CHP Units for Biogas: Technology, Efficiency, Sizing

A combined heat and power unit (Blockheizkraftwerk, BHKW) for biogas is a combustion engine coupled to a generator that produces electricity and usable heat from biogas at the same time. Today almost all new units use spark-ignition gas Otto engines. Depending on size, biogas CHP units reach electrical efficiencies of roughly 35 to 43 per cent and thermal efficiencies of roughly 40 to 50 per cent. The right size depends on the available gas volume, the target rated output and whether the plant runs continuously or flexibly according to power prices.

How a biogas CHP unit is built

A biogas CHP unit consists of the gas engine, the generator, heat exchangers for cooling water and exhaust gas, the gas control train, the control system and usually a soundproof container. The biogas has to be conditioned before it reaches the engine: it is cooled and dried to remove condensate, and desulphurised because hydrogen sulphide attacks the engine and the oil. An activated carbon filter is often added.

Heat is available at two temperature levels: engine cooling water at around 80 to 90 degrees Celsius and hot exhaust gas, which can be used via an exhaust heat exchanger. Part of the heat is needed to warm the digester; the rest can supply barns, the farmhouse, drying or a district heating network. The guide to biogas heat use explains the options.

Gas Otto or dual fuel?

Dual fuel engines were common in the early years of farm biogas because they are derived from robust diesel engines and achieved good efficiencies at small outputs. The need to buy and store a second fuel, stricter exhaust requirements and shorter service intervals mean that new plants are now almost always fitted with gas Otto engines. For operators of older plants, replacing a dual fuel engine with a gas Otto unit at the next major overhaul can be an option.

Efficiency and full load hours

Electrical efficiency is the key figure because electricity is the more valuable part of the energy. It rises with engine size: small units up to 100 kilowatts sit at the lower end of the range, megawatt-class engines at the upper end. Including heat, overall efficiencies of roughly 80 to 90 per cent are achievable, provided the heat is actually used.

A worked example for a small manure plant: the plant produces 300,000 cubic metres of biogas a year at 55 per cent methane, i.e. 165,000 cubic metres of methane. At roughly 10 kilowatt hours per cubic metre of methane, that is about 1.65 million kilowatt hours of fuel energy. At 38 per cent electrical efficiency this yields roughly 627,000 kilowatt hours of electricity. Divided by the 8,760 hours of the year, that gives a rated output of just over 70 kilowatts. At 45 per cent thermal efficiency, roughly 740,000 kilowatt hours of heat are produced at the same time. The biogas calculator works this out for your own figures.

In baseload operation, roughly 7,500 to 8,300 full load hours a year are usual. Every hour of downtime means unused gas that has to be buffered in the gas store or, in an emergency, burned in the flare. High availability is therefore more important for profitability than one extra percentage point of efficiency.

Safeguards include a gas store large enough to bridge short stoppages and an emergency flare that burns surplus gas safely if the CHP unit is down for longer. Flared gas earns nothing, but it is far better than methane escaping unburned. Choosing two engines or a service contract with a short response time reduces the risk of long outages.

Choosing the right size

Two values need to be distinguished. Installed capacity is the rated electrical output of the CHP unit. Rated output (Bemessungsleistung) is the electricity actually generated in a year divided by the hours of the calendar year. For small manure plants the tariff is based on rated output, while total installed capacity at the site must not exceed 150 kilowatts. Details are in the guide to the EEG feed-in tariff.

These questions help with sizing:

1. How much gas is reliably available? Base this on realistic feedstock planning, not on the best case in a brochure.
2. Baseload or flexibility? For continuous operation the unit is chosen slightly larger than the rated output to make up for maintenance hours. For flexible operation, installed capacity can be considerably higher.
3. One or two engines? Two smaller units raise availability because one keeps running while the other is serviced. Investment and maintenance effort rise accordingly.
4. Part load behaviour: an engine that runs well below rated load for long periods loses efficiency and wears faster.
5. Heat concept: heat output should match demand on the farm or in the heating network.

Flexible operation: more capacity, fewer hours

With flexible operation, the plant generates electricity when wholesale prices are high and stores gas during the remaining hours. This requires more installed engine capacity, larger gas storage, often a heat store and controls that follow schedules. The EEG supports this extra capacity with the flexibility supplement under Section 50a EEG, which under the current wording amounts to 100 euros per kilowatt of installed capacity per year. It applies to biogas plants above 100 kilowatts installed capacity whose tariff is set by law, and to plants awarded in tenders. Existing plants may be eligible for the flexibility premium under Section 50b EEG.

For small manure plants, flexibility is usually secondary; the focus is on reliable continuous operation and good heat use. For larger plants, and for plants considering their prospects after the first EEG payment period ends, flexible operation is often the main lever.

Operation, maintenance and suppliers

The CHP unit is the component of a biogas plant with the highest maintenance requirement. Oil changes, spark plugs, valve clearance and, at longer intervals, a major overhaul are routine. Many operators therefore sign a service contract with the manufacturer or a service company. The guide to biogas plant maintenance covers what to look out for. Companies supplying and servicing CHP units are listed under service and maintenance.

When buying, make sure spare parts and service technicians are available in your region. A cheap engine is of little use if it waits days for a part after a breakdown. Also ask for biogas references, because sulphur, moisture and fluctuating methane content place higher demands on an engine than natural gas.