Syngas biomethanation: Current state and future perspectives

Bio esou ce Technology 358 (2022) 127436 A ailable online 6 June 2022 0960-8524/© 2022 The Au ho s. Published by Else ie L d. This is an open access a icle unde he CC BY-NC-ND license (h p://c ea i ecommons.o g/licenses/by- nc-nd/4.0/). Syngas biome hana ion: Cu en s a e and u u e pe spec i es Se gio Paniagua a , b , Raquel Leb e o a , b , Raúl Mu˜ noz a , b , * a Ins i u e o Sus ainable P ocesses, D . Me gelina s/n, 47011 Valladolid, Spain b Depa men o Chemical Enginee ing and En i onmen al Technology, School o Indus ial Enginee ing, Uni e si y o Valladolid, D . Me gelina s/n, 47011 Valladolid, Spain HIGHLIGHTS GRAPHICAL ABSTRACT •Biome hane om syngas is an al e na- i e o na u al gas in a bio-ci cula economy. •O ganic was e gasi ica ion om se e al sou ces can p oduce quali y syngas. •Bio ickling il e s can eplace ca aly ic eac o s o bioCH 4 syn hesis om syngas. •Syngas biocon e sion o biome hane needs op imiza ion o CO and H 2 mass ans e . •Me hanogenic mic obiology mus be boos ed o op imize syngas biocon e sion. ARTICLE INFO Keywo ds: Biogas upg ading Biomass gasi ica ion Biome hana ion Me hane Syn hesis gas ABSTRACT In egions highly dependen on ossil uels impo s, biome hane ep esen s a p omising bio uel o he ansi ion o a bio-based ci cula economy. While biome hane is ypically p oduced ia anae obic diges ion and upg ading, biome hana ion o he syn hesis gas (syngas) de i ed om he gasi ica ion o ecalci an solid was e has eme ged as a p omising al e na i e. This wo k p esen s a comp ehensi e and in-dep h analysis o he s a e-o - he- a and mos ecen ad ances in he ield, compiling he po en ial o his echnology along wi h he bo lenecks equi ing u he esea ch. The key design and ope a ional pa ame e s go e ning syngas p oduc ion and bio- me hana ion (e.g. o ganic eeds ock, gasi ie design, mic obiology, bio eac o con igu a ion, e c.) a e c i ically analysed. 1. In oduc ion An h opogenic ac i i ies ha e signi ican ly changed he dynamics o he plane and caused many en i onmen al p oblems in ecen decades (Seo e al., 2022). The human popula ion is expec ed o g ow om 6.8 billion o >9 billion by 2050, while ene gy demand is expec ed o nea ly double. Fu he mo e, he p oduc ion o o ganic was e will con inue o ise, posing a global p oblem. The la ge olume o o ganic was e, i no p ope ly handled, may deg ade ai , wa e , and soil quali y, causing de imen al consequences o he en i onmen (Lin e al., 2018). An a ac i e al e na i e o he use o hese o ganic was es is hei ene ge ic alo iza ion. This ac , oge he wi h he dec easing p oduc ion cos s * Co esponding au ho a : Depa men o Chemical Enginee ing and En i onmen al Technology, School o Indus ial Enginee ing, Uni e si y o Valladolid, D . Me gelina s/n, 47011 Valladolid, Spain. E-mail add ess: [email p o ec ed] (R. Mu˜ noz). Con en s lis s a ailable a ScienceDi ec Bio esou ce Technology jou nal homepage: www.else ie .com/loca e/bio ech h ps://doi.o g/10.1016/j.bio ech.2022.127436 Recei ed 30 Ap il 2022; Recei ed in e ised o m 3 June 2022; Accep ed 4 June 2022 Bio esou ce Technology 358 (2022) 127436 2 and en o cemen o g eene en i onmen al ene gy egula ions, may suppo a s eady inc ease in enewable ene gy consump ion (e.g. 3% in 2020) along wi h a slowdown in he demand o ossil uels (Dua ah e al., 2022). Sola , biomass, wind, geo he mal and hyd opowe a e he main enewable ene gy sou ces in he ene gy pool o mos coun ies (Singh e al., 2022). Bioene gy cu en ly makes up abou 10% o he global p ima y ene gy supply, and holds he po en ial o o e >60% o he wo ld’s ene gy supply. The p oduc ion o bioene gy is expec ed o iple by 2060 (Sca la and Dallemand, 2019), (Paniagua e al., 2019). The cu en geopoli ical scena io and in e na ional en i onmen al ag eemen s equi e in he Eu opean Union no el ene gy p oduc ion sys ems non-based on ossil uels, capable o simul aneously educing g eenhouse gas emissions and en i onmen al impac s and aligned wi h he p omo ion o ci cula economy (Co ea e al., 2019). In his con ex , enewable ene gy echnologies based on biomass u iliza ion can play a key ole. The a o emen ioned o ganic was es a e one o he mos com- mon ypes o biomass uels ecei ing special a en ion as a po en ial sou ce o enewable ene gy (Sa a ian and Unn ho sson, 2018). Recen es ima ions de e mine a wo ld annual p oduc ion o 2.01 billion onnes o hese esidues, he hi d pa s ill being mismanaged and se iously h ea ening he en i onmen (Szulc e al., 2021). Wi hin he exis ing biomass- o-ene gy con e sion echnologies, he mochemical p ocesses exhibi a high un apped po en ial (Ayub e al., 2022). These echnolo- gies include di ec combus ion, lique ac ion, py olysis and gasi ica ion p ocesses. In compa ison o comme cially a ailable echnologies like incine a ion, gasi ica ion is a no el bu p omising echnology. Was e gasi ica ion can be used as a mo e eliable ene gy supply echnology o places ha a e emo e om cen al ene gy ne wo ks and equi e a dis- ic hea ing and powe sys em. In addi ion, gasi ica ion shows consid- e ably lowe en i onmen al impac s due o he educed wa e equi emen s and lowe emissions o gaseous pollu an s in o he a mo- sphe e (Sa a ian e al., 2020). The gas s eam esul ing om was e gasi ica ion p ocess, ypically e e ed o as syngas, cons i u es an ene gy ec o ha can be upg aded, s o ed and dis ibu ed (expo ed/impo ed) globally using he al eady exis ing in as uc u es (M˘ a culescu e al., 2022). This syngas can be upg aded in o biome hane, which exhibi s compa able applica ions han na u al gas in powe and hea gene a ion, anspo a ion, and chemical sec o . Mo eo e , biome hane holds a c i ical ad an age o e liquid bio uels since i is o ally miscible wi h na u al gas (G imal - Alemany e al., 2018). While na u al gas consump ion will emain cons an o a leas a ew decades, biome hane is expec ed o co e he new gas demand due o hei enewable na u e and low CO 2 oo p in (Sko ek-Osikowska, 2022). Thus, he biome hana ion p ocess has been ex ensi ely s udied in he pas yea s as a p omising ene gy al e na i e, al hough se e al limi a ions associa ed wi h he di e en a iables in ol ed migh s ill be o e come (Figue as e al., 2021). Thus, his e iew compiles and c i ically discusses mos ecen da a published on he opic o syngas biome hana ion, no only om a mic oscopical pe spec i e bu also paying especial a en ion o he gas bio il a ion p ocess. To his aim, he e iew includes i s a de ailed desc ip ion o he con e sion o o ganic was e in o syngas and he impac o di e en gasi ica ion a iables on he syngas composi ion. The syngas biome hana ion p ocess is hen ackled om bo h a mic oscop- ical and mac oscopical pe spec i es, wi h special emphasis on bio- eac o s con igu a ion and p ocess limi a ions. Finally, u u e p ospec s o his sec o a e discussed. 2. O ganic was e con e sion in o syngas ia gasi ica ion Gasi ica ion p o ides an e icien and obus ou e o he mochemi- cally con e ing a b oad po olio o was es in o an ene gy ec o h ough an indi ec combus ion (Di Giuliano e al., 2022). Thus, he gasi ied o ganic esidue is con e ed in o a aluable syn hesis gas (called syngas) ia pa ial oxida ion a high empe a u es. This pa ial oxida ion can be unde aken wi h ai , oxygen o s eam (Saleem e al., 2020). In he co-p oduc ion o bio- e ilize (bio-cha and ash) and syngas, o ganic was e gasi ica ion is a single-s ep he mo-chemical p ocess widely accep ed as ene gy-e icien and cos -compe i i e (Ansa i e al., 2020). D ying, py olysis, pa ial oxida ion, and gasi ica ion a e all examples o complex he mochemical eac ions ha esul in he simul aneous in e con e sion o solid and gaseous species. This pa ial oxida ion gene a es he hea ha powe s he o he eac ions while also lowe ing he o ganic was e eeds ock’s ini ial mois u e con en . The hea om he oxida ion zone and he limi ed oxidising agen also cause was e py ol- ysis a 200 o 700 ◦C (O 2 o ai ), wi h he concomi an o ma ion o a gas s eam consis ing o a mix u e o hyd oca bons, N 2 , H 2 , CO, CO 2 , H 2 O and o he mino compounds (Chen e al., 2019). Cha , which u he akes pa in gasi ica ion, is p edominan ly o med du ing he py olysis p ocess (Na nawa e and Panwa , 2022). The gasi ica ion p ocess, on he o he hand, occu s allo he mally a a speci ic empe a u e and p essu e, which a e e e ed o as gasi ica ion empe a u e and gasi ica ion p essu e. The he mal gasi ica ion p ocess can be ca ied ou using di e en gasi ying agen s. This pa ame e g ea ly in luences he composi ion o he inal syngas. The global gasi ica ion eac ions o 1 mol o o ganic was e using ai , s eam and oxygen can be desc ibed by Eqs. (1), (2) and (3), espec i ely (Khalila ya e al., 2021): CH α Oβ+ ω H2O+γ(O2+3.76N2)→nH2H2+nCOCO +nCO2CO2+nH2OH2O +nCH4CH4+nN2N2(1) CH α Oβ+ ( ω + ε )H2O→nH2H2+nCOCO +nCO2CO2+nH2OH2O+nCH4CH4 (2) CH α Oβ+ ω H2O+δO2→nH2H2+nCOCO +nCO2CO2+nH2OH2O+nCH4CH4 (3) Whe e CH α Oβ is he o ganic was e chemical o mula. α and β, a e, espec i ely, he hyd ogen and oxygen mola a ios. nH2, nCO, nCO2, nH2O, nCH4 and nN2 is he mola numbe o hyd ogen (H 2 ), ca bon monoxide (CO), ca bon dioxide (CO 2 ), s eam (H 2 O), me hane (CH 4 ) and ni ogen (N 2 ), espec i ely. γ, ε , and δ a e he inpu ai , s eam and oxygen, espec i ely, e e ed o 1 mol o d y ash- ee o ganic was e. Finally, w is he mois u e/d y ash- ee o ganic was e a io desc ibed acco ding o Eq. (4): ω =Mwas e⋅MC MH2O⋅(1−MC)(4) MC is he mois u e con en o he o ganic was e, and Mwas e and MH2O s and o he molecula weigh o he o ganic was e and wa e , espec i ely. Gasi ica ion is one o he classical me hods o H 2 p oduc ion ia he mal decomposi ion o coal and biomass, and i is conside ed mo e ene gy e icien han combus ion p ocesses o p oduce ene gy. An o e all explana ion o he di e en ypes o gasi ie s employed wi h hei ope a ing condi ions is shown in Fig. 1. In he con ex o he ci cula economy needed o gua an ee he sus ainabili y o an h opogenic ac i i ies, gasi ica ion can help educing he olume o o ganic was es while gene a ing ene gy in he o m o syngas. P omising s udies ha e ecen ly shown ha he e ogeneous was es can be con e ed o syngas ia gasi ica ion (Ayub e al., 2022; Di Giuliano e al., 2022; Lee, 2022). Howe e , syngas has a ela i ely low calo i ic alue (in pa icula when p oduced ia ai gasi ica ion) and a high con en o a , which hinde s i s di ec chemical o bio echnological con e sion. 3. Syngas composi ion The syngas composi ion a ies acco ding o bo h he composi ion o he o ganic was e and he expe imen al condi ions o he gasi ica ion p ocess. Ope a ional pa ame e s such as he gasi ica ion ime, he ype S. Paniagua e al. Bio esou ce Technology 358 (2022) 127436 3 and low a e o gasi ying agen (GA), he empe a u e, o he mois u e con en o he eeds ock in luence he syngas composi ion (A yal e al., 2021). Simila ly, syngas composi ion can a y depending on he size, shape and densi y o he o ganic was e (Ce one e al., 2020). The mos impo an pa ame e s in luencing syngas composi ion a e desc ibed below: 3.1. Was e composi ion The composi ion o he o ganic was e gasi ied clea ly in luences syngas composi ion. T adi ionally, coal was used as a eeds ock o syngas p oduc ion, which en ailed mul iple echnical and en i onmen al issues such as pa icle agglome a ion, used-ash slagging and emission o SO x , NO x , and H 2 S (Gup a and De, 2022). Hence, esea ch has been de o ed o explo e he po en ial o al e na i e eed- s ock such as biomass, municipal was e, biosolids (s abilized esidues de i ed om he ea men o biological sewage sludge) o plas ic was e. The syngas composi ion o he mos common o ganic was es is sum- ma ized in Table 1. The composi ion o syngas om lignocellulosic biomass di e s depending on he GA employed, especially in e ms o H 2 con en . Fo ins ance, gasi ica ion o pine wood wi h s eam suppo s a H 2 con en o 60.3% (Ka al and ¨ Oz e en, 2021), while gasi ica ion o wood chips wi h ai canno p o ide H 2 con en s highe han 20% using BFB eac o s (Banda a e al., 2021). (Rasmussen and A yal, 2020) Fig. 1. Main cha ac e is ics and schemca ic ep esen a ion o he di e en ypes o gasi ie s. Adap ed om (Basu, 2018; Be mudez and Fidalgo, 2016; Pio and Ta elho, 2021). Gasi ying agen (GA), Fuel a io =Fixed ca bon/ Vola ile ma e , Bubbling luidized bed (BFB), Ci cula ing luidized bed (CFB), En ained low eac o s (EFR). Table 1 Syngas composi ion o se e al eeds ocks depending on he gasi ica ion agen employed. Raw ma e ial H 2 (%) CH 4 (%) CO (%) CO 2 (%) N 2 (%) Reac o ype Re e ence Gasi ica ion agen : S eam Pine Wood 60.3 1.6 15.3 22.4 – BFB (Ka al and ¨ Oz e en, 2021) Co n s aw 26–29 13–16 33–35 22–25 – FBR (Hu e al., 2019) Sewage sludge 58–63 1–3 13–18 14–17 – FB (Hu e al., 2020) Municipal solid was e 42–45 14–17 15–18 23–26 – FBR (Fu e al., 2022) Gasi ica ion agen : Ai Almond shells 14.3 2.3 30.8 8.4 43.7 FBR (Ce one e al., 2020) Plas ic was e 18–22 0–4 12–18 8–12 60–63 FBR (Moja e e al., 2022) Food was e 1–5 1–6 3–8 12–18 57–68 FBR (M˘ a culescu e al., 2022) Wood chips 10–20 1–5 13–18 12–16 36–60 BFB (Banda a e al., 2021) Wood Pelle 16–20 2–4 12–16 12–16 42–50 BFB (Banda a e al., 2021 S. Paniagua e al. Bio esou ce Technology 358 (2022) 127436 4 compa ed he syngas ob ained o s aw and wood pelle wi h simila esul s in e ms o CO, H 2 and CH 4 . Howe e , he gasi ica ion o s aw esul ed in ope a ing p oblems due o he agglome a ion de i ed om he highe alkali con en . The gasi ica ion o plas ic ma e ials p oduces combus ible gasses like H 2 , CH 4 , C 2 H 6 and C 3 H 8 (Shadangi, 2022). A educ ion o H 2 p oduc ion by a ac o o en was howe e obse ed when plas ic was combined wi h biomass a 800 ◦C (M˘ a culescu e al., 2022). (Al-asadi e al., 2020) also demons a ed ha he addi ion o Me/ Ni/ZSM-5 ca alys s o he use o mo e oxygen in he N 2 /O 2 mix u e can imp o e syngas p oduc ion (H 2 and CO). Municipal solid was e (MSW) has been success ully gasi ied unde se e al ope a ing condi ions wi h accep able esul s in e ms o high con en s o CO, H 2 and CH 4 in he syngas (Khalila ya e al., 2021) as well as highe LHV (lowe hea ing alue) (16 MJ/Nm 3 ) (Veses e al., 2020). As a esul , MSW gasi ica ion is a iable and cos -e ec i e op ion o he inal disposal o hese was es (Lee, 2022). Sewage sludge, bo h we and d ied, has been al eady s udied o bio-syngas p oduc ion. (Yang e al., 2021) p oposed a wo- s age so p ion-enhanced s eam gasi ica ion o sewage sludge o syn- gas p oduc ion wi h a H 2 p oduc ion 3 imes highe when compa ed wi h no s eam addi ion and also a highe pu i y o H 2 and CO gases. O he s udies ha e mixed sewage sludge wi h pine sawdus , ob aining a maximum d y gas yield (1.23 Nm 3 /kg), H 2 yield (14.44 mol/kg) and a ca bon con e sion e iciency (84.56%) using 60% o sewage sludge (Hu e al., 2016). I he gasi ica ion p ocess is ocused on he p oduc ion o me hane, he gasi ica ion o ag icul u al was e suppo s a CH 4 con en o 45–75% (Gao e al., 2018), ha o u ban sewage sludge, 60–65% CH 4, and ha o land ill was es, 35–65% CH 4 (Gue e o e al., 2020). 3.2. Tempe a u e Tempe a u e also in luences syngas quali y (Fuchs e al., 2020). Acco ding o he li e a u e, as he empe a u e o gasi ica ion ises, he concen a ion o he esul ing H 2 and ca bon con e sion e iciency ise, while he concen a ion o a in he syngas alls (Mülle e al., 2017). Recen in es iga ions ha e demons a ed ha empe a u e is a key pa ame e when PET (polye hylene e eph hala e) was gasi ied. Thus, he yields o H 2 (+87.7%), he dominan gas p oduc CO 2 (+40.3%), and biphenyl (+123%) all imp o ed when he empe a u e was aised om 750 o 800 ◦C. The gasi ica ion p oduc s o MSW we e also a unc ion o empe a u e (Ka dani e al., 2021). As a esul , aising he gasi ica ion empe a u e inc eased syngas p oduc ion and H 2 con en (Lee, 2022). In addi ion, (Wu e al., 2019) epo ed ha he op imal empe a u e o H 2 p oduc ion om lignocellulosic biomass was 850 ◦C, which minimized he ac i a ion ene gy o H 2 o ma ion. Recen s udies ha e a emp ed o model he in luence o empe a u e on he composi ion o he inal syngas. Thus, (Mikuland i´ c e al., 2020) accu a ely modelled he composi ion o he syngas wi h a 90% ma ch wi h sho ime a ia ions (up o 5 min). 3.3. Gasi ying agen The gasi ying agen is a key ope a ional pa ame e ha ema kably in luences syngas composi ion. Table 1 shows how he p esence o N 2 in he syngas is linked o he use o ai and s eam as GA o se e al eed- s ocks. When lignocellulosic biomass is gasi ied, he GA and he equi - alence a io (ER) play a c i ical ole on syngas composi ion. The ER is de ined as he a io o he ac ual ai - o- uel a io and he s oichiome ic ai - o- uel a io. Thus, ER accoun s o he ne e ec o ai low a e, eed supply a e and he esidence ime (Ma ínez e al., 2011). When using ai as gasi ying agen , he composi ion and LHV o he syngas linea ly changes wi h ER. Thus, he LHV inc eases wi h he dec ease in ER. In e es ingly, when using s eam as gasi ying agen , he LHV emains nea ly cons an when a ying he s eam o uel a io (Ka a as and Akgun, 2018). The yield in bo h luidized bed and mo ing bed eac o s is di ec ly p opo ional o he ER a ia ion and he gases esidence ime in he educ ion zone, acco ding o a se ies o s udies (She h and Babu, 2009). On he o he hand, he syngas yield o d ied sewage sludge is ypically g ea e when using ai as GA compa ed o s eam/O 2 du ing he gasi ica ion. This is a ibu ed o bo h he high ni ogen con en in he syngas and he high cha gasi ica ion a e media ed by ai (Jeong e al., 2022). Despi e he low hea ing alue o syngas p oduced by ai gasi i- ca ion p ocesses, i has he lowes p oduc ion cos (Pio e al., 2018), which is key o he comme cializa ion o MSW gasi ica ion. Fo PET gasi ica ion, he inc ease in he s eam o uel a io does no suppo a signi ican inc ease in H 2 yield (Li e al., 2022). Howe e , he use o s eam ins ead o ai inc eased H 2 le els by a ac o o 3 du ing sewage sludge gasi ica ion (Nipa ummakul e al., 2010). 3.4. Mois u e Mois u e con en o he eeds ock also in luences he composi ion o he syngas. The gasi ica ion p ocess can employ uel wi h a mois u e le el ≥40%, al hough mois u e le els >30% hinde igni ion and educe he syngas hea ing alue (McKend y, 2002). O e all, he dec ease in he mois u e o he eeds ock biomass implies a posi i e e ec on biomass gasi ica ion (Jah omi e al., 2021) and enhances he cold gas e iciency, CGE (chemical ene gy in he p oduc gas e sus he ene gy in he ini ial solid uel) (Niu e al., 2013). Howe e , he supe c i ical wa e gasi i- ca ion p ocess and he gas shi eac ion in he gasi ie bene i om inc eased mois u e con en . Syngas composi ion is also a ec ed by his pa ame e . In his sense, based on accep ed models (Ki sano s and Zandeckis, 2015) wo king wi h lignocellulosic biomass, i can be s a ed ha he inc ease in he mois u e con en can exe a posi i e impac on syngas CH 4 con en . Thus, he amoun o CH 4 ob ained in his ype o uel inc eased om 1.72% o 40% when inc easing he mois u e con en om 0% o 40%. Howe e , a highe uel mois u e con en media ed a de imen al impac on CO con en du ing he gasi ica ion o he same was e. Indeed, he CO concen a ion d opped signi ican ly om 30.5% o 6.20% when he mois u e con en inc eased om 0 o 40% likely due o he d op in empe a u e in he gasi ie eac o . On he o he hand, CO 2 con en in he syngas aised om 5.63% o 19.23% wi h inc easing mois u e le els. Was e biomass mois u e con en also in luences de calo i ic alues o syngas ob ained om gasi ica ion. The highe he mois u e con en , he lowe he ene gy o he syngas. Fo example, a 7% educ ion in he mois u e con en ( om 29 o 27%) o an he baceous biomass caused a nea ly double calo i ic alue o syngas (2.63 MJ/Nm 3 s 4.95 MJ/Nm 3 ) (A naw e al., 2014). 3.5. Gas pa ial p essu e This pa ame e a ec s he gas composi ion depending on he was e gasi ied. Hence, H 2 p oduc ion om he gasi ica ion o plas ic ma e ials dec eased when inc easing he ope a ional p essu e in he gasi ie . This dec ease was mo e p ominen han o polyp opylene (~5% dec ease in H 2 p oduc ion wi h an inc ease in he p essu e o 900 kPa). Simila ly, CO con en dec eased by ~3% du ing polyp opylene gasi ica ion a a simila p essu e inc ease. Howe e , he e ec s o ope a ional p essu e in he gasi ie on CO 2 p oduc ion we e negligible (Moja e e al., 2021). Inc eases in CO pa ial p essu e (P CO ) in syngas biome hana ion p o- cesses ypically esul in pa ial inhibi ion, which a ec s CH 4 yield and p oduc i i y. Al hough changes in P H2 ha e been epo ed o a ec mic obial ac i i y (as highe P H2 educes mic obial di e si y), he con- cen a ion o H 2 exe s a milde e ec on he conso ium’s pe o mance (G imal -Alemany e al., 2018). Pa icula ly, o coal gasi ica ion using s eam as GA, he composi ion o he gas changed as he s eam pa ial p essu e inc eased. Wi h lowe s eam pa ial p essu es, H 2 and CO 2 con en dec eased, while CO con en inc eased. Indeed, changing he pa ial p essu e o he s eam can con ol he H 2 /CO a io o he syn hesis gas (Sha ma e al., 2009). (Han oko e al., 2019) s udied he composi ion o he syn hesis gas ob ained om he gasi ica ion o sewage sludge and epo ed ha , despi e he cons an composi ion o CO and CO 2 , H 2 S. Paniagua e al. Bio esou ce Technology 358 (2022) 127436 5 con en dec eased sligh ly and CH 4 inc eased (in bo h cases wi h a i- a ions o less han 5%) when he p essu e was inc eased by 10 poin s ( om 25 o 35 MPa). 3.6. Syngas eed impu i ies Apa om CO, CO 2 , H 2 , H 2 O, and CH 4 , aw syngas commonly con ains solid pa icles (mos ly ash), condensable ola iles, and gases p oduced a e gasi ica ion, such as ace ylene (C 2 H 2 ), e hylene (C 2 H 4 ), e hane (C 2 H 6 ), benzene (C 6 H 6 ), hyd ogen sul ide (H 2 S), sul u dioxide (SO 2 ), ammonia (NH 3 ), ni ogen (N 2 ), hyd ogen cyanide (HCN) o ca bonyl sul ide (COS) among o he s. The ype and concen a ion o syngas impu i ies, and hei impac on mic obial p ocesses, can be in luenced by a numbe o ac o s, including gasi ie design and pe - o mance and/o gas clean-up me hods. Impu i ies can cause cell oxici y o enzyme inhibi ion, a ying edox po en ial, osmolali y, and pH (Xu e al., 2011). P io o he syngas biome hana ion, syngas pol- lu an s mus be elimina ed o a oid de imen al e ec s on biocon e sion pe o mance. Fo ins ance, se e al enzymes in ace ogenic bac e ia a e inhibi ed by a s, NOx and NH 3 . Despi e he ac ha low le els o im- pu i ies ha e been shown o ha e no e ec on biome hana ion pe o - mance, mo e esea ch is needed o de e mine aw syngas minimum clean-up equi emen s. In his con ex , mos exis ing in es iga ion a labo a o y scale use syn he ic syngas commonly composed o ca bon monoxide (CO), ca bon dioxide (CO 2 ), and hyd ogen (H 2 ), he compo- si ion o his “clean” syngas clea ly di e ing om indus ially p oduced syngas. To imp o e he economic iabili y o was e- o-biome hane ia gasi ica ion-biome hana ion, he cos o syngas cleaning mus be educed (San os and Alenca , 2020). A ypical syngas pu i ica ion scheme consis s o a quench owe ollowed by a washing s ep wi h wa e solu ion and hen an alkaline solu ion. Pa icula es, me als, and HCl a e all emo ed du ing his ea men . A second upg ading s ep is equi ed o comple e pu i ica ion and sulphu emo al ollowing a we elec os a ic p ecipi a o o emo e ly ashes oge he wi h a homoge- niza ion ank o bu e luc ua ions in syngas low and composi ion (due o he e ogenei y in he gasi ie eed). The esidual con aminan s in he syngas a e a ppm o pp le els a e his ea men sequence, bu hey a e s ill able o deac i a e con en ional downs eam chemical ca alys s. Ad anced gas cleaning echnologies, such as hose based on ca aly ic dus il e s o analogous, could imp o e syngas cleaning and dec ease cos s (Cen i and Pe a hone , 2020). 4. Syngas biome hana ion A g ea a ie y o was es wi h high o ganic ca bon con en can be gasi ied and he esul ing syngas be upg aded (Demey e al., 2019). This p ocess is limi ed by he low p oduc i i y, CO inhibi ion (Li e al., 2022) and he ex eme ope a ional condi ions. Se e al ca aly ic sys ems, commonly based on nickel, ha e been epo ed in he li e a u e (S an- geland e al., 2017). Among he main p oblems o be ackled, lowe ing he p ocess empe a u e while boos ing con e sion yield, manu ac u ing eliable ca aly ic sys ems, and egula ing eac ion hea a e poin ed ou as he mos impo an challenges in his ma u e echnology. In his sense, he enginee ing o he eac o and he op imiza ion o he ca alys composi ion and o mula ion a e c ucial in o de o con ol he abo e men ioned di icul ies. As p e iously s a ed, Ni is by a he mos commonly used ca alys due o, among o he s, i s high ac i i y, s ong CH 4 selec i i y and low cos (Al-Timimi and Yaakob, 2022). Howe e , o he wo ks ha e es ed Ru and Fe ca alys s wi h good esul s (Ki chne e al., 2018). The usage o nanoscale ca alys s may aid in imp o ing hei ac i i y and s abili y. Dispe sion o nanopa icles on di e en suppo s has been explo ed, wi h hyd o alci es and lan hanum oxide ecei ing cu en in e es (A es a e al., 2018). On he o he hand, he biological me hod con e s syngas o me hane h ough he me abolism o me hanogenic mic oo ganisms (biome hana ion) a milde empe a u es (35–75 ◦C) and a mosphe ic p essu e. The biome hana ion o syngas by mic oo ganisms can ake place in wo ways. As a me hane p ecu so , he i s uses an ace a e pa hway. Mic obial cells ha pe o m his eac ion include Ace obac e- ium woodii and Enbac e ium limosum. Following ha , me hanogenic bac e ia like Me hanosa cina ba ke i con e ace a e o me hane. The H 2 / CO 2 pa hway is used in he o he pa hway. Mic oo ganisms such as Me hano he mobac e he moau o ophicus and Clos idium he moace i- cum can con e CO in o CO 2 . Some mic oo ganisms, such as Me h- anosa cina o micicum, con e he H 2 and CO 2 p oduced and ini ially p esen in he syngas in o me hane. When compa ed o ca alys agen s, he use o mic oo ganism makes he p ocess mo e esilien o impu i ies in he eed gas and is mo e en i onmen ally iendly (Ba e al., 2020). In addi ion, biological syngas me hana ion can con e CO/CO 2 and H 2 in o CH 4 using di e en biological ou es ha bou ed by bac e ia and a chaea (Fig. 2), which suppo s syngas biome hana ion independen ly om he CO/CO 2 /H 2 a io (Figue as e al., 2021). Thus, he exploi a ion o he biological ou es o syngas con e sion in o me hane has ecen ly a ac ed a g ea in e es . In o de o upg ade he esul ing syngas molecules like CO and CO 2 in o biome hane using H 2 as elec on dono ( ypically p esen in syngas), mul iple s a egies can be implemen ed. CH 4 can be p oduced om non-con e ed CO 2 using adi ional biogas sc ubbing o by me hanizing CO 2 wi h addi ional H 2 (Angelidaki e al., 2018) . Du ing gasi ica ion, a CO 2 so ben can be used o educe he concen a ion o CO 2 , esul ing in a syngas wi h a highe concen a ion o H 2 (Salaudeen e al., 2020). H 2 can also be p oduced using enewable elec ici y and wa e elec olysis (A yal e al., 2021). 4.1. Mic oo ganisms in ol ed in syngas biome hana ion The wo main ca bon sou ces o syngas a e CO and CO 2 , which a e used by me hanogenic mic oo ganisms o build-up new biomass and CH 4 . The biological con e sion o CO 2 o CH 4 wi h H 2 addi ion is a well- known p ocess implemen ed o biogas upg ading (Kougias e al., 2017). CO con e sion o CH 4 is, on he o he hand, much less well-s udied, and CO has been shown o be oxic o mic oo ganisms (Wang e al., 2021). One o he ou es o con e ing CO in o me hane consis s o an ini ial con e sion o ace a e ca alyzed by ace ogenic bac e ia. This CO me aboliza ion can be ca ied ou by some species om gene a Clos- idium, Ace obac e ium and Spo omusa, which a e capable o p oducing ace a e and alcohols (No ak e al., 2021; Renaudie e al., 2022; Song e al., 2021). The eac ion is ollowed by ace oclas ic me honagenesis. The second ou e o syngas biome hana ion in ol es me abolizing CO o H 2 /CO 2 ia ca boxydo ophic hyd ogenogenesis, also known as wa e shi eac ion. Ce ain species o he gen es Rhodospi illum, The mincola, Desul o omaculum, Ca boxydo he mus, Caboxydocella and Moo ella con e CO o H 2 /CO 2 (Ka o e al., 2021; Liu e al., 2020). This eac ion is ollowed by hyd ogeno h ophic me hanogenesis. In me hanogenic en i onmen s, ace a e is ei he consumed by ace oclas ic me hanogens ha di ec ly used o me hanogenesis o i is oxidized by syn ophic ace a e-oxidizing bac e ia. Syn ophic ace a e oxida ion (SAO) is dependen on he in e species ans e o hyd ogen and/o o ma e, whe e he syn ophic pa ne (e.g., a hyd ogeno ophic me hanogen) consumes he e men a ion p oduc s (Dyksma e al., 2020; Sun e al., 2014). SAO has been iden i ied as a signi ican anae obic pa hway when combined wi h hyd ogeno ophic me hanogenesis unde he mophilic condi ions (55 ◦C) (Dol ing, 2014). Al hough pu e mic obial cul u es ha e suppo ed a good bio- me hana ion pe o mance, ecen s udies ha e e ealed he key ole o mic obial conso ia wi hin he o e all biome hana ion p ocess (Log o˜ no e al., 2022; Szuhaj e al., 2021). Thus, mixed cul u e-based bio- me hana ion has mul iple ad an ages o e monocul u e e men a ion in e ms o esilience and sensi i i y o inhibi ion. Howe e , mixed cul u e e men a ion o en equi e a g ea e le el o con ol and a ho ough unde s anding o how mic obial composi ion go e ns syngas bio- me hana ion, and pa icula ly CO con e sion. Fu he mo e, H 2 has a S. Paniagua e al. Bio esou ce Technology 358 (2022) 127436 6 signi ican in luence on syngas biome hana ion, since i ul ima ely de- e mines he maximum CH 4 le el in biome hane. Indeed, H 2 is ypically equi ed o comple ely con e CO and CO 2 p esen in syngas o i ually pu e me hane, a p ocess known as syngas upg ading (Li e al., 2020). Simila ly, he use o a chaeal bio ilms cul i a ed on memb ane su aces in a cus om-made memb ane bio ilm eac o o hyd ogeno ophic me hana ion has al eady been posi i ely es ed (P a o io i o e al., 2021) eached a maximum me hane p oduc ion pe eac o up o 1.17 Nm 3 / (m 3 ⋅d), exhibi ing ha he concep o memb ane bound bio ilms im- p o es mass ans e by di ec ly deli e ing subs a e gases o he bio ilm. When designing he biome hana ion p ocess, modelling he beha iou and pe o mance o mixed cul u es unde con en ional ope a ional scena ios migh be qui e aluable. P ocess modelling ecen ly e ealed ha di e ences in biome hane p oduc i i y we e due o he p e ailing ca abolic ou es, a he han o he kine ic pa ame e s o he mic obial conso ium (G imal -Alemany e al., 2020a). As a esul o his inding, he s udy o mic obial kine ics, which was p e iously o c i ical impo - ance, has been pushed o he backg ound. In his con ex , he mos ecen wo ks in syngas biome hana ion ocused on he use o a i icial hype he mophilic a chaeal co-cul u es capable o ans o ming syn he ic ca bon monoxide (CO) om lue gases o me hane. Syn he ic co-cul u es ep esen a no el app oach o he syn hesis o bio-based p oduc s whe e in e species in e ac ions occu wi hou he complexi y o open mixed cul u es, hus minimizing side eac ions and inc easing p oduc selec i i y. When g own as co- cul u es, mic oo ganisms can ac mo e e icien ly han when cul i- a ed independen ly. Indeed, an e ec i e ans e o me aboli es and mac omolecules, such as p o eins and RNA, occu s in co-cul u es, causing he s ains o po en ially in luence each o he ’s me abolism di ec ly (Diende e al., 2021). The co-cul u e o Ca boxydo he mus hyd ogeno o mans (a ca boxydo ophic hyd ogenogen) wi h Me h- ano he mobac e he moau o ophicus (a hyd ogeno ophic me hanogen) can e ec i ely biocon e syngas in o biome hane. Thus, C. hyd ogeno o mans bio ans o ms he oxic CO in o H 2 and CO 2 (was e -gas shi eac ion) suppo ing he g ow h o M. he moau o ophicus, which is capable o apidly assimila ing H 2 and CO 2 om he en i onmen , c ea ing he modynamically mo e a ou - able condi ions o C. hyd ogeno o mans g ow h. This p ocess should be conduc ed unde he mophilic condi ions, whe e he S anda d Gibbs ee-ene gy change (ΔG 0 ′) o he wa e –gas shi eac ion becomes mo e nega i e. The supe io pe o mance o he symbio ic C. hyd ogeno o mans and M. he moau o ophicus co-cul u e has been ecen ly alida ed by he esea ch g oup o D . Souza a Wageningen Uni e si y & Resea ch (The Ne he lands) unde suspended g ow h in s i ed ank e men e s (Diende e al., 2018). Likewise, co-cul u es o The mococcus onnu iqqqqneus and Me hanocaldococcus jannaschii, Me h- anocaldococcus ulcanius, o Me hanocaldococcus illosus ha e been suc- cess ully es ed by (Zippe le e al., 2021). In his s udy, up o 10 mol% CH 4 was p oduced by con e ing pu e CO o syn he ic CO-con aining indus ial was e gases employing he a o emen ioned co-cul u e in closed ba ch bio eac o . 5. P ocess limi a ions Syngas bio-con e sion o me hane is go e ned by en i onmen al, design and ope a ional pa ame e s. The mos ele an limi a ions encoun e ed du ing syngas biome hana ion a e desc ibed below. pH. The syngas biome hana ion ou e and p ocess e iciency a e also a ec ed by his pa ame e (Li e al., 2022). pH can in luence he ac i i y o mic oo ganisms in ol ed du ing biome hana ion. While a chaea ha e a ela i ely na ow pH ange o g ow h, anging om 6.0 o 8.0, wi h op imal g ow h ac i i y a 7.0, bac e ia exhibi a much wide pH ange (Ga cia e al., 2000). The pH o he syngas biome hana ion p ocess would he e o e depend on he bioca alys s in ol ed, pe o ming bes a neu al pH (Li e al., 2022). The accumula ion o ola ile a y acids (VFA) could lowe he pH o he eac o , causing inhibi ion and ul i- ma ely p ocess ailu e (Yuan e al., 2019). I H 2 is added o syngas, i will p e e en ially eac wi h CO 2 a he han CO, esul ing in highe H 2 consump ion and hus an inc ease in he pH, which inhibi s he ac i i y o CO consuming bac e ia (Li e al., 2020). When he e is a high con- e sion a e o VFAs o biogas (me hane and CO 2 ), as well as an addi- ional CO 2 con en in he syngas, special a en ion mus be paid o Fig. 2. Rou es o syngas biocon e sion in o CH 4 . Adap ed om (G imal -Alemany e al., 2018; Ra a i e al., 2021). SAO: syn ophic ace a e oxida ion. S. Paniagua e al. Bio esou ce Technology 358 (2022) 127436 7 main aining a neu al pH (Wes man e al., 2016). Ope a ional empe a u e in luences bo h gas–liquid mass ans e and mic obial kine ics du ing syngas biocon e sion. Despi e he ela- i ely low gas solubili y unde he mophilic condi ions, a p e ious s udy demons a ed ha he mophilic condi ions ou pe o med mesophilic condi ions in syngas biome hana ion (Al es e al., 2013). Fu he mo e, he ope a ional empe a u e has an impac on CO me abolism. Fo ins ance, (Sipma e al., 2003) in es iga ed CO me abolism a 30 and 50 ◦C in se en di e en anae obic sludge inocula. A 30 ◦C and 55 ◦C, he esul s showed ha ace a e and H 2 /CO 2 we e he main p ecu so s o me hanogenesis, espec i ely. Howe e , because o he highe ace a e yield and syngas con e sion a e, mesophilic condi ions we e ound o be mo e sui able o he con e sion o syngas o ace a e han he mo- philic and ambien condi ions a pH 5.5. Unde he mophilic condi ions, CO was e icien ly con e ed, bu i was mos ly con e ed o H 2 , which was hen ans o med o ace a e. (Luo e al., 2018) . Tempe a u e de- e mines he kine ics o mixed mic obial conso ia and ep esen s one o he mos impo an pa ame e s du ing syngas biome hana ion. The empe a u e o he cul u e can in luence he mic obial in e ac ions among membe s o he mic obial conso ium and go e n i s majo me abolic pa hways. Thus, ace a e is he p incipal p ecu so o me h- anogenesis unde mesophilic condi ions, acco ding o se e al esea ch on CO biome hana ion. On he o he hand, H 2 is he mos ele an p ecu so unde he mophilic condi ions, as he highe di e si y o ca boxydo ophic hyd ogenogenic bac e ia in he mophilic bio eac o s sugges ed. I has been also hypo hesized ha hyd ogenogenic p ocesses become mo e exe gonic a highe empe a u es, hus p omo ing a s onge hyd ogenogenic con e sion o CO unde he mophilic condi- ions (Con ad and We e , 1990). In addi ion, i has been demons a ed ha g ea e empe a u es cause al e a ions in conso ia mic obial s uc u e, which ul ima ely leads o highe con e sion a es du ing syngas biome hana ion (G imal -Alemany e al., 2018). The ype o cul u e also in luences he biome hana ion p ocess. Due o i s inc eased unc ionali y, obus ness and lowe ulne abili y o en i onmen al inhibi o s, mixed cul u es enable a mo e success ul syngas biome hana ion pe o mance han pu e cul u es (Esqui el-Eliz- ondo e al., 2017; Hill e al., 2017; Weiss e al., 2017). Membe s o he mic obial co-cul u e syn ophically coope a e and allow a “di ision o labou ” du ing he o e all biop oduc ion/biocon e sion p ocess (Lin- demann e al., 2016). Recen s udies (G imal -Alemany e al., 2020b) ha e in es iga ed he pe o mance o Ace obac e ium sp., Me hanospi - illum hunga eii, Me hanospi illum s amsii and Me hano h ix sp. a meso- philic condi ions, and The mincola ca boxydiphila and Me hano he mobac e sp. a he mophilic condi ions. These s udies concluded ha he mic obial selec ion was no d i en only by kine ic compe i ion, since he modynamic limi a ions also played a key ole de ining he dominan ca abolic ou es. Low gas–liquid mass ans e ypically limi biome hana ion p ocess (And eides e al., 2022). The poo mass anspo o gaseous CO and H 2 o he anae obic cells due o hei low aqueous solubili y (Hen y’s law cons an s, H CO =42 and H H2 =52 a 25 ◦C). The olume ic mass ans e a e o CO and H 2 om he syngas (G) o he mic oo ganism- con aining aqueous phase (A) can be exp essed as a unc ion o Kla G/A (o e all olume ic gas–liquid mass ans e coe icien , s −1 ), and he CO o H 2 concen a ion g adien (g m-3) in he liquid side ([Pollu an ] G/H - [Pollu an ] A ). Thus, hese high H alues ypically esul in a low d i ing o ce o he mass anspo o CO and H 2 om he syngas o he aqueous phase su ounding/embedding he anae obic mic obial cul u e, and he e o e in a limi ed CO and H 2 biocon e sion. A low gas–liquid mass anspo en ails p ocess ope a ion in la ge gas-phase bio eac o s, which signi ican ly inc eases bo h in es men and ope a ing cos s (Asimako- poulos e al., 2018). The e o e, he de elopmen o nex gene a ion biome hana ion p ocesses based on syngas biocon e sion equi es he enginee ing o compac high mass- ans e bio eac o s capable o sup- po ing an e icien mass anspo o CO and H 2 (Figue as e al., 2021). The pe o mance o syngas biome hana ion is mainly de e mined by mass ans e p ocesses o syngas componen s, which a e dependen on bo h he olume ic mass ans e coe icien (de e mined by he cha - ac e is ics o he bio eac o ) and he pa ial p essu e o hese gases as he d i ing o ce o hei anspo o he mic obial communi y (G imal - Alemany e al., 2018). The mass ans e o gas subs a es o he liquid medium is p opo ional o he subs a e pa ial p essu e in he head space, because di e ence concen a ion is he d i ing ac o o mass ans e (Mohammadi e al., 2011). The p essu e o CO, P CO , in luences cell p oli e a ion and p oduc syn hesis. An inc ease in P CO can esul in an inc ease in cell concen a ion as a esul o he enhanced mass ans e , bu also mic obial inhibi ion due o CO oxici y. Fu he mo e, when he P CO was inc eased abo e 1.35 ba he pa hway o ace ic acid owa d e hanol gene a ion was boos ed (Hu s and Lewis, 2010). Simi- la ly, (Skidmo e e al., 2013) epo ed an enhanced con e sion o ace ic acid o e hanol when inc easing he amoun o CO as a esul in he highe a ailabili y o educing equi alen s. 6. P ocess con igu a ions C oss d a gasi ie s ha e been widely used o he p oduc ion o syngas de o ed o biological o ca aly ic me hana ion (Hause e al., 2021). Recen in es iga ions ha e employed his gasi ie , ob aining a high me hane p oduc i i y and a good CO and H 2 con e sion wi h eal syngas (Asimakopoulos e al., 2021). FBR gasi ie s ha e also been used o he simul aneous biome hana ion o exogenous and endogenous CO 2 . In an unique wo-s age me hod o biological con e sion o syngas o biome hane, FBR eac o s ha e also been success ully in eg a ed wi h he mophilic anae obic diges ion o sewage sludge (And eides e al., 2021). The au ho s concluded ha he amoun o hyd ogen in syngas was he mos impo an elemen in in luencing he amoun o CH 4 in biome hane. The also s a ed ha H 2 concen a ion in syngas was he main ac o de e mining he CH 4 con en in biome hane. This sec ion o he e iew will ocus on he mos common bio eac o s o syngas bio- me hana ion. Table 2 summa izes he main esul s om he mos com- mon bio eac o con igu a ions de o ed o he biome hana ion o syngas. 6.1. S i ed- ank bio eac o s S i ed- ank bio eac o s (STBRs) a e he mos common eac o s o cul u ing biological agen s like cells, enzymes, and an ibodies. They a e con ac o s ha ely on in e nal mechanical agi a ion o keep he phases (gas, mine al medium and mic oo ganisms) well mixed. The impelle mus agi a e a a as -enough a e o dispe se all phases and achie e a uni o m concen a ion inside he bio eac o . The olume equi ed o a biop ocess is de e mined by he echnical design o an STBR, which is dependen on he p oduc ion (Ja a inejad, 2017). STBRs a e equen ly used o he in ensi ica ion o gas–liquid mass ans e in mic obial e men a ions and can be e en ually used o suppo syngas bio- me hana ion. The main pa ame e desc ibing he in ensi y o CO and H 2 mass ans e in s i ed ank eac o s is he olume ic mass ans e coe icien k L a (Pe ˇ íˇ cek e al., 2018). Se e al ac o s de e mine he k L a, including he geome y, impelle con igu a ion, agi a ion speed, and gas low a e o he eac o . Highe gas–liquid mass- ans e a es a e o en achie ed a high agi a ion speeds and syngas low a es, which inc eases he gas–liquid in e acial a ea due o bubble b eak-up. The main limi- a ion o his bio eac o con igu a ion is he high shea s ess caused by mechanical agi a ion, which can damage cell in eg i y and ul ima ely de e io a e syngas biocon e sion (Diende e al., 2018). 6.2. T ickling bed il e s T ickling bed il e s (TBF) consis o a column packed wi h ine ma e ials o high speci ic su ace a ea, on which bio ilm is de eloped. Syngas is pumped h ough he packed bed ei he downwa ds o upwa ds and a nu i ious liquid media is ickled and ecycled o e he packing ma e ial o p o ide mois u e and nu ien s, o ming a hin liquid laye S. Paniagua e al. Bio esou ce Technology 358 (2022) 127436 8 o e he bio ilm. The bio ilm is composed o a speci ic a angemen o immobilised cells wi hin a ma ix o ex acellula polyme ic subs ances (Po ´ e e al., 2019). These ype o bio eac o s a e a sui able al e na i e o STBRs, being an e icien sys em o achie e high CH 4 quali y and p oduc ion capaci ies a lowe ene gy demands (S übing e al., 2019). In TBRs he mic obes a e immobilized on he packing ma e ial, which should ha e a high su ace-a ea o gas–liquid mass ans e in o de o a ou a high densi y and ac i i y o me hanogenic a chaea (Dupnock and Deshusses, 2017). The ea u es ha e suppo ed high H 2 con e sion and CH 4 p oduc ion a es in lab scale es (Siebo g e al., 2020). TBRs unde con inuous mode inocula ed wi h en iched mixed mi- c obial conso ia o syngas biome hana ion ha e been poo ly s udied (Asimakopoulos e al., 2020). Due o he ac ha hyd ogeno ophic communi ies a e capable o de eloping s able bio ilms suppo ing a obus me hanogenic ac i i y, ecen esea ch ac i i y has been ocused on dedica ed s udies o ickling bed eac o s o biological syngas me hana ion and biogas upg ading by mixed mic obial cul u es (Thema e al., 2019). Howe e , ope a ional limi a ions such as he high liquid eci cula ion cos s o he high cos o syn he ic packing media mus be o e come o acili a e. (Thapa e al., 2022) ha e ecen ly alida ed CO 2 biome hana ion in hese bio eac o con igu a ions wi h a high CH 4 p oduc ion a e (up o 2.65 L⋅L esidue −1 ⋅d −1 and maximum 98% CH 4 con- en ) wi h a 100% H 2 u iliza ion e iciency and Me hanoculleus bou gensis as he dominan species in he liquid and bio ilm phases. 6.3. Bubble column and gas-li bio eac o s The use o bubble columns and gas-li eac o s in syngas bio- me hana ion p ocesses has also been in es iga ed based on hei la ge gas–liquid in e acial a eas, high olume ic mass ans e coe icien s, non-mechanical mixing, and low ope a ing cos s. The gas–liquid mass ans e coe icien in hese bio eac o con igu a ions is la gely de e - mined by he gas low a e and he size o he bubbles (G imal -Alemany e al., 2018). The in luence o hese ope a ional pa ame e s on CO mass ans e was in es iga ed in bo h a bubble column and a gas-li eac o , wi h he K L a inc easing as he gas low a e and he po e size o he column di use dec eased (Munasinghe and Khanal, 2010). (L´ ea e al., 2022) ha e ecen ly e alua ed he pe o mance o a pilo scale bubble column eac o o ex si u biological me hana ion o syngas wi h mixed mic obial cul u e, achie ing a 94% syngas con e sion (H 2 /CO 2 ) in o me hane a 4 N⋅L −1 ⋅d −1 . Simila ly, (Kougias e al., 2017) epo ed he supe io CH 4 p oduc ion pe o mance o bubble column eac o s compa ed o TBR (73% s 66% espec i ely) unde he same wo king condi ions. Gas-li bio eac o s use he powe c ea ed by gas injec ion and he densi y di e ence be ween gas–liquid mix u e and he liquid o ci cula e he gas–liquid mix u e. This pa icula gas–liquid ci cula ion pa e n will enhance mass ans e , hea ans e and mixing (Li, 2017). Un o una ely, he numbe o s udies assessing he pe o mance o gas- li eac o s o syngas biome hana ion is e y limi ed and he wo ks we e ca ied ou ew decades ago. Fo ins ance (Guio e al., 2010) bio- upg aded syngas in o me hane employing mic obial g anules de i ed om a was ewa e ea men plan in a 30 L gas-li eac o supplied wi h a gas mix u e con aining ca bon monoxide a di e en gas eeding and eci cula ion a es. The yields achie ed in his s udy (i.e. 60% CH 4 ) we e lowe han hose epo ed in ecen s udies wi h al e na i e bio eac o con igu a ions. 7. Resea ch needs and u u e di ec ions The complex geopoli ical si ua ion and he high dependency o na- ional economies on coun ies ha expo ossil uels (especially na u al gas and oil) is igge ing esea ch boos ing biome hane p oduc ion om biomass was e. In his con ex , echnological ad ances ha i e yea s ago we e a long- e m p ojec , s a o become a eali y oday. Thus, no el echnologies o syngas p e- ea men ope a ing a low ene gy demands a e nowadays needed. A a ie y o syngas cleaning echniques ha e been de eloped, including cyclones, elec os a ic p ecipi a o s (Jeong e al., 2022), il e s, o a ing pa icle sepa a o s and wa e sc ubbe s (Tsai e al., 2021). Despi e subs an ial ad ancemen s, hese echnologies mus o e come he mass ans e limi a ion imposed by he hyd ophobic na u e o mos a componen s. In addi ion, a new gene - a ion o compac and high mass ans e bio eac o s able o p o iding Table 2 Summa y o he mos ecen syngas biome hana ion s udies. Cul u e Reac o Ope a ion mode Feed syngas composi ion (%) Vol. (L) pH T (◦C) GRT (h) Final CH 4 composi ion (%) CH 4 Yield (mol CH 4 / mol syngas) Re e ence Co-cul u e C.hyd ogeno o mans and M. he moau o ophicus CSTR Ba ch & Con . H 2 /CO 2 (66.6/ 33.3) 1.50 7.2 65 2.08 70.0 Nd (Diende e al., 2018) ADS CSTR Ba ch. H 2 /CO 2 /CH 4 /N 2 (80/20/0/14.7) 9.50 8.1 55 1.19 49.0 0.16 (Voelklein e al., 2019) ADS CSTR Con . H 2 /CO 2 /CH 4 /N 2 (54/14/32/0) 9.50 8.1 55 1.14 61.0 0.22 (Voelklein e al., 2019) Mixed mic obial conso ia (WWTP anae obic sludges) TBR Con . H 2 /CO 2 /CO/N 2 (45/25/25/10) 0.18 7.0 37 3.00 67.0 Nd (Asimakopoulos e al., 2020) Mixed mic obial conso ia (WWTP anae obic sludges) TBR Con . H 2 /CO 2 /CO/N 2 (45/25/25/10) 0.18 7.0 60 1.00 86.0 Nd (Asimakopoulos e al., 2020) Pseudomonas sp, Me hanobac e ium o micicum, Pep ococcaceae TBR Con . H 2 /CO 2 /CH 4 /N 2 (62/15/23/0) 0.80 7.1 54 4.00 96.0 Nd (Gho ani-Is ahani e al., 2022) Diges a e om biogas plan (mainly M. he mau o ophicus & Clos idia sp.) TBR Con . N 2 /CO 2 /CH 4 (65/15/23) 1.20 8.0 52 8.00 66.0 Nd (Kougias e al., 2017) Diges a e om biogas plan (mainly M. he mau o ophicus & Clos idia sp.) BC Con . N 2 /CO 2 /CH 4 (65/15/23) 1.40 8.0 52 8.00 73.0 Nd (Kougias e al., 2017) MACE BC Con . H 2 /CO 2 /CH 4 /N 2 (65/17.5/0/ 17.5) 0.29 7.5 52 1.32 67.1 0.18 (Siebo g e al., 2020) MACE BC Con . H 2 /CO 2 /CH 4 /N 2 (62/15/23/0) 1.00 8.3 54 2.1 95.1 0.25 (Po ´ e e al., 2019) ADS - Cop o he mobac e & Me hanobac e ium BC Con . H 2 /CO 2 (4.2 a io) 22.00 7.5 55 Nd 94.0 Nd (L´ ea e al., 2022) ADS: anae obic-diges e sludge; BC: bubble column; Con .: con inuous; CSTR: Con inuous s i ed- ank eac o ; GRT: gas esidence ime; MACE: mixed anae obic cul u e en iched wi h hyd ogeno ophic me hanogens; Nd: no de ined; TBR: ickling bed eac o ; T: empe a u e; Vol.: olume. WWTP: Was e wa e ea men plan . S. Paniagua e al. Bio esou ce Technology 358 (2022) 127436 9 high syngas con e sion e iciencies and me hane p oduc i i ies is needed. Gas-phase hollow ib e memb ane eac o s, bubble sp inkle s, Taylo low eac o s o bio eac o s based on bioac i e coa ings ha e been la ely p oposed o enhance he biocon e sion o poo ly wa e soluble subs a es (Yasin e al., 2019). Bioac i e coa ing-based bio il e s ep esen an inno a i e biome hana ion pla o m based on polyme ic coa ings (e.g. la ex o hyd ogels) enginee ed as nanopo ous ma ices wi h dense popula ions o anae obes esul ing in bioac i e packing ma e ials. The lab-scale p oduc ion o H 2 using a i icial pho osyn he ic lea es and indoo ai ea men has been success ully demons a ed using his inno a i e bioca aly ic app oach (Gonz´ alez-Ma ín e al., 2022). Bioac i e polyme ic coa ings wi h a high a ini y o CO and H 2 could be used in packed bed bio eac o s o imp o e syngas up ake om he gas phase, a oiding he mass ans e limi a ions caused by he wa e laye ha co e s adi ional bio ilms. Mos ecen publica ions a e based on he use o mic obial conso ia, whose symbio ic ac ion is capable o inc easing he yield o me hane p oduced. Howe e , a mo e de ailed s udy o he di e en mic oo ganisms in ol ed as well as he me abolic ou es associa ed would be o i al impo ance in o de o op imize p ocess pe o mance. In addi ion, he op imiza ion o he design and ope a ion o bio eac o s wi h immobilized co-cul u es will also b ing signi ican ad ances in he ield o syngas biome hana ion. The possi- bili y o using mesophilic mic obial cul u es would elimina e all he disad an ages linked o he mophilic bac e ia. In his con ex , (Mou ahi e al., 2020) ecen ly epo ed ha he biogas and bio-me hane yield a 35 ◦C in e ms o me hane p oduc ion pe kg o ola ile solids (~0.384 Nm 3 me hane/kg) was enhanced du ing he co-diges ion o h ee bio- was es. The de elopmen o modelling ools capable o op imizing he a iables in ol ed in he syngas biome hana ion p ocess would en ail a signi ican imp o emen o he was e o biome hane p ocess. The in e- g a ion o wa e elec olysis using enewable ene gies and bio- me hana ion o he syngas p oduced om o ganic was e gasi ica ion will inc ease sec o al compe i i eness and lowe he oo p in o bio-based indus ies. 8. S a egic impo ance, bo lenecks and po en ial solu ions o biome hane The EU has se mo e se e e a ge s in e ms o en i onmen al p o- ec ion, aiming a a nea -ze o emissions economy and 100% enewable ene gy p oduc ion by 2050, which is c i ical o os e biogas echnology (Cook, 2021). The ele ance o biogas, especially biome hane, as a sus ainable ene gy al e na i e is demons a ed in he inc easing numbe o publica ions in he pas 20 yea s (Calise e al., 2021). In his con ex , gasi ica ion ep esen s a s ep o wa d owa ds a educ ion in was e olume ia con e sion in o ene gy in he o m o syngas and o he mino chemicals, which pa es he way o he ci cula i y o he p ocess. Syngas mus be upg aded o ob ain a biome hane wi h an adequa e quali y o injec ion in o he na u al gas g id (Figue as e al., 2021). Thus, se e al bio e ine ies a e planning o cu en ly unde aken he upg ading o syngas o ob ain a g een subs i u e o na u al gas while complying wi h cu en egula ions encou aging a ze o-was e concep –based bio- ci cula economy (Chak a a y and Manda gane, 2022). Despi e i s po en iali y, he e a e s ill impo an limi a ions in he scale up and comme cializa ion o his echnology, such as: (i) he p esence o ola- ile siloxanes, (ii) he p esence o o ganic compounds in he was e ha a e only pa ially deg aded, (iii) he low eac ion a es, which en ails a la ge-capaci y and mo e cos ly bio eac o s, and (i ) he p esence o excess CO 2 , H 2 S, and mois u e oge he wi h me hane, which makes he p ocess less cos -e ec i e (Jacob e al., 2020). Ne e heless, hese inhe en bo lenecks can be sol ed o a la ge ex en by implemen ing di e en p e- ea men s a egies (physical, chemical, biological and combined echnologies) (Pascual e al., 2021). Ano he impo an lim- i a ion o he implemen a ion o he biome hana ion p ocess is associ- a ed o he mic obiology in ol ed. In his sense, i is c ucial o p ope ly selec he echnology con igu a ion and o ca e ully moni o and con ol c i ical ope a ing pa ame e s such as empe a u e, pH, mixing, e en ion ime, o he p esence o inhibi o y subs ances (Adnan e al., 2019). 9. Conclusions O ganic was e gasi ica ion can p oduce a syngas wi h a composi ion go e ned by he na u e o he was e, he ype o gasi ie , empe a u e, gasi ying agen , e c. This syngas may be upg aded o biome hane using bio eac o s ope a ed a a low empe a u e and p essu e. This p ocess s ill exhibi s se e al limi a ions in e m o CO/H 2 mass ans e and mic obiology, which a e he basis o u u e s udies in his a ea. The e olu ion o new bio eac o designs wi h a high mass ans e capaci y a low ope a ing cos s, and based on syne gis ic co-cul u es should pa e he way o his no el was e- o-biome hane ou e. CRediT au ho ship con ibu ion s a emen Se gio Paniagua: W i ing – e iew & edi ing, W i ing – o iginal d a . Raquel Leb e o: W i ing – e iew & edi ing. Raúl Mu˜ noz: W i ing – e iew & edi ing, P ojec adminis a ion, Supe ision. Decla a ion o Compe ing In e es The au ho s decla e ha hey ha e no known compe ing inancial in e es s o pe sonal ela ionships ha could ha e appea ed o in luence he wo k epo ed in his pape . Da a a ailabili y No da a was used o he esea ch desc ibed in he a icle. Acknowledgmen s This wo k was suppo ed by he Regional Go e nmen o Cas illa y Le´ on and he EU-FEDER (CLU 2017-09, CL-EI-2021-07, UIC 315). The Spanish Minis y o Science and Inno a ion is g a e ully acknowledged o he Juan de la Cie a-Fo ma ion con ac o D . Se gio Paniagua (FJC2020-043479-I). Re e ences Adnan, A.I., Ong, M.Y., Nomanbhay, S., Chew, K.W., Show, P.L., 2019. Technologies o biogas upg ading o biome hane: a e iew. Bioeng. (Basel, Swi ze land) 6. h ps:// doi.o g/10.3390/BIOENGINEERING6040092. Al-asadi, M., Miskolczi, N., Elle , Z., 2020. 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