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