaq b 4247 1000 / 200 455000 4900000 3.37 Biological system BioSulfurex a 500 (30 °C. 35vol.%CO2) 5000 / 200 25540 160500 1.18 Biological method a 500 500 / 200 na 121600 0.45 Caustic scrubber Sulfurex a 500 (30 °C. 35vol.%CO2) 10000 / 200 91490 132800 2.56 a [36] Considering 1€ = 7.4 kr, depreciation 10 years at 5% interest (year 2011) 390 b [37] Considering 1$ = 0.7 €, annualized over 10-year life cycle (year 2013) 391 ‘na’ means ‘not available’ 392 Table 8. H2S removal costs from biogas using different technologies 393 Technology Advantages Disadvantages Activated carbon Siloxa Simple process Fast and efficient High consumption of activated carbon for large flows Adsorption with iron oxides Sulfatreat Simple and effective Regeneration is possible Expensive process for its large consumption of adsorbent (only feasible for small installations) Produce waste stream of spent media Caustic scrubber Sulfurex High removal efficiency Suitable for large gas volumes or high concentrations of H2S Simultaneous removal of CO2 and H2S High initial investment costs High consumption of water and chemicals High technical requirement to deal with the caustic solution Biological scrubber Thiopaq Injecting of air into the biogas stream is not required The spent caustic solution is partially regenerated in a bioreactor Higher specific cost than other biological systems A continuous bleed stream is required to avoid accumulation of sulfate Biological system BioSulfurex Low investment and operating costs Addition of air/oxygen, which reduces the quality of the biogas Acidification of the media due to sulfuric acid formation by the degradation of the H2S (pH drop) Biological method Low investment and operating costs Chemicals are not required Addition of air/oxygen Low H2S removal rate A second cleaning step must be used pH drop, control of microorganisms’ growth 394