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