Turning waste cooking oils into biofuels - valorization technologies: a review

  Ci a ion: Nascimen o, L.; Ribei o, A.; Fe ei a, A.; Valé io, N.; Pinhei o, V.; A aújo, J.; Vila inho, C.; Ca alho, J. Tu ning Was e Cooking Oils in o Bio uels—Valo iza ion Technologies: A Re iew. Ene gies 2022,15, 116. h ps://doi.o g/10.3390/en15010116 Academic Edi o : A ilio Con e i Recei ed: 18 No embe 2021 Accep ed: 10 Decembe 2021 Published: 24 Decembe 2021 Publishe ’s No e: MDPI s ays neu al wi h ega d o ju isdic ional claims in published maps and ins i u ional a il- ia ions. Copy igh : © 2021 by he au ho s. Licensee MDPI, Basel, Swi ze land. This a icle is an open access a icle dis ibu ed unde he e ms and condi ions o he C ea i e Commons A ibu ion (CC BY) license (h ps:// c ea i ecommons.o g/licenses/by/ 4.0/). ene gies Re iew Tu ning Was e Cooking Oils in o Bio uels—Valo iza ion Technologies: A Re iew Lucas Nascimen o 1, And éRibei o 1, Ana Fe ei a 1, Nádia Valé io 1, Vânia Pinhei o 1, Jo ge A aújo 1, Cândida Vila inho 2and Joana Ca alho 1,* 1CVR—Cen e o Was e Valo isa ion, Uni e si y o Minho, 4800042 Guima aes, Po ugal; [email p o ec ed] (L.N.); a ibei o@c esiduos.p (A.R.); a e ei a@c esiduos.p (A.F.); [email p o ec ed] (N.V.); [email p o ec ed] (V.P.); [email p o ec ed] (J.A.) 2Depa amen o de Engenha ia Mecânica, Campus de Azu ém, Uni e sidade do Minho, 4800058 Guima aes, Po ugal; [email p o ec ed] *Co espondence: jca alho@c esiduos.p Abs ac : In sea ch o a mo e sus ainable socie y, humani y has been looking o educe he en i on- men al impac s caused by i s a ious ac i i ies. The ene gy sec o co esponds o one o he mos impac ul ac i i ies since mos ene gies p oduced come om ossil uels, such as oil and coal, which a e ini e esou ces. Mo eo e , hei inhe en p ocesses o con e ene gy in o elec ici y emi a ious pollu an s, which a e esponsible o global wa ming, eu ophica ion, and acidi ica ion o soil and ma ine en i onmen s. Bio uels a e one o he al e na i es o ossil uels, and he aw ma e ial used o hei p oduc ion includes ege able oils, wood and ag icul u al was e, municipal was e, and was e cooking oils (WCOs). The con en ional ou e o WCO alo iza ion is he p oduc ion o biodiesel, which, as all eco e y echnologies, p esen s ad an ages and disad an ages ha mus be explo ed om a echnical and economic pe spec i e. Despi e i s success ul use in he p oduc ion o biodiesel, i should be no iced ha he e a e o he app oaches o use WCO. Among hem, he mochemical echnologies can be applied o p oduce al e na i e uels h ough c acking o hyd oc acking, py oly- sis, and gasi ica ion p ocesses. Fo each echnology, he bes condi ions we e iden i ied, and inally, p ojec s and companies ha wo k wi h his ype o echnology and use WCO we e iden i ied. Keywo ds: uel; was e cooking oils; ene gy; con e sion echnologies 1. In oduc ion 1.1. Consump ion o Edible Oils in Eu ope I is es ima ed ha abou 90% o cooking oils and a s used in he EU come om ege able oils [ 1 ]. Since he sou ces and da a e e o he low o new and used edible oils in Po ugal and Eu ope, he in o ma ion is sca ce, speci ically on he quan i ies consumed, p oduced, collec ed, impo ed, and p ocessed. Se e al sou ces o in o ma ion, bo h a Eu opean and na ional le els, we e consul ed so ha alues can be es ima ed in some way. In gene al, in he analysis ca ied ou in his a icle, se e al ypes o edible oils (soybean, sun lowe oils, e c.), ege able a s, and oli e oils we e included since he e is o en no e ec i e con ol o e he esidues ha a e deposi ed in con aine s o was e oil collec ed. The in o ma ion collec ed e e s, essen ially, o he pe iod om 2014 o 2018, making i possible o ace a empo al e olu ion whene e da a a e a ailable [2]. Thus, Figu e 1and Table 1show he app oxima e amoun s o ege able oils and a s consumed a he Eu opean le el by including ege able oils and a s p oduced wi hin he Eu opean Union and impo ed ege able oils and a s, based on da a ob ained om he Eu opean Union Oil and P o einmeal Indus y, In e na ional Oli e Council, and he Na ional Ins i u e o S a is ics [2]. Ene gies 2022,15, 116. h ps://doi.o g/10.3390/en15010116 h ps://www.mdpi.com/jou nal/ene gies Ene gies 2022,15, 116 2 o 15 Ene gies 2022, 14, x FOR PEER REVIEW 2 o 15 Table 1. Calcula ion o he o al ege able oils and a s consumed in he EU, in he yea s 2015–2018 [2]. Yea 2015 2016 2017 2018 To al ege able oils and a s consumed in he EU 28 (×103 ons) 26,625 26,523 28,312 27,978 P oduc ion o EU28 ege able oils and a s EU 28 (×103 ons) 17,399 16,942 17,341 17,635 Impo o ege able oils and a s EU28 (×103 ons) 17,808 10,175 11,223 10,728 Expo o ege able oils and a s EU28 (×103 ons) 10,257 1999 1836 1885 Figu e 1. Da a on new ege able oils and a s in he Eu opean Union [3]. 1.2. Was e Cooking Oils In gene al, i is di icul o es ima e he global p oduc ion o WCO due o he lack o eliable epo s and he consequen di icul y in aceabili y (especially om he domes ic sec o ) and he di icul ies associa ed wi h es ima ing he p oduc ion o WCO om con- sump ion pa e ns. Ne e heless, se e al s udies use indi ec me hods o es ima e quan i- ies based on consump ion da a o edible oils. Es ima es published in 2008 p edic ed ha a leas 16.54 million ons (M ) o WCO would be p oduced each yea among he la ges p oducing coun ies and egions: China, Malaysia, he Uni ed S a es o Ame ica, Eu ope, Taiwan, Canada, and Japan [4]. Acco ding o he BioDieNe P ojec (2007–2009), he es ima ed annual WCO p oduc- ion o EU27 was 3.55 M , a ying be ween 6.3–8.0 kg/capi a and 5.6–7.2 kg/capi a [5]. The es ima ed annual WCO con ibu ion in he domes ic sec o would be 1.748 M [6,7], app oxima ely 49% o o al WCO p oduc ion, wi h he emainde being a ibu ed o he HORECA sec o and a lesse ex en o he indus ial sec o . Es ima es published in 2016 p edic ed ha 1.66 M o WCO would be a ailable, 0.854 M om he domes ic sec o and 0.806 M om he comme cial sec o [8]. Acco ding o he Eu opean Biomass Indus y Associa ion, he po en ial es ima ed WCO o be collec ed is a ound 8 L WCO/capi a/yea . Ex apola ed o he o al EU popu- la ion o a ound 500 million, his would mean an annual WCO p oduc ion capaci y o 4 M , app oxima ely se en imes mo e han he amoun cu en ly collec ed [9]. In a 2018 publica ion, he au ho s ga he ed da a on he quan i ies o WCO p oduced, which can be subjec ed o eco e y pe yea in di e en coun ies. Despi e he ela i e deg ee o unce ain y esul ing om he lack o e e ence in he s udies consul ed o he Figu e 1. Da a on new ege able oils and a s in he Eu opean Union [3]. Table 1. Calcula ion o he o al ege able oils and a s consumed in he EU, in he yea s 2015–2018 [ 2 ]. Yea 2015 2016 2017 2018 To al ege able oils and a s consumed in he EU 28 (×103 ons) 26,625 26,523 28,312 27,978 P oduc ion o EU28 ege able oils and a s EU 28 (×103 ons) 17,399 16,942 17,341 17,635 Impo o ege able oils and a s EU28 (×103 ons) 17,808 10,175 11,223 10,728 Expo o ege able oils and a s EU28 (×103 ons) 10,257 1999 1836 1885 1.2. Was e Cooking Oils In gene al, i is di icul o es ima e he global p oduc ion o WCO due o he lack o eliable epo s and he consequen di icul y in aceabili y (especially om he domes- ic sec o ) and he di icul ies associa ed wi h es ima ing he p oduc ion o WCO om consump ion pa e ns. Ne e heless, se e al s udies use indi ec me hods o es ima e quan i ies based on consump ion da a o edible oils. Es ima es published in 2008 p edic ed ha a leas 16.54 million ons (M ) o WCO would be p oduced each yea among he la ges p oducing coun ies and egions: China, Malaysia, he Uni ed S a es o Ame ica, Eu ope, Taiwan, Canada, and Japan [4]. Acco ding o he BioDieNe P ojec (2007–2009), he es ima ed annual WCO p oduc- ion o EU27 was 3.55 M , a ying be ween 6.3–8.0 kg/capi a and 5.6–7.2 kg/capi a [ 5 ]. The es ima ed annual WCO con ibu ion in he domes ic sec o would be 1.748 M [ 6 , 7 ], app oxima ely 49% o o al WCO p oduc ion, wi h he emainde being a ibu ed o he HORECA sec o and a lesse ex en o he indus ial sec o . Es ima es published in 2016 p edic ed ha 1.66 M o WCO would be a ailable, 0.854 M om he domes ic sec o and 0.806 M om he comme cial sec o [8]. Acco ding o he Eu opean Biomass Indus y Associa ion, he po en ial es ima ed WCO o be collec ed is a ound 8 L WCO/capi a/yea . Ex apola ed o he o al EU popula- ion o a ound 500 million, his would mean an annual WCO p oduc ion capaci y o 4 M , app oxima ely se en imes mo e han he amoun cu en ly collec ed [9]. In a 2018 publica ion, he au ho s ga he ed da a on he quan i ies o WCO p oduced, which can be subjec ed o eco e y pe yea in di e en coun ies. Despi e he ela i e deg ee o unce ain y esul ing om he lack o e e ence in he s udies consul ed o he me hods o ob aining he p e ious es ima ions, he igu es gi e ele an in o ma ion. Acco ding o he da a p esen ed, Eu opean coun ies, such as Denma k, Spain, I aly, and Ene gies 2022,15, 116 3 o 15 he Uni ed Kingdom, had annual p oduc ions be ween 0.1 M and 0.5 M o WCO. All o he EU coun ies had annual p oduc ion below 0.1 M . Analyzing he same da a on a pe capi a basis, some EU coun ies, such as Po ugal (up o 6.5 kg/capi a/yea ), become majo p oduce s [10]. Due o his la ge amoun o WCO p oduced, i is essen ial o ecycle his was e o ans o m i in o a “new p oduc ” wi h economic alue. One way o ecycle his was e is o con e i in o bio uel [11–13]. 2. P oduc ion o Al e na i e Fuels h ough The mochemical Con e sion P ocesses The plane Ea h aces se e al p oblems in he social, economic, and en i onmen al sphe es. Those a e c i ical p oblems ha co ela e, o example, wi h he consump ion o ossil uels, which is esponsible o causing nume ous en i onmen al impac s, such as he g eenhouse e ec , wi h di ec impac s on human heal h. T anspo depends on ossil uels, speci ically uels de i ed om pe oleum, gasoline, diesel, lique ied pe oleum gas, and na u al gas. In u n, au omobiles a e he leading pollu ing gases, wi h high associa ed uel cos s [14]. Recen ly, he e has been widesp ead in e es in lea ning mo e abou ob aining liquid uels om non- ossil sou ces, such as bio uels. Bio uels a e an al e na i e way o eplace ossil uels since hey a e enewable and cause less en i onmen al pollu ion. Bio uels a e less pollu ing han ossil uels as hey emi ewe chemicals ha m ul o he en i onmen du ing combus ion and hei p oduc ion p ocesses end o be mo e sus ainable [15]. The use o e hanol, a ype o bio uel, can educe global wa ming caused by ossil uels. I educes he emission o ca bon dioxide (CO 2 ), and much o he CO 2 emi ed by ca s ueled wi h e hanol is eabso bed in suga cane plan a ions, which makes he CO 2 emissions pa ially o se . On he o he hand, biodiesel has ema kable en i onmen al ad an ages when compa ed o diesel. Acco ding o a s udy pe o med by he Uni ed S a es Na ional Biodiesel Council, bu ning biodiesel can emi , on a e age, 48% less ca bon monoxide, 47% less pa icula e ma e , and 67% less hyd oca bon han diesel om pe oleum [15]. F om he poin o iew o aw ma e ials, bio uels a e an asse because hey use ma e ials wi h cyclical a ailabili y, such as some plan s (suga cane, soybeans, palm, and o he s), algae, and se e al o he esidues. Howe e , some expe imen al s udies and p ojec s in es iga e using was e s eams o p oduce bio uel, e en hough hey ha e high le els o con amina ion and he e ogenei y. In addi ion, hey ha e a mo e signi ican and as e con inui y o aw ma e ial supplies o bio uel p oduc ion. Thus, he li e a u e gene ally classi ies bio uels as i s -, second-, and hi d-gene a ion. Such classi ica ion is no ye en i ely accep ed, as i appea s o conside he classi ica ion acco ding o he echnology used o p oduce he bio uel and he na u e o he aw ma e ial [11]. Fi s -gene a ion bio uels a e p oduced om suga , s a ch, o ege able oil, and hei aw ma e ial is om ege able o animal o igin, compe ing wi h he p oduc ion and dis ibu ion o ood. Second-gene a ion p oduc s a e by-p oduc s, such as was e, biomass om ee p uning, used cooking oils, and o he s; hese a e mo e sus ainable because hey do no compe e wi h ood p oduc ion [16]. Table 2shows he ela ionship o aw ma e ial, echnology, and ypes o bio uels p oduced. Ene gies 2022,15, 116 4 o 15 Table 2. Rela ionship o aw ma e ial, echnology, and ypes o bio uels p oduced (adap ed om [ 17 ]). Fi s Gene a ion Second Gene a ion Thi d Gene a ion Biodiesel om ege able and animal oils and a s by anses e i ica ion. E hanol om cellulosic ma e ials by e men a ion. Bio uels de i ed om algae. E hanol om suga c ops and ce eals ia con en ional e men a ion. Biomass alcohol ia gasi ica ion and ca aly ic o enzyma ic con e sion. Hyd ogen bio uel. Biogas om biodeg adable was e by anae obic diges ion. Fuels om biomass ia gasi ica ion and Fische –T opsch. Bio uels de i ed om mixed was e (e.g., MSW). Biodiesel ia py olysis, depolyme iza ion, and c acks. Bio uels om ege able oils and a s h ough exis ing chemical and pe ochemical p ocesses. Due o he lack o in o ma ion, his was e is disca ded i egula ly in he sewage collec ion sys em, which causes nume ous en i onmen al impac s, such as sea and lake con amina ion. I also has economic e ec s, as i inc eases he cos s o was ewa e ea men p ocesses and can also damage he domes ic pipe sys em [18,19]. Acco ding o Dias (2013), Junipe (2009), and Nascimen o (2011), WCO can be alued in he manu ac u e o p oduc s in a ious segmen s o he indus y, such as he p oduc ion o soap, oil pain s, and bio uels. Th ough he alo iza ion o WCO, he p oduc ion o bio uels appea s as a solu ion o he ene gy p oblem o educe he amoun o was e gene a ed and i egula deposi ion [17,20,21]. In addi ion, se e al s udies e eal he po en ial o WCO o supplemen he bio uel supply chain. Mo eo e , i is a good al e na i e ha could con ibu e o sol ing uel supply p oblems in mo e isola ed places, such as u al a eas [1,17,18,22]. Rega ding he echnologies and bio uel ypes p oduced, he s a e o he a pe o med indica es a p edominance o s udies o biodiesel p oduc ion h ough anses e i ica ion. Howe e , some s udies use di e en echnologies wi h o he ypes o bio uels p oduced h ough he alo iza ion o WCO, such as he p oduc ion o hyd ogen- ich syngas in gasi ica ion, bio-oils h ough a ious ypes o py olysis, and bioke osene, among o he s. Table 3shows he leading echnologies used o p oduce bio uel and he p oduc o ha echnology [23–34]. Table 3. Al e na i e echnologies used o bio uel p oduc ion h ough WCO. Technology P oduc Re e ence Hid oc acking Biodiesel, bio-oil; bioke osene [23,24,27,28] Gasi ica ion Syngas + H [29,30] Fas py olysis Bioke osene; Bio-oil [31–34] Py olysis Bio-oil [25,26] Ca aly ic py olysis Bio-oil [25] 2.1. Hyd oc acking Hyd oc acking is commonly used in he oil e ine y o b eak down la ge molecules and emo e sul u (S), ni ogen (N), and me als om pe oleum p oduc s [ 35 ]. This p ocess equi es a high amoun o hyd ogen o desul u iza ion. Howe e , i also uses hyd ogen (H2) o sa u a e he molecules, inc easing he H2/C a io o he compounds [36]. Acco ding o Tiwa i (2011) and Melo (2016), he hyd oc acking p ocess is a good al e na i e o was e chemical ecycling because o ca alys use. The was e can be used o deoxygena ion, a complemen a y p ocess ha emo es O 2 a oms om he glyce ide molecules and a y acids in ege able and animal oils, such as animal a , used cooking Ene gies 2022,15, 116 5 o 15 oils, and o he s, o c ea e pu i ied and be e -quali y hyd oca bons [ 35 , 37 ]. In Figu e 2, he hyd oc acking p ocess is ep esen ed. Ene gies 2022, 14, x FOR PEER REVIEW 5 o 15 and o he s, o c ea e pu i ied and be e -quali y hyd oca bons [35,37]. In igu e 2, he hy- d oc acking p ocess is ep esen ed. The hyd oc acking p ocedu e is ca ied ou a high empe a u es and p essu es (52– 300 °C and 0.1–20 MPa), being powe ed by H2, esul ing in anspo uels and je uel, wi h he desi ed iscosi y, low oxygen con en , be e a omiza ion, and lub ica ion. Fo example, in he expe imen al s udy o Dujjanu a (2020), he esea che s p oduced bio- hyd ogena ed ke osene wi h he desi ed molecula leng h, sa u a ion, and b anching le el ha had simila cha ac e is ics o he mine al ke osene [28]. Li e al. (2015) epo ed in hei s udy ha hyd oc acking p oduces s aigh -chain alkanes om animal a y acid iglyce ides, WCO, and o he ege able oils. Howe e , o ha e a be e yield, lowe p oduc ion cos s, and gua an ee he emo al o he e oa oms, i is necessa y o use ca alys s, such as Ni-Mo/Al2O3 and Co-Mo/Al2O3 [38]. Figu e 2. Rep esen a ion o he hyd oc acking p ocess. (1) Reac o ; (2) acuum sepa a ion o p od- uc s. (Adap ed om [24].) Melo (2016) epo ed ha empe a u e is a signi ican ac o o be con olled as i di- ec ly in luences he eac ion and ex en o he o ma ion o alkane chains. I he empe a- u e is oo high, he e will be a signi ican loss o CO2 and CO, p oducing lowe hyd oca - bons [37]. In he s udy led by Beze gianni (2009), bio uel p oduc ion was ca ied ou by he hyd oc acking p ocess, using WCO as aw ma e ial. Resea che s ealized ha a he lowe empe a u e (350 °C), mo e ke osene/je uel and naph ha we e p oduced. On he o he hand, when hey inc eased he empe a u e o 390 °C, hey obse ed a educ ion in uel p oduc ion [23]. In ano he expe imen al s udy by Beze gianni (2009), he esea che s concluded ha high liquid hou ly space eloci y (LHSV) in he eac o associa ed wi h a mode a e em- pe a u e (350 °C) p omo es biodiesel p oduc ion. On he o he hand, he inc ease in em- pe a u e o 370 °C induces he p oduc ion o o he bio-oils [24]. Li (2015) es ed h ee ypes o zeoli es (Meso-Y, SAPO-34, and HY) wi h nickel, which we e used o con e WCO as je bio uel h ough hyd oc acking. The expe imen al esul s show ha he used cooking oil and he Meso-Y-ca alys ob ained he bes esul s since he combina ion mainly deoxygena ed hep adecane (C17H36) and pen adecane (C15H30) chains by deca bonyla ion du ing he i s h ee hou s. The chains o long alkanes we e b oken in o a ange o C8–C16 alkanes. The esea che also e i ied he p oduc ion o cycloal- kanes and a oma ic hyd oca bons h ough cycliza ion and dehyd ogena ion pa hways [38]. Wijaya (2014) es ed a Ni-ben oni e ca alys o pe o m hyd oc acking o WCO by empe a u e a ia ion o 300, 350, 400, and 450 °C. The esul s show ha he ideal empe - a u e o bio uel p oduc ion om cooking oil used wi h Ni-ben oni e ca alys was 300 °C. The hyd oc acking p oduc s con ained 2-p opanone, dodecane, oc anoic acid, decanoic acid, dodecanoic acid, and 1,2 benzene dica boxylic acid. These compounds a e simila o hose ound in ossil gasoline and diesel [39]. Figu e 2. Rep esen a ion o he hyd oc acking p ocess. (1) Reac o ; (2) acuum sepa a ion o p oduc s. (Adap ed om [24]). The hyd oc acking p ocedu e is ca ied ou a high empe a u es and p essu es ( 52–300 ◦C and 0.1–20 MPa), being powe ed by H 2 , esul ing in anspo uels and je uel, wi h he desi ed iscosi y, low oxygen con en , be e a omiza ion, and lub ica ion. Fo example, in he expe imen al s udy o Dujjanu a (2020), he esea che s p oduced bio-hyd ogena ed ke osene wi h he desi ed molecula leng h, sa u a ion, and b anching le el ha had simila cha ac e is ics o he mine al ke osene [28]. Li e al. (2015) epo ed in hei s udy ha hyd oc acking p oduces s aigh -chain alkanes om animal a y acid iglyce ides, WCO, and o he ege able oils. Howe e , o ha e a be e yield, lowe p oduc ion cos s, and gua an ee he emo al o he e oa oms, i is necessa y o use ca alys s, such as Ni-Mo/Al2O3and Co-Mo/Al2O3[38]. Melo (2016) epo ed ha empe a u e is a signi ican ac o o be con olled as i di ec ly in luences he eac ion and ex en o he o ma ion o alkane chains. I he em- pe a u e is oo high, he e will be a signi ican loss o CO 2 and CO, p oducing lowe hyd oca bons [37]. In he s udy led by Beze gianni (2009), bio uel p oduc ion was ca ied ou by he hyd oc acking p ocess, using WCO as aw ma e ial. Resea che s ealized ha a he lowe empe a u e (350 ◦ C), mo e ke osene/je uel and naph ha we e p oduced. On he o he hand, when hey inc eased he empe a u e o 390 ◦ C, hey obse ed a educ ion in uel p oduc ion [23]. In ano he expe imen al s udy by Beze gianni (2009), he esea che s concluded ha high liquid hou ly space eloci y (LHSV) in he eac o associa ed wi h a mode a e empe a- u e (350 ◦ C) p omo es biodiesel p oduc ion. On he o he hand, he inc ease in empe a u e o 370 ◦C induces he p oduc ion o o he bio-oils [24]. Li (2015) es ed h ee ypes o zeoli es (Meso-Y, SAPO-34, and HY) wi h nickel, which we e used o con e WCO as je bio uel h ough hyd oc acking. The expe imen al esul s show ha he used cooking oil and he Meso-Y-ca alys ob ained he bes esul s since he combina ion mainly deoxygena ed hep adecane (C 17 H 36 ) and pen adecane (C 15 H 30 ) chains by deca bonyla ion du ing he i s h ee hou s. The chains o long alkanes we e b oken in o a ange o C8–C16 alkanes. The esea che also e i ied he p oduc ion o cycloalkanes and a oma ic hyd oca bons h ough cycliza ion and dehyd ogena ion pa hways [38]. Wijaya (2014) es ed a Ni-ben oni e ca alys o pe o m hyd oc acking o WCO by em- pe a u e a ia ion o 300, 350, 400, and 450 ◦ C. The esul s show ha he ideal empe a u e o bio uel p oduc ion om cooking oil used wi h Ni-ben oni e ca alys was 300 ◦ C. The hyd oc acking p oduc s con ained 2-p opanone, dodecane, oc anoic acid, decanoic acid, dodecanoic acid, and 1,2 benzene dica boxylic acid. These compounds a e simila o hose ound in ossil gasoline and diesel [39]. Ene gies 2022,15, 116 6 o 15 2.2. Gasi ica ion Gasi ica ion is a he mochemical con e sion p ocess in which subs ances con aining ca bon and hyd ogen, such as biomass and solid u ban was e, a e pa ially oxidized a high empe a u es (800–1100 ◦ C). This p ocess occu ed in he p esence o a gasi ying agen (ai , s eam, and oxygen) and con e ed in o gaseous p oduc s [40]. Gasi ica ion p oduces gas om ossil uels, biomass, and was e, usually known as syn hesis gas. The syn hesis gas (syngas) consis s mainly o CO, H 2 , CO 2 CH 2 , and H 2 O [ 41 ]. This p ocess may con ain aces o hyd oca bons o high molecula weigh , ine gases, and o he con aminan s. This gas is used in he p oduc ion o elec ici y o s eam. The gasi ica ion p ocess educes he possibili y o dioxin and u an p oduc s and p e en s compounds such as SO2and NOX[42–46]. The H 2 and CO con en o he gasi ica ion eac ions can be modi ied depending on he eac ion condi ions. To p omo e he hyd ogen p oduc , se e al ca aly ic ea men s wi h Ni-based ca alys s ha e been s udied. These ca alys s ha e p o ed o be he mos e ec i e in p oducing hyd ogen om biowas e gasi ica ion so a [40,47]. Usually, syngas is used as a uel o s a iona y powe and hea gene a ion o associa ed wi h ca aly ic con e sion o p oduce o he liquid uels and chemical in e media es. Syngas can also p oduce biomass uels in liquid h ough he Fische –T opsch syn hesis, which equi es an H 2 /CO a io o a ound 2.15. Howe e , he H 2 /CO a io o syngas p oduced is ypically be ween 1.0 and 2.2 and may equi e addi ional adjus men s o be achie ed by gas exchange eac ions in wa e o e e se exchange [40,43,48]. A signi ican challenge o biomass gasi ica ion is a o ma ion, leading o co osion, scale, and blockage. Biomass a is a ligh mix u e o hyd oca bons and phenolic com- pounds ha can be con e ed in o gaseous p oduc s h ough s eam injec ion and se e al ypes o ca alys s o p omo e a con e sion. Ca alys s a e classi ied in o h ee g oups: (1) na u ally occu ing ca alys s (i.e., dolomi e, oli ine); (2) me al ca alys s (i.e., nickel and alkali me als); and (3) alkaline ca alys s (po as- sium hyd oxide (KOH), po assium bica bona e (KHCO 3 )). To educe he biocha o ma ion in syngas, ope a ing pa ame e s mus be op imized [40,49,50]. Rega ding he gasi ie , he li e a u e e iew desc ibes h ee commonly used models: luidized bed gasi ie s, ixed bed gasi ie s, and plasma gasi ie s. 2.2.1. Fluidized Bed Gasi ie The e a e no dis inc eac ion zones in he luidized bed gasi ie (Figu e 3). In his case, d ying, py olysis, and gasi ica ion occu oge he , ac oss he eac o bed, which, being well luidized, leads o p ac ically iso he mal condi ions. The bed is egula ly made up o silica walls o main ain he abili y o e ain hea [51]. Acco ding o Tamoši ¯ unas (2019), luidized bed gasi ica ion was de eloped o sol e ope a ional p oblems, such as a mix u e o gas–solid con ac , uni o mi y, empe a u e con ol, and e iciency. This gasi ie model has he ad an age o eaching a highe coal con e sion a e, p oducing a gas wi h a lowe concen a ion o a [53]. The incoming biomass is p e- ea ed and added ia an en ance inle o he gasi ie . A e eeding, mixing wi h he bed ma e ial occu s and he d ying, py olysis, and gasi i- ca ion phases ake place. An oxidizing agen is added o he bo om o he gasi ie , being ed a a speed su icien o keep he bed ma e ial in a suspended s a e, hus acqui ing cha ac e is ics like a luid. The bed empe a u e depends on he ype o ma e ial o be gasi ied, o example, in he case o biomass, due o high ola ile con en and wi h a low mel ing poin , he bed empe a u e mus be be ween 800 and 900 ◦ C. In he ea men o was e, some p ocesses ope a e a highe empe a u es o gua an ee hei comple e decom- posi ion. The ashes p oduced du ing his p ocess a e emo ed om he bo om o he gasi ie [40,43,51,54]. Ene gies 2022,15, 116 7 o 15 Ene gies 2022, 14, x FOR PEER REVIEW 7 o 15 Figu e 3. Fluidized bed gasi ie model [52]. Acco ding o Tamošiūnas (2019), luidized bed gasi ica ion was de eloped o sol e ope a ional p oblems, such as a mix u e o gas–solid con ac , uni o mi y, empe a u e con ol, and e iciency. This gasi ie model has he ad an age o eaching a highe coal con e sion a e, p oducing a gas wi h a lowe concen a ion o a [53]. The incoming biomass is p e- ea ed and added ia an en ance inle o he gasi ie . A e eeding, mixing wi h he bed ma e ial occu s and he d ying, py olysis, and gasi i- ca ion phases ake place. An oxidizing agen is added o he bo om o he gasi ie , being ed a a speed su icien o keep he bed ma e ial in a suspended s a e, hus acqui ing cha ac e is ics like a luid. The bed empe a u e depends on he ype o ma e ial o be gasi ied, o example, in he case o biomass, due o high ola ile con en and wi h a low mel ing poin , he bed empe a u e mus be be ween 800 and 900 °C. In he ea men o was e, some p ocesses ope a e a highe empe a u es o gua an ee hei comple e decom- posi ion. The ashes p oduced du ing his p ocess a e emo ed om he bo om o he gasi ie [40,43,51,54]. Kim (2015) ca ied ou WCO alua ion s udies o soy oil o p oduce syngas gasi ied wi h ai in a luidized bed eac o . The gas p oduced was il e ed wi h ac i a ed cha coal. I was obse ed ha oxida ion a low empe a u e (808.5 °C) signi ican ly changed he composi ion o he uel, making he WCO mo e a o able o he gasi ica ion eac ion, p o- ducing a mo e signi ican amoun o hyd ogen and ca bon monoxide. In addi ion, wi h he il a ion o ac i a ed cha coal, he gas (WCO p oduc ) ul illed he a equi emen (<0.1 g/Nm3) o he ene gy p oduc ion o an engine by syngas [30]. Li (2009) s udied he alo iza ion o used palm oil o ob ain syngas ich in hyd ogen in a luidized bed. The au ho s ealized ha he highe he empe a u e, he g ea e he quan i y o syngas p oduced. Howe e , his loses calo i ic powe [55]. Sakaguchi (2010) s udied he alo iza ion o WCO, cha coal, and bio-oil om he py- olysis o wood biomass o p oduce syngas in a luidized bed gasi ie wi h a maximum empe a u e o 840 °C. The au ho s concluded ha o hese ma e ials and a his empe - a u e, he ca bon con e sion is incomple e, and some mixed hyd oca bons a e p esen in he gases p oduced. This occu ed due o he ca alysis o he wa e –gas displacemen e- ac ion, and s eam gasi ica ion signi ican ly a ec ed he p oduc ’s gas yields, which led o highe H2 yields and lowe ed CO and hyd oca bon yields [56]. Figu e 3. Fluidized bed gasi ie model [52]. Kim (2015) ca ied ou WCO alua ion s udies o soy oil o p oduce syngas gasi ied wi h ai in a luidized bed eac o . The gas p oduced was il e ed wi h ac i a ed cha coal. I was obse ed ha oxida ion a low empe a u e (808.5 ◦ C) signi ican ly changed he composi ion o he uel, making he WCO mo e a o able o he gasi ica ion eac ion, p oducing a mo e signi ican amoun o hyd ogen and ca bon monoxide. In addi ion, wi h he il a ion o ac i a ed cha coal, he gas (WCO p oduc ) ul illed he a equi emen (<0.1 g/Nm3) o he ene gy p oduc ion o an engine by syngas [30]. Li (2009) s udied he alo iza ion o used palm oil o ob ain syngas ich in hyd ogen in a luidized bed. The au ho s ealized ha he highe he empe a u e, he g ea e he quan i y o syngas p oduced. Howe e , his loses calo i ic powe [55]. Sakaguchi (2010) s udied he alo iza ion o WCO, cha coal, and bio-oil om he py olysis o wood biomass o p oduce syngas in a luidized bed gasi ie wi h a maximum empe a u e o 840 ◦ C. The au ho s concluded ha o hese ma e ials and a his empe a- u e, he ca bon con e sion is incomple e, and some mixed hyd oca bons a e p esen in he gases p oduced. This occu ed due o he ca alysis o he wa e –gas displacemen eac ion, and s eam gasi ica ion signi ican ly a ec ed he p oduc ’s gas yields, which led o highe H2yields and lowe ed CO and hyd oca bon yields [56]. 2.2.2. Fixed Bed Gasi ie The ixed bed gasi ie (Figu e 4) e e s o he condi ions a he ends o he bed, which do no change unde s a iona y condi ions. The combus ible ma e ial is ed h ough he uppe pa o he gasi ie , mo ing o he lowe pa o he ae a o h ough he o ce o g a i y, while he oxidizing ma e ial is added h ough he lowe pa o he eac o . In ela ion o he p oduc s gene a ed in he sys em, he gas ises h ough he bed, and he ashes a e ex ac ed om he bo om o he bed. Thus, his con igu a ion cha ac e izes his eac o model as he coun e cu en gasi ie because he was e and he oxidizing agen oam in opposi e di ec ions [40,51]. Ene gies 2022,15, 116 8 o 15 Ene gies 2022, 14, x FOR PEER REVIEW 8 o 15 2.2.2. Fixed Bed Gasi ie The ixed bed gasi ie (Figu e 4) e e s o he condi ions a he ends o he bed, which do no change unde s a iona y condi ions. The combus ible ma e ial is ed h ough he uppe pa o he gasi ie , mo ing o he lowe pa o he ae a o h ough he o ce o g a i y, while he oxidizing ma e ial is added h ough he lowe pa o he eac o . In ela ion o he p oduc s gene a ed in he sys em, he gas ises h ough he bed, and he ashes a e ex ac ed om he bo om o he bed. Thus, his con igu a ion cha ac e izes his eac o model as he coun e cu en gasi ie because he was e and he oxidizing agen oam in opposi e di ec ions [40,51]. Fixed bed gasi ie s a e a simple, eliable, and obus small-size echnology. Due o he small size o he eac o and he low cos o i s cons uc ion, i becomes an a ac i e solu ion in small-scale applica ions [53]. Nanda (2019) used WCO in gasi ica ion a a iable empe a u es (375–675 °C), eed concen a ion o 25–40% by weigh , and eac ion ime o 15–60 min o in es iga e hei e ec s on syngas yield and composi ion. The maximum hyd ogen yields ob ained we e 5.16 mol/kg and o al gases 10.5 mol/kg. I was obse ed ha he ideal empe a u e, eed concen a ion, and eac ion ime wi h he bes yields we e 675 °C, 25% by weigh , and 60 min, espec i ely [29]. Figu e 4. Fixed bed gasi ie [57]. Li (2013) e alua ed he use o ca alys s o p oduce syngas ich in H2. The ca alys s Fe2O3, Al2O3, CaO, and ac i a ed cha coal we e es ed o e alua e he ca aly ic b eakdown pe o mance o he oil used in a ixed bed eac o . Fe2O3 was conside ed he mos e ec i e ca alys compa ed o he o he s o p oduce hyd ogen, ca bon deposi , and con e sion o WCO. When he empe a u e ose, he syngas con en and he con e sion o used oil o Fe2O3 inc eased. The con e sion o WCO o syngas occu ed a 750 °C and eached 100%, and, a he same ime, he con en o H2 and CO was 48% and 10%, espec i ely [27]. Gi en ha empe a u e in luences he esul s o syngas p oduc ion, [58] ca ied ou s udies o alue he ash p oduced in he gasi ica ion p ocess as ca alys s o p oduce syngas wi h mo e quali y and o alue he ashes p oduced. The eac ion condi ions ca ied ou Figu e 4. Fixed bed gasi ie [57]. Fixed bed gasi ie s a e a simple, eliable, and obus small-size echnology. Due o he small size o he eac o and he low cos o i s cons uc ion, i becomes an a ac i e solu ion in small-scale applica ions [53]. Nanda (2019) used WCO in gasi ica ion a a iable empe a u es (375–675 ◦ C), eed concen a ion o 25–40% by weigh , and eac ion ime o 15–60 min o in es iga e hei e ec s on syngas yield and composi ion. The maximum hyd ogen yields ob ained we e 5.16 mol/kg and o al gases 10.5 mol/kg. I was obse ed ha he ideal empe a u e, eed concen a ion, and eac ion ime wi h he bes yields we e 675 ◦ C, 25% by weigh , and 60 min, espec i ely [29]. Li (2013) e alua ed he use o ca alys s o p oduce syngas ich in H 2 . The ca alys s Fe 2 O 3 , Al 2 O 3 , CaO, and ac i a ed cha coal we e es ed o e alua e he ca aly ic b eakdown pe o mance o he oil used in a ixed bed eac o . Fe 2 O 3 was conside ed he mos e ec i e ca alys compa ed o he o he s o p oduce hyd ogen, ca bon deposi , and con e sion o WCO. When he empe a u e ose, he syngas con en and he con e sion o used oil o Fe 2 O 3 inc eased. The con e sion o WCO o syngas occu ed a 750 ◦ C and eached 100%, and, a he same ime, he con en o H2and CO was 48% and 10%, espec i ely [27]. Gi en ha empe a u e in luences he esul s o syngas p oduc ion, [ 58 ] ca ied ou s udies o alue he ash p oduced in he gasi ica ion p ocess as ca alys s o p oduce syngas wi h mo e quali y and o alue he ashes p oduced. The eac ion condi ions ca ied ou we e a ca alys concen a ion o 4% by weigh , 12:1 mola a io o me hanol/oil, and ope a ing ime o 90 min. These ca alys s op imized he con e sion o a y acids and me hyl es e s o hyd oca bons, con e ing 97% o he a y acids. Ahmad (2018) e alua ed he use o an acid ca alys om cha coal gasi ica ion ashes o WCO me hanolysis and o p oduce mo e liquid bio uel. The ideal eac ion condi ions we e a 9:1 a io o me hanol/oil, 6% by weigh o ca alys load (g), eac ion o 130 min a 65 ◦C, and allowed he con e sion o 96% o es e [59]. 2.2.3. Plasma Gasi ie Plasma gasi ica ion (Figu e 5) is a new and p omising he mochemical ea men me hod. The co e o he echnology is in he ins alla ion o he gasi ie , called he plasma o ch, which hea s he aw ma e ial o high empe a u es ( om se e al housand o ens Ene gies 2022,15, 116 9 o 15 o housands o deg ees). These high empe a u es a e eached due o he bu ning o he elec ic a c be ween wo elec odes in he discha ge chambe o plasma. The cen al pa o he gasi ie is ed in o he discha ge chambe passing h ough he egion o he bu ning a c, whe e i is hea ed. The ma e ial o be ea ed will be in con ac wi h a plasma s eam, whe e empe a u es will each 2000–5000 ◦ C. The e o e, one o he main ad an ages o plasma gasi ica ion is i s abili y o ea any dange ous o non-dange ous o ganic and ino ganic aw ma e ial, b eaking i down in o elemen a y molecules. The o ganic ma e is con e ed in o high-quali y syngas wi h a small amoun o a , while he ino ganic ac ion is glazed in ine slag and lea es he eac o a he bo om [38]. Ene gies 2022, 14, x FOR PEER REVIEW 9 o 15 we e a ca alys concen a ion o 4% by weigh , 12:1 mola a io o me hanol/oil, and ope - a ing ime o 90 min. These ca alys s op imized he con e sion o a y acids and me hyl es e s o hyd oca bons, con e ing 97% o he a y acids. Ahmad (2018) e alua ed he use o an acid ca alys om cha coal gasi ica ion ashes o WCO me hanolysis and o p oduce mo e liquid bio uel. The ideal eac ion condi ions we e a 9:1 a io o me hanol/oil, 6% by weigh o ca alys load (g), eac ion o 130 min a 65 °C, and allowed he con e sion o 96% o es e [59]. 2.2.3. Plasma Gasi ie Plasma gasi ica ion (Figu e 5) is a new and p omising he mochemical ea men me hod. The co e o he echnology is in he ins alla ion o he gasi ie , called he plasma o ch, which hea s he aw ma e ial o high empe a u es ( om se e al housand o ens o housands o deg ees). These high empe a u es a e eached due o he bu ning o he elec ic a c be ween wo elec odes in he discha ge chambe o plasma. The cen al pa o he gasi ie is ed in o he discha ge chambe passing h ough he egion o he bu ning a c, whe e i is hea ed. The ma e ial o be ea ed will be in con ac wi h a plasma s eam, whe e empe a u es will each 2000–5000 °C. The e o e, one o he main ad an ages o plasma gasi ica ion is i s abili y o ea any dange ous o non-dange ous o ganic and in- o ganic aw ma e ial, b eaking i down in o elemen a y molecules. The o ganic ma e is con e ed in o high-quali y syngas wi h a small amoun o a , while he ino ganic ac- ion is glazed in ine slag and lea es he eac o a he bo om [38]. Figu e 5. Plasma gasi ica ion model [53]. Tamošiūnas (2019) in es iga ed he alo iza ion o WCO by gasi ica ion o plasma o ob aining syngas. The bes e iciency o he gasi ica ion p ocess was ob ained a a a io o gasi ying agen / aw ma e ial (S/WCO) o 2.33. Fo his eason, he highes concen a ion o hyd ogen and ca bon monoxide, he H2/CO a io, he lowes hea ing alue, ca bon con- e sion e iciency, ene gy con e sion e iciency, speci ic ene gy equi emen s, and he a con en in he syngas we e 47.9%, 22.42%, 2.14, 12.7 MJ/Nm3, 41.3% 85.42%, 196.2 kJ/mol (o 1.8 kWh/kg), and 0.18 g/Nm3, espec i ely. The s udy concluded ha he he mal a c plasma me hod used in his s udy can be e ec i ely used o he gasi ica ion o cooking oil esidues in high-quali y syngas wi h an inc edibly low a con en [53]. Ra iq (2011) s udied he alua ion o WCO o p oduce a bio syngas h ough a plasma gasi ie ; a Ni-based ca alys was used and p opane and ai as oxidan s. The esul s show Figu e 5. Plasma gasi ica ion model [53]. Tamoši ¯ unas (2019) in es iga ed he alo iza ion o WCO by gasi ica ion o plasma o ob aining syngas. The bes e iciency o he gasi ica ion p ocess was ob ained a a a io o gasi ying agen / aw ma e ial (S/WCO) o 2.33. Fo his eason, he highes concen a ion o hyd ogen and ca bon monoxide, he H 2 /CO a io, he lowes hea ing alue, ca bon con e sion e iciency, ene gy con e sion e iciency, speci ic ene gy equi emen s, and he a con en in he syngas we e 47.9%, 22.42%, 2.14, 12.7 MJ/Nm 3 , 41.3% 85.42%, 196.2 kJ/mol (o 1.8 kWh/kg), and 0.18 g/Nm 3 , espec i ely. The s udy concluded ha he he mal a c plasma me hod used in his s udy can be e ec i ely used o he gasi ica ion o cooking oil esidues in high-quali y syngas wi h an inc edibly low a con en [53]. Ra iq (2011) s udied he alua ion o WCO o p oduce a bio syngas h ough a plasma gasi ie ; a Ni-based ca alys was used and p opane and ai as oxidan s. The esul s show ha in he es ha las ed 10 h a a empe a u e o 920 ◦ C, wi h a WCO low o 38 mL/h whe e he plasma s abilized, he concen a ions o CO, H 2 , CO 2 , and CH 4 inc eased om 8.4 o 13.8%, 7.1 o 15%, 3.2 o 4.2%, and 0.6 o 2.5%, espec i ely, and he concen a ed CO 2 dec eased om 5.32 o 2.7%. Howe e , a high p opo ion o H 2 /CO indica es syngas sui able o p oduce hyd ogen o biome hane [60]. Table 4shows some global companies ha use gasi ica ion o p oduce bio uels h ough was e, o es biomass, MSW, and o he s. Gasi ica ion is a ecommended echnology because i can be used in a wide a ie y o o ganic oods (biodeg adable and non-biodeg adable), he ma e ials p oduced ha e good quali ies, and his echnology is highly ecommended o p oduce bio syngas and bio-oil.