Methanisation: “good for the environment and energy autonomy”

Emit­ting methane (CH₄) and nitrous oxide (N₂O), live­stock efflu­ents (waste­water) are respon­si­ble for about 10% of green­house gas (GHG) emis­sions from live­stock farm­ing¹. These emis­sions are linked to the way in which they are stored and treat­ed. Mit­i­ga­tion solu­tions include reduc­ing stor­age time, sep­a­rat­ing the sol­id and liq­uid phas­es, cov­er­ing pits or using nitri­fi­ca­tion or ure­ase (urine trans­for­ma­tion) inhibitors. 

The last method cur­rent­ly under devel­op­ment is methani­sa­tion. This con­sists of recov­er­ing the methane pro­duced by the anaer­o­bic decom­po­si­tion – with­out oxy­gen – of organ­ic mat­ter. The bio­gas can be trans­formed into elec­tric­i­ty and heat in cogen­er­a­tion units or inject­ed direct­ly into the gas net­work. The residue, called diges­tate, is used to fer­tilise and improve the soil. Methani­sa­tion units using live­stock efflu­ents can be installed on the farm, on top of exist­ing slur­ry pits or cen­tralised with­in a region.

On 1^st Jan­u­ary 2022, there were 1175 methani­sa­tion units in France, 805 of which were using agri­cul­tur­al efflu­ents in 2021. Methani­sa­tion units treat­ing house­hold and indus­tri­al waste are not real­ly evolv­ing, while those at waste­water treat­ment plants are mov­ing towards bio­methane injec­tion. On the oth­er hand, on-farm and cen­tralised instal­la­tions are increas­ing sig­nif­i­cant­ly and have the great­est poten­tial in France: the num­ber of on-farm instal­la­tions has risen from 38 units in 2010 to 661 in 2020. There are a few large-scale anaer­o­bic diges­tion projects, but the new facil­i­ties are main­ly small to medi­um-sized, pro­cess­ing between 10,000 and 20,000 tonnes of waste per year.

His­tor­i­cal­ly, the anaer­o­bic diges­tion sec­tor has focused on the cogen­er­a­tion of elec­tric­i­ty and heat, which rep­re­sents 72% of on-farm and cen­tralised units in 2020² . How­ev­er, for the past five years, the trend has been towards inject­ing bio­methane into the gas net­work. At the farm lev­el, the use of bio­methane for injec­tion rose from 41 to 114 units between 2018 and 2020. The same dynam­ic can be observed for cen­tralised instal­la­tions, which went from 15 to 55 injec­tion units over the same period.

First­ly, the desire to sub­sti­tute import­ed nat­ur­al gas with bio­methane rather than elec­tric­i­ty. In the cur­rent con­text, bio­methane has the major advan­tage of pro­vid­ing a renew­able and inde­pen­dent ener­gy source for France and Europe. Anoth­er argu­ment in favour of bio­methane injec­tion is ener­gy effi­cien­cy. Elec­tric­i­ty cogen­er­a­tion alone is 35% effi­cient, ris­ing to 50% or even 55% if we also make use of the heat. Bio­methane injec­tion has an ener­gy effi­cien­cy close to 85%.

Val­i­da­tion of the tech­nol­o­gy’s per­for­mance has opened the way to wider dis­sem­i­na­tion of this type of project. Today, reg­u­la­tions require that the fea­si­bil­i­ty of an injec­tion instal­la­tion be stud­ied as a pri­or­i­ty. How­ev­er, these projects are more expen­sive: the aver­age invest­ment is €5.5 mil­lion com­pared to €2 mil­lion for cogeneration.

It has been iden­ti­fied as one of the main meth­ods for reduc­ing the GHGs asso­ci­at­ed with live­stock farm­ing. On aver­age, each project pre­vents the release of 2,600 tonnes of CO₂ equiv­a­lent into the atmos­phere. INRAE has car­ried out the first life cycle analy­sis (LCA) of bio­methane from agri­cul­tur­al methani­sa­tion³. It estab­lish­es an envi­ron­men­tal eval­u­a­tion tak­ing into account ener­gy pro­duc­tion, efflu­ent man­age­ment and soil fer­til­i­sa­tion. Envi­ron­men­tal per­for­mance is improved by 60–85% for 16 of the indi­ca­tors con­sid­ered. No improve­ment is observed for 5 indi­ca­tors, and the sys­tem is less effi­cient for some indi­ca­tors, notably because of the increased use of elec­tri­cal ener­gy. The points for con­sid­er­a­tion are the con­trol of fugi­tive bio­gas emis­sions and the respect of good prac­tices for the spread­ing of digestates.

Methani­sa­tion also offers a solu­tion for treat­ing the ter­ri­to­ry’s bio-waste, diver­si­fy­ing agri­cul­ture, allow­ing organ­ic mat­ter to be returned to the soil and reduc­ing the use of min­er­al fer­tilis­ers. It is a major advan­tage for increas­ing our auton­o­my regard­ing soil fertilisation.

Yes, we have seen a strong momen­tum over the past three years, with 1.5 TWh of addi­tion­al annu­al capac­i­ty installed each year. The objec­tive of the Mul­ti­an­nu­al Ener­gy Plan to reach 6 TWh in 2023 will be exceed­ed this year! We esti­mate the pro­duc­tion poten­tial at 30–35 TWh of bio­methane in 2030, and 90–130 TWh in 2050⁴. There is also a high poten­tial in rela­tion to slur­ry pit cov­ers, these micro-methani­sa­tion units that only treat live­stock efflu­ents and use the bio­gas autonomous­ly on the farms. There are cur­rent­ly 40 of these units, while there are sev­er­al tens of thou­sands of slur­ry pits.

Live­stock manure is the pri­or­i­ty resource to be used. This source must be sup­ple­ment­ed by more methanogenic sub­strates (such as plants) in order to find a good tech­ni­cal and eco­nom­ic com­pro­mise. In our prospec­tive work, the mobil­is­able resource in gross ton­nage is made up of 50% live­stock manure, 30% inter­me­di­ate crops and 20% oth­er waste. Inter­me­di­ate crops or cov­er crops are essen­tial to achieve our renew­able ener­gy pro­duc­tion objectives.

Some coun­tries, such as Ger­many, have cho­sen to pro­duce annu­al crops ded­i­cat­ed to ener­gy pro­duc­tion. It is essen­tial to reg­u­late this use to avoid com­pe­ti­tion with food. In France, this share is lim­it­ed by law to 15% of cul­ti­vat­ed areas and is cur­rent­ly between 3 and 6%.

Methani­sa­tion units are clas­si­fied instal­la­tions for envi­ron­men­tal pro­tec­tion and are there­fore sub­ject to numer­ous reg­u­la­tions for the pre­ven­tion of envi­ron­men­tal and pub­lic risks. Because of their orig­i­nal agri­cul­tur­al activ­i­ty, farm­ers are already used to man­ag­ing live­stock efflu­ents: only the man­age­ment of bio­gas is new and requires train­ing and rig­or­ous oper­a­tion. The risks in the event of an acci­dent main­ly con­cern farm staff and not local residents. 

I am more con­cerned about very large instal­la­tions, where the sus­tain­abil­i­ty of sup­plies and envi­ron­men­tal per­for­mance may be ques­tioned. The local inte­gra­tion of projects is not in favour of these installations.

Inter­view by Anaïs Marechal