Methanisation: converting CO2 from biomethane plants into useful products

Methani­sa­tion, also called anaer­o­bic diges­tion, is a nat­ur­al bio­log­i­cal process where­by mate­r­i­al is bro­ken down by micro-organ­isms. In doing so, it releas­es ener­gy from organ­ic mat­ter. Indus­tri­al methani­sa­tion is the exploita­tion of this process, often in agri­cul­tur­al set­tings, as sus­tain­able source of ener­gy. Whilst methani­sa­tion is still grow­ing in usage, there remain sev­er­al unex­ploit­ed solu­tions to be explored, which could fur­ther improve its envi­ron­men­tal per­for­mance¹. 

The bio­gas pro­duced by methani­sa­tion gen­er­al­ly con­tains 50–70% methane (CH₄) and around 30–50% CO₂. Dur­ing the process that pro­duces the bio­methane, CO₂ is sep­a­rat­ed from the gas to deliv­er a flow with high methane con­cen­tra­tion. In this prac­tice of bio­gas purifi­ca­tion, off-gas­es are emit­ted to the atmos­phere. They main­ly con­tain 98% CO₂ and 1–2% resid­ual methane². Accord­ing to the indus­try, this CO₂, called bio­genic CO₂, does not con­tribute to a net addi­tion of cli­mate gases.

Nev­er­the­less, the resid­ual methane released into the atmos­phere does con­tribute to glob­al warm­ing. There­fore, to reduce the envi­ron­men­tal impact of the plants in France, a new reg­u­la­tion has been issued. It requires bio­gas-to-bio­methane plants to lim­it their emis­sion of methane in the off-gas­es to 0.5–1% by 2025³.

To abide by the new reg­u­la­tion, mature tech­nolo­gies (such as cryo-dis­til­la­tion, absorp­tion by sol­vent, pres­sure swing adsorp­tion) are avail­able and allow the sep­a­ra­tion of bio­genic CO₂  from the resid­ual methane. Puri­fy­ing the bio­genic CO₂  would allow plants to meet objec­tives and enhance the per­for­mance of the plant by rein­ject­ing the resid­ual CH₄into the grid. Also, an addi­tion­al vir­tu­ous loop takes place. Indeed, the puri­fied bio­genic CO₂ can be used to cre­ate val­ue through mul­ti­ple outlets.

Using CO₂  in spe­cif­ic set­tings can con­tribute to the cir­cu­lar econ­o­my⁴. The CO₂  can be inject­ed into green­hous­es or applied direct­ly to crops. It can also be used in var­i­ous indus­tri­al appli­ca­tions, such as in the pro­duc­tion of plas­tics, chem­i­cals, and build­ing mate­ri­als like con­crete. CO₂ can also be used for oth­er pur­pos­es, such as in the food and bev­er­age indus­try for car­bon­a­tion, in the med­ical indus­try for imag­ing, and in fire­fight­ing as a dry ice sub­sti­tute. E‑fuels and microal­gae are oth­er poten­tial out­lets that are also being investigated.

There­fore, cap­tur­ing the bio­genic CO₂ that is oth­er­wise emit­ted into the atmos­phere could cre­ate nov­el syn­er­gies between bio­methane plants and CO₂ users and thus pro­vide a sus­tain­able alter­na­tive to fos­sil-based CO2 in prod­ucts. There are sea­son­al ten­sions on the CO₂ mar­ket, induc­ing price volatil­i­ty asso­ci­at­ed with short­ages that can great­ly affect some con­sumers. In fact, the mar­ket price of CO₂ is high­ly vari­able: approx­i­mate­ly €50 to €200/t⁵. Thus, hav­ing local bio­genic CO₂ pro­duced by french bio­methane plants would make it pos­si­ble to meet these cur­rent challenges.

France is home to Europe’s fastest-grow­ing bio­methane sec­tor⁶. The num­ber of units pro­duc­ing bio­methane was a lit­tle over 500 at the end of 2022 in France⁷. There is a grow­ing inter­est in find­ing ways to fur­ther improve sus­tain­abil­i­ty of these plants. Puri­fy­ing bio­genic CO₂  would help meet the envi­ron­men­tal guide­lines in 2025 and sub­sti­tute fos­sil-based CO₂  use in France. In France, 800 Kt/year of CO₂, are con­sumed, of which 70% by the agri-food indus­try⁸. The poten­tial of bio­genic CO₂ that can be val­orised through these plants has been esti­mat­ed at around 700 to 800 Kt CO₂/y. To date in France, less than a dozen bio­methane plants have imple­ment­ed this cir­cu­lar activ­i­ty^9,10. 

While the poten­tial ben­e­fits of CO₂ val­ori­sa­tion in bio­methane plants are sig­nif­i­cant and pro­mote the devel­op­ment of a cir­cu­lar econ­o­my, there are still tech­ni­cal and eco­nom­ic chal­lenges to over­come in France. Depend­ing on the out­let cho­sen and the recov­ery tech­nol­o­gy of CO₂, the process will impact the over­all ener­gy effi­cien­cy of the bio­methane chain and the cost of CO₂ cap­ture. Also, the biggest user of CO₂ in France remains the agri-food sec­tor. Spe­cif­ic qual­i­ty cer­ti­fi­ca­tions, such as the EIGA stan­dard, are need­ed for bio­genic CO₂ to be com­pli­ant with this indus­try. Fur­ther­more, in France, bio­methane units have an aver­age pro­duc­tion capac­i­ty of 1.8–2 kt CO₂/year, where­as con­ven­tion­al sources of CO₂ can sup­ply up to 200 kt CO₂/year. Hence, mutu­al­iza­tion is need­ed in bio­genic CO₂ trans­porta­tion to enhance economies of scale¹¹.

Over the past years, the imple­men­ta­tion of a favourable reg­u­la­to­ry frame­work has sup­port­ed the expan­sion of the French bio­methane sec­tor. Case in point, the reg­u­la­tion to reduce CH4 in off gas­es will help improve its envi­ron­men­tal impact. Addi­tion­al­ly, it might cre­ate a poten­tial mar­ket for bio­genic CO₂ that could sub­sti­tute the fos­sil-based one. Nev­er­the­less, if the process would make sense envi­ron­men­tal­ly, the busi­ness mod­el is still to be cre­at­ed. Efforts are required to co-cre­ate a viable val­ue propo­si­tion. Sure­ly enough, the local demand side is yet to be stim­u­lat­ed. Efforts from indus­tri­al play­ers can be pur­sued to pro­pose bio­genic CO₂ at a com­pet­i­tive cost while improv­ing logis­tics and cost cap­ture tech­nolo­gies. Oth­er levers can be iden­ti­fied, such as imple­ment­ing a reg­u­la­to­ry incen­tive for the pro­duc­ers and for the con­sumers of bio­genic CO₂. How­ev­er, the lat­ter can be a dou­ble-edged sword because it can deter the reduc­tion of con­ven­tion­al CO₂ by imply­ing there is a need from the demand side. Last­ly, indus­tri­al play­ers are inves­ti­gat­ing nov­el finan­cial val­u­a­tion, such as the sell­ing of car­bon cred­its asso­ci­at­ed with the seques­tra­tion of the bio­genic CO₂ on the long term, through spe­cif­ic prod­ucts¹².