Bio­gas in the agri­cul­tur­al sector 

Over the past few years, elec­tric­i­ty gen­er­a­tion using bio­gas in the agri­cul­tur­al sec­tor has become increas­ing­ly impor­tant, not least because today the sec­tor plays a vital role as a sup­pli­er of raw mate­ri­als. From 2007 to 2014 in par­tic­u­lar, the num­ber of bio­gas plants in Ger­many grew rapid­ly, part­ly due to the fund­ing guide­lines imple­ment­ed as part of the Ger­man Renew­able Ener­gy Sources Act (EEG) which came into force in 2000. At a Euro­pean Union (EU) lev­el, there are action plans and guide­lines for the grow­ing pro­por­tion of renew­able ener­gy sources that can be used by the agri­cul­tur­al sec­tor to pro­duce bio­gas. For exam­ple, the methane strat­e­gy of the EU Green Deal has result­ed in an accel­er­at­ed mar­ket devel­op­ment for bio­gas pro­duced from sus­tain­able sources in the agri­cul­tur­al sec­tor. The EU’s “Fit for 55” plan has also played a role here. As a result, the impor­tance of bio­gas plants has increased because the pro­duc­tion of bio­gas in the agri­cul­tur­al sec­tor is essen­tial for renew­able ener­gy and heat sup­plies. Main­tain­ing a rel­e­vant size of bio­gas plants requires good crop man­age­ment in order to design and mon­i­tor the grow­ing con­di­tions effi­cient­ly. Site-adapt­ed and bal­anced crop rota­tion sys­tems are ide­al for ensur­ing a sus­tain­able pro­duc­tion of bio­gas in the agri­cul­tur­al sec­tor. A care­ful­ly thought-through crop and seed plan­ning can there­fore be cru­cial. In the next sec­tion, we’re going to take a clos­er look at the crops that can be used to pro­duce biogas.

Plant sub­strates

In the past, maize was often used to pro­duce bio­gas in the agri­cul­tur­al sec­tor. The crop is very well-suit­ed to this because of its good yield per­for­mance. How­ev­er, despite the action plans and guide­lines in place, an ever-chang­ing crop rota­tion will be required in the future in order to main­tain fer­tile soils. This also requires good pro­fes­sion­al agri­cul­ture prac­tices, if pos­si­ble, with­out a sec­ond crop of the same plant being grown in succession.

To avoid this, there are oth­er plants that can be used in addi­tion to maize. The “food vs fuel” debate has put for­ward dif­fer­ent alter­na­tives for pro­duc­ing enough bio­mass out­side of the cul­ti­va­tion peri­ods of the main crops. There is a wide range of crops avail­able that can be used as ener­gy crops and that are par­tic­u­lar­ly suit­able for the pro­duc­tion of bio­gas in the agri­cul­tur­al sec­tor. By pick­ing the right catch crop, farm­ers can pro­duce a rel­a­tive­ly large amount of bio­mass in a short amount of time. The ener­getic out­put per hectare is impor­tant here. In addi­tion, the dry mat­ter con­tent of the vari­eties must be tak­en into account. Flow­er­ing and stem-form­ing plants are also a good alter­na­tive for pro­duc­ing bio­gas. When main, catch and sec­ondary crops are mixed, the effects of the crop rota­tions play a vital role. There are a range of dig­i­tal tools avail­able to help with the analy­sis and trans­par­ent­ly map the yield per­for­mance, such as the Crop and Seed Plan­ning com­po­nent from 365FarmNet for example.

Maize silage

Just like in the past, today maize silage is most com­mon­ly used to pro­duce bio­gas in the agri­cul­tur­al. It is par­tic­u­lar­ly pop­u­lar because of its sta­ble yields and high methane yields. Spe­cial, site-adapt­ed maize vari­eties, such as late-ripen­ing ones, are also avail­able. As the reten­tion time required to break down the maize silage in the digesters of the bio­gas plants to pro­duce bio­gas is sig­nif­i­cant­ly dif­fer­ent to when the maize is used for ani­mal feed, the aim is to have a high con­tent of slow­ly degrad­able car­bo­hy­drates in the maize silage.

Grass silage

With grass silage, the fol­low­ing applies: a high­er crude pro­tein con­tent means a low­er methane yield. The num­ber of cuts has a sim­i­lar effect, the first cut being the most suit­able for the pro­duc­tion of bio­gas in the agri­cul­tur­al sec­tor. How­ev­er, the eco­nom­ic ben­e­fits of using grass silage for the farm should be checked by means of a prof­itabil­i­ty cal­cu­la­tion. In addi­tion to cul­ti­va­tion on per­ma­nent grass­land, arable land can also be used to grow dif­fer­ent crop mix­tures. One exam­ple of this is clover grass which con­sists of a mix­ture of legumes added to grass seed. The mix­tures can also be adapt­ed to the local con­di­tions on indi­vid­ual farms.

Whole crop cere­al silage

As a gen­er­al rule, whole crop cere­al silage is a mate­r­i­al that ensiles well. Peren­ni­al rye is par­tic­u­lar­ly suit­able for pro­duc­ing bio­gas in the agri­cul­tur­al sec­tor. In total, the crop pro­duces yields of approx. six tonnes of dry mat­ter per hectare. Peren­ni­al rye should be plant­ed before the main crop and should not real­ly affect its growth. This can be ensured with a good crop man­age­ment and a suf­fi­cient doc­u­men­ta­tion of crop cul­ti­va­tion activ­i­ties. The har­vest­ing time also has a sig­nif­i­cant impact on the yield per­for­mance and methane yield. With peren­ni­al rye, high ener­gy yields are achieved from the “ear emer­gence” to “lac­tic ripeness” veg­e­ta­tive stages. Even though maize offers more advan­tages, whole crop cere­al silage is a good choice for bio­gas pro­duc­tion in the agri­cul­tur­al sec­tor as it can be used to break up crop rota­tions. This is par­tic­u­lar­ly the case when humus-drain­ing crops such as sweet grass­es (sorghum and oth­er vari­eties) are being used.

Sug­ar beet

Sug­ar beet has been used by many in the agri­cul­tur­al sec­tor to pro­duce bio­gas for sev­er­al years now. In par­tic­u­lar, farm­ers have been using excess beets that they didn’t need to meet their sug­ar beet quo­ta. Fol­low­ing the reform of the Euro­pean Sug­ar Regime in 2017, there has been a decline in the amount of sug­ar beet direct­ly reach­ing the digesters of the bio­gas plants. It is now much more com­mon for shred­ded beets to be used to pro­duce bio­gas in the agri­cul­tur­al sector.

Catch crops

Mus­tard, sum­mer rape­seed, oil radish and wild plant mix­es can be used as catch crops to pro­duce bio­gas in the agri­cul­tur­al sec­tor. The aim is for farm­ers to obtain a high dry mat­ter yield. The catch crops must be able to estab­lish them­selves well in the crop rota­tion and should not cause any prob­lems dur­ing the har­vest. The cul­ti­va­tion of turnips is there­fore only suit­able for spe­cial­ist farms that have the right har­vest­ing equip­ment to har­vest this crop.
Resid­ual mate­ri­als, by-prod­ucts and co-substrates

With regards to the resid­ual mate­ri­als result­ing from bio­gas pro­duc­tion in the agri­cul­tur­al sec­tor, there are many require­ments that apply and that must be observed. By-prod­ucts from live­stock farm­ing can pose a major risk through infec­tion chains between ani­mals and ani­mals and ani­mals and humans. As such, appro­pri­ate mea­sures must be tak­en to ensure a safe pro­duc­tion of bio­gas in the agri­cul­tur­al sec­tor. Even co-sub­strates from the indus­try should not con­tain sub­stances that can reach the soils once the fer­men­ta­tion sub­strate has been spread on the ground and then dam­age the environment.

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