biogas from anaerobic digestion

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Technology update

Biogas from anaerobic digestionCO2 savings and economics

IntroductionBiogas from anaerobic digestion (AD) has a range of potential energy uses across heat, transport and electricity. This technology update describes these different options and compares the carbon savings potential for each. It also discusses the impact incentives might have on the economics of biogas production.

Key points The Carbon Trust has developed a model that allows comparison of the carbon savings and economics of different energy uses of biogas from AD. It is important to consider parasitic energy loads when comparing carbon benefits of different end uses, as these, along with fugitive methane emissions, can have an unexpectedly high impact on the carbon savings. Based on the assumptions used in the model, biomethane as a transport fuel has higher carbon savings potential compared to use for heat and electricity. Biomethane to the gas grid used for renewable heat has lower carbon saving impact than using the biogas for electricity generation, but it will have an advantage over electricity generation in the future when the electricity grid has been decarbonised, and injection to the gas grid provides flexibility to use the biomethane as a transport fuel elsewhere. Incentives such as feed in tariffs and the renewable heat incentive have a major effect on the economics of biogas production and the cost effectiveness between different energy end uses can vary.

Biogas from anaerobic digestion

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Figure 1

Resource Food waste

Use CHP/ electricity

Manure Anaerobic Digestion Agricultural waste H2S etc. removal

Biogas CH4/CO2

On-site heat

CO2 removal

Biogas-togridBiomethane CH4

Sewage sludge

Transport fuel

DefinitionsAnaerobic digestion (AD) is a process in which bacteria break down organic material in the absence of oxygen. Anaerobic digestion of sewage sludge is in widespread use in the water industry in the UK. There are also around 20 AD plants operating on food waste (from domestic and industrial sources) and farm waste (slurry, agricultural residues, crops) with more in development. Biogas, the gas produced directly from anaerobic digestion, contains a mixture of methane and CO2 with some other impurities. This can be burnt directly in a CHP engine or specialised boiler. Biomethane is purified biogas which has had the CO2 removed and is >97% methane. This can be injected into the gas grid (after odorisation and adjustment of calorific value by adding propane) or compressed and used as a transport fuel.

Different uses of biogas and biomethaneThere are four alternative options for the use of biogas produced by anaerobic digestion, shown diagrammatically in Figure 1. Following minimal cleaning (to remove hydrogen sulphide and other impurities that could damage equipment): The biogas can be burnt in an engine to produce electricity and heat. The biogas can be burnt in a boiler to provide on-site heating or steam/ hot water for a local heating scheme. Or CO2 and other impurities are removed to leave a pure biomethane: The biomethane can be injected into the natural gas grid. The biomethane can be compressed or liquefied and used as a transport fuel. Compressed biomethane is the same as compressed natural gas and can be used in internal combustion engines with suitable adjustments. Currently most AD plants producing energy in the UK combust the gas to generate electricity (frequently the only available use for the heat produced is within the AD plant and only electricity is exported). Trials are planned for the first injection of biomethane into the gas grid (DECC published guidance on this in December 2009) and biomethane has been used on a small scale as a transport fuel.

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Figure 2 Model inputs and outputs

Feedstock Type (manure, sewage, food waste, energy crops) Flowrate

Financial assumptions Energy prices Incentives Gate fees

Biogas modelling tool Selects AD plant type Calculates biogas yield

Outputs: Carbon savings Economic potential (returns, NPV etc)

Technical data Carbon conversion factors Efficiencies

Tool developed with assistance from Organic with assistance from ORA (Organic Resource Agency Ltd) Tool developed Resource Agency Ltd (ORA)

Carbon Trust modelThe Carbon Trust commissioned work to compare different uses of biogas for electricity generation, heat and as a transport fuel. Anaerobic digestion experts ORA built a spreadsheet model which allows the carbon savings and economics of different waste streams and different uses of the biogas to be compared and this has subsequently been developed further by the Carbon Trust.

Gas clean-up technologiesA number of different clean-up technologies are available to convert biogas to biomethane by removing CO2 and other impurities. Commonly used processes include water washing, chemical scrubbing and pressure swing absorption. The costs and energy use for gas clean up in the Carbon Trust model were based on chemical scrubbing technology. A review of suppliers prices suggested that these costs are representative of other gas cleaning processes, but which technology is most cost effective is likely to depend on the biogas flow rates.

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Chart 1 Electricity production parasitic loads3,500 3,133 3,000 2,500 2,000471

Chart 2 Biomethane production parasitic loads3,500 3,000 2,500 3,059

471 489 33553 1,567 144

100

537

2,025

2,000

t CO2e pa

t CO2e pa

1,500 1,000 500 0CO2 displaced AD plant elec Gas Methane Net clean up lost saving

1,500 1,000 500 0

CO2 displaced

AD plant elec

AD plant heat

Gas Methane Added Net clean up lost propane saving

Carbon savings from different optionsThe model sizes an AD plant for a selected waste stream and calculates the CO2 savings for different end uses of the biogas produced by the plant, based on the following assumptions: A CHP plant replacing grid electricity (the kg CO2/kWh of the electricity produced can be varied in the model) and producing heat used locally, which is assumed to replace natural gas heating. The electricity only case is where the CHP heat usage is set to 0% and the only use for the heat produced is in heating the AD plant. The baseline assumption was that electricity was generated at an efficiency of 35%. Biomethane to gas grid (replacing natural gas with a factor of 0.185 kg CO2/kWh). Transport fuel displacing diesel, assuming vehicle engine efficiency is unchanged i.e. the km per MJ of fuel are the same for diesel and compressed biomethane. (In practice the actual efficiency will vary depending on engine type.) In the discussion that follows figures are presented based on an AD plant processing 25,000 tonnes pa of commercial food waste. This is a typical scale for plants proposed for industrial and commercial food waste in the UK and is processing an amount equivalent to the food waste from around 70,000 households.

Parasitic energy useThe model includes consideration of parasitic power; the heat to run the AD plant and electricity to run the gas clean up process are subtracted from the energy produced. The CO2 equivalent of fugitive methane emissions for each process are also included and these vary for different energy production routes. Charts 1 and 2 above show the breakdown in CO2e for two cases electricity generation and biomethane to grid. For electricity production the effect of the parasitic load and fugitive methane is to reduce the carbon benefit by about a third, for biomethane production the reduction is nearly half.

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Chart 3 2010 net CO2 savings for 25,000tpa food waste3,000 2,500 2,000

Chart 4 2020 net CO2 savings for 25,000tpa food waste3,000 2,500 2,000

t CO2e pa

1,500 1,000 500 0 -500 -1,000 -1,500 -2,000

t CO2e pa

1,500 1,000 500 0 -500 -1,000 -1,500 -2,000

Transport fuel

CHP 50% heat use

Electricity only

Biomethane to gas grid net CO2

Transport fuel

CHP 50% heat use

Electricity only Biomethane to gas grid

parasitic CO2

parasitic CO2

net CO2

2010 carbon savingsChart 3 shows a comparison of net CO2 savings (after parasitic power and fugitive methane has been taken into account) assuming electricity is displaced at the current grid electricity generation mix of 0.54kg CO2/kWh. The hierarchy of savings, based on the assumptions listed above, is: Biomethane as a transport fuel, or a CHP plant with heat utilisation of 50% or more, saves most carbon. Electricity only generation is the next best option. Biomethane to grid savings are the lowest, as it displaces gas (a comparatively low carbon fossil fuel) and has a relatively high parasitic load. The carbon savings for electricity generation are very sensitive to the value assumed for the electrical efficiency of the generator. If this is increased to 40% from the baseline assumption of 35%, electricity generation saves the same amount of carbon as biomethane transport fuel. Another important assumption is the engine efficiency for the vehicle using compressed biomethane. If, instead of taking the same efficiency for diesel and methane, a 10% drop in efficiency is assumed, the transport fuel net CO2 savings in 2010 are slightly higher than the electricity only option.

2020 carbon savingsIn future it is expected that the carbon intensity of the electricity grid will reduce, so the impact of generating electricity will be less. Using the Committee on Climate Changes target of 0.31 kg CO2/kWh for 2020, and assuming at t

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