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.