Technology: Biogas heating and CHP

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Picture: Lemvig Biogas Plant, Denmark. Large, thermophilic biogas plant that supplt biogas for district heating. Photo by INFORSE-Europe.

Biogas for District Heating

Biogas plants produce in a bacterial, anaerobic process a gas with around 60% methane(CH4), 40% carbon dioxide (CO2) and smaller amounts of nitrogen (N2), hydrogen sulfide(H2S), ammonia (NH3), hydrogene (H2), and water vapour. H2S is a very corrosive gas and is usually removed before the gas is used. The gas can be upgraded with removal of CO2 and other gases to obtain natural gas quality with almost pure CH4, but for district heating, this is not necessary as both boilers and motors run well on biogas after H2S removal.

Biogas plants work with a biological process with a combination of natural bacteria in an anaerobic (oxygene free) environment. The bacteria can be psychrophilic (below 20’C), mesophilic (20-40’C) or thermophilic (above 40’C). In biogas for district heating is usually used mesophilic bacteria at around 37’C and thermophilic around 55’C. The thermophilic process is the fastest, but is harder to manage than the mesophilic. Biogas digestor tanks need to be warmed and kept at a constant temperature to maintain a stable production yields. Often 20% of the produced gas is used for heating the digestor tanks and electricity use in the plant for stirring the digestor tanks and for pumping.

Feed-stock for biogas is often manure mixed with waste from food and food industries as well as soft agricultural residues. The resulting biogas sludge (digestate) is a good fertilizer with more plant nutrient in mineral forms than in the input manure and other feed-stock. The higher degree of mineralization makes the plant nutrients faster for plants to use. The biogas sludge will contain the same heavy metals and persistence organic pollutants (POPs) than in the feed-stock in a somewhat concentrated form, but it will contain much less harmful bacteria and will also have less smell. The main pollutants from biogas plants are losses of methane that is a strong greenhouse gas and smell from the feed-stock, both of which can be reduced with good technology and good management. To reduce methane emissions, post digestion tanks can be used with methane collection.

Main components of a biogas plant for district heating:

  • Building to receive manure and other feed-stock, often facilities to receive liquid and solid feed-stock. The building is constructed to limit smell from feed-stock.
  • Inlet storage to ensure a steady supply of feed-stock to the digestors.
  • Eventual Hygieinisation of feed-stock with bacteriological risk.
  • Mixing of feeds-stocks. If feed-stock is too dry,also addition of water.
  • Heat exchanger to pre-heat prepared feed-stock with degassed biogas sludge.
  • Digestor tanks with heating and stirring (stirring of tank content can be with equipment in tank or with pumping between different layers of tank).
  • Outlet storage for degassed biogas sludge. This can be covered to collect methane.
  • Eventual separation of degassed biogas sludge in (thin) liquid fraction and thick/fiber fraction.
  • Gas cleaning and drying.
  • Eventual gas storage to match gas production and demand on a daily basis (day-storage).
  • Eventual local CHP plant to supply heat and electricity for the plant (if district heating station is far from biogas plant).

Examples

Lemvig Biogas Plant

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This is the largest Danish thermophilic plant that produce gas for district heating (larger plants produce for the public gas grid). It has 4 digestor tanks with a combined capacity of 14.000 m3, operated with thermophilic bacteria at a temperature of 52’C The plant is owned by farmers and is delivering gas via biogas pipes to the CHP plants in the town of Lemvig as well as to 5 villages that also have district heating. The CHP plants produce 33 GWh electricity and 55 GWh heat annually, supplying heat for more than 3,000 households while the electricity is sold on the electricity grid.

The feed-stock is animal manure and a number of organic waste streams from food factories and from waste water treatment plants. Waste must have a limited content of heavy metals as the output is used as fertilizer. The feed-stock is delivered by truck and the biogas sludge is sent to the farmers that own the plant. Large tanks for the biogas sludge allow farmers keep it during the winter and only apply it as fertilizer during the plant growing season.

Technical parameters for new biogas plants

  • Sizes, typically 1000 tons input/day, biogas production capacity 9 MW (typical for Denmark).
  • Efficiency: Of the produced gas, typically heat use is 9% and electricity use is 3.8%, allowing 86% of gas to be used for other purposes.
  • Yield, typically 0,75 GJ (0,2 MWh) gas per ton of input.
  • Lifetime, technical lifetime is typically 20 years.
  • Space requirements: Because of smell in case of problems with the process, it is recommended not to size a biogas plant near dwellings or business that has a problem with odours.
  • Emissions: Methane emissions from below 1% to 2% for larger plant supplying district heating (Danish experience, 1% is aim for all plants, 2% is present average in 2021). In addition, biogas boilers and CHP plants have similar NOx and methane emissions as natural gas boilers and CHP plants. Also biogas upgrading plants to produce natural gas quality can have methane emissions.

Financial parameters for new biogas plants

  • Nominal investment: 1.7 mill. €/MW of biogas plant, not including CHP unit to district heating or pipes outside plant premises. Plants much smaller than 9 MW will be more expensive per MW.
  • Fixed O&M 160,000 €/MW biogas capacity / year excluding heat and electricity demand that can be produced from the biogas, and assuming the plant is operating.
  • Variable O&M: with above value for fixed O&M, no additional variable O&M should be added.

Sources for more information

Calculation of energy and economy
Cases
  • For examples of biogas plants, see list of Danish biogas plants here: ens.dk
    (links to websites of plants can be found with an internet search engine)
Technology suppliers and installers (coming)
Technical advice (coming)
Research and Development (coming)

Sources for this chapter

  • Technology Catalogue for Renewable Fuel, Danish Energy Agency, April 2022 (both Technology Catalogue and accompanying data sheets)
  • Lemvig Biogas: lemvigbiogas.com
  • Danish Energy Agency online information on biogas: ens.dk
  • Biomethanation under psychrophilic conditions: A review (only reference for definition of temperature ranges for psychrophilic, mesophilic, and thermophilic temperature ranges), Published May 2003 in Bioresource Technology 87(2):147-53. Authors: Kashyap, Des; Dadhich, K; Sharma, S. researchgate.net
  • Metantab fra biogasanlæg skal bringes ned (only about methane emissions). Article by the association Biogas Danmark, online, September 2021: biogas.dk