The profitability of Biogas

For a biomethanization unit in piggery with its electric cogenerator to be profitable, it is necessary to closely study fermental yields in methane production, a yield notably conditioned by the dry matter content of the liquid to be fermented. Then there is a power generator module adapted to the gas emission volume of the biogas unit.

Finally, potential electricity buyers in France should pay for electricity at a price per kW in line with the European price.

Potential buyers cite better profitability of nuclear power, omitting to include in its cost of production, the dismantling of future plants, a situation that penalizes the development of renewable energies in general and the energy exploitation of pig manure in particular.

Key words: biogas, biomethanation, anaerobic digestion, manure, waste, agriculture, valorization, kwh, redemption, cost, investments.

Failure to comply with European regulations

At the European electricity tariff paid to the producer at 0,10 € / kWh, a cogenerator on slurry biogas of a unit of 350 sows and fattening could be profitable. But at the French purchase price 0,059 € / kW, about half, it becomes more difficult ...

Unlike their neighbors in Luxembourg, Germany, Belgium and Switzerland, French pork farmers and farmers have so far been confronted with a blockage when they proposed projects to install biogas from manure. This situation would, however, partly solve many problems, including greenhouse gas emissions, nitrate emissions and the strengthening of the financial solidity of farms, which are constantly being tugged by burdens. Keep in mind that methane naturally released by manure pits, if not recovered, is 21 more damaging as a greenhouse gas than CO2. A white paper published by the European Commission in November 97 (Com (97) 559) set the target for 12%, the share of renewable energy in the total consumption of 2010.

This implicit blocking, of a financial nature, is linked to the profitability of a cogenerator, itself dependent on the price of electricity paid to the farmer.

The principle

Recently, a group of farmers, pig farmers in Lorraine, met in a project of profitability study of anaerobic digestion and cogeneration units. The feasibility study is on a farm of 170 ha including piggery, birth and fattening of 350 sows. The Career EARL at Cléry (55)

Let's remember in two words what it is. In the slurry tank an anaerobic (airless) fermentation occurs. The anaerobic process leads to the degradation of carbon molecules into small single-carbon molecules, CH4 methane and C02 carbon dioxide, which is an energy gas of choice.

The principle of transforming a slurry pit into a digester is simple but expensive. The pit can simply be covered with a biogas-tight tarpaulin to collect the gases. It is necessary to adapt a stirrer and heating to 35 - 45 ° C - ideal for slurry pits - to activate fermentation and to degas. The volume of the pits can range from 300 to 1 m000. Modern, more efficient mixers can mix up to 3 m1700 of slurry containing up to 3% dry matter.

The theoretical slurry retention time is about 28 days. It usually takes 50 days to optimize performance. This involves installing a new pit for digestate retention and a pre-pit, all of which are covered to collect the gases.

The methane collected must first be purified of its sulfur compounds, type H2S, deemed corrosive, this by injecting 3 with 6% of air. The gas can finally supply a motor coupled to an electricity generator. Part of the energy will be used to heat slurry and pigsty.

Dry matter, limiting factor

The pigsty studied in Meuse, with 350 sows and fattening, or 415 UGB, produces 16,4 m3 of slurry (6000 m3 / year) daily. According to various studies, the optimum fermentation gas yield of 0,925 m3 of biogas / UGB / day is obtained with slurry at about 9% DM (dry matter). This DM level depends in particular on the type of feed the pigs are fed, in solid or soup form. The soup leads to slurry less loaded with 4 to 5% DM. In this case, to optimize the fermentation yield, it is necessary to supplement the liquid manure with DM - from fallow land, green waste from communities or food industries, etc. The DM rate being a factor limiting the profitability of biogas production.

In Luxembourg, biogas plants operate with fallow silage and intercropping. The European regulations in force (Nr: 1251 / 1999) make it possible to use fallows for this energy objective. Corn grain straw, currently buried in plowing, could also be valued.

From biogas to electricity

Once the optimum volume of biogas production has been reached (0,925 m3 of biogas / UGB / day), the question arises of the choice of the electricity generating module. It is also from this choice that will result the profitability of the investment. In the case of the La Carrière farm in Cléry, two cases were studied: a plant operating with a module of 65 kW with slurry at 6% MS and another with a module of 100 kW and slurry brought to 9 % of MS, thanks in particular to the crops of the farm.

A module of 65 kW produces 465 000 kWh annually. 390 000 kWh's annual requirement for hog barns and slurry heaters precludes any profitability.

In the second case, that of the module of 100 kWh with slurry at 9% of MS, the annual production reaches 840 000 kWh and ensures an effective surplus of 450 000 kWh.

Cost effectiveness of the installation

In the case of this farm, the cost of the investment from the existing facilities, to which must be added a tank of 800 m3 for the gas, the transformation of the pit into a digester, the pre-pit and the digestate pit ( 2500 m3), a cogeneration module of 100 kW, and lastly electrical equipment and slurry pipes, would amount to about 280 000 €.

The European sales price applied in Germany, Belgium and Luxembourg is 0,10 € / kWh. In France, the tariff order relating to non-fossil materials of plant or animal origin as part of the obligation to purchase electricity from renewable sources (published on 16 April 2002) provides a purchase price of 0,059 € / kWh, almost half.

According to this feasibility study, the installation on this farm could be profitable from a purchase price to 0,09 € / kWh. At the price offered by the French electricity operators, it would almost double the pigsty, 800 sows plus fattening to make the facility profitable.

Potential buyers cite the greater profitability of nuclear electricity, failing to include precisely in their cost-effectiveness calculations the cost of dismantling and reprocessing dismantling waste when it is at the end of its life.

While it must be admitted that nuclear power is unavoidable, it should not penalize the development of renewable energy on the pretext of insufficient profitability in the face of nuclear power, a profitability can not, for the moment, not be objectively evaluated.

David Lefebvre

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