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.