Boosting Biogas with Heat Bonus: How Combined Heat and Power Optimizes Biogas Utilization

Renewable energy sources include water and wind power, solar and geothermal energy, as well as energy from biomass. The technical achievability and the actual usage of these energy sources are different around Europe, but biomass is seen to have a great potential in many of them. An efficient method for the conversion of biomass to energy, is the production of biogas by microbial degradation of organic matter under the absence of oxygen (anaerobic digestion).

Biogas Production

Biogas is a mixture containing predominantly methane (50%–65% by volume) and carbon dioxide, and in a natural setting it is formed in swamps and anaerobic sediments, for instance. Due to its high methane concentration, biogas is a valuable fuel. Wet (40%–95%) organic materials with low lignin and cellulose content are generally suitable for anaerobic digestion. Examples include agricultural materials like liquid manure, dung, fresh plants or plant parts (for example grass, clover, sugar beet leaves), silages (for instance from grass or maize), industrial residues from food processing like stillage (grains and liquid effluent remaining after distillation), pomace (the solid remains of fruit after pressing), whey, grease trap contents, and the organic fraction of municipal waste.

Due to this wide spectrum of raw materials, biogas production can either be used for energy production from agricultural land and for the utilization of waste streams and by-products in food processing, or bio-refineries.

Figure 1, below, shows a schematic of an agricultural biogas plant, with the anaerobic digester at the ‘heart’ of it. Pre-treatment steps, which could for example include chopping, grinding, mixing or hygienization, largely depend on the origin of the raw materials.

Downstream processing steps are rarely installed because the digester effluent can be used as a valuable liquid fertilizer – in order for this to be used in accordance with regulations, a storage tank is needed. Some German agricultural biogas plants use gas-tight covered storage tanks, thus considerably reducing methane loss from the digester effluent into the atmosphere and increasing the amount of biogas for energy production.

Prior to its utilization, the biogas has to be cleaned. Coming out of the digester, biogas is saturated with water which would cause corrosion problems upon condensation – it therefore has to be removed. And, depending on the type of raw material in the biogas plant, hydrogen sulphide is also generated as a by-product in the microbial conversion process. Because of its corrosive character the concentration of this constituent also has to be reduced considerably in the cleaning process.

Biogas Utilization

Biogas can be converted to energy in several ways. The predominant utilization in Germany is combined heat and power (CHP) generation in a gas engine installed at the place of biogas production.

Gas engines are available in a wide range of sizes from around 60 kWe up to more than 2 MWe in a single unit. They have a proven reliable operation in more than 4000 biogas plants in Germany and their average annual operating hours are around 7500. This indicates that German biogas CHP engines are being operated for electricity production in baseload.

There are two main reasons for this. First, biogas production is an almost continuous process; it is rather difficult or, in the short-term, even impossible, to control the operation of anaerobic digesters according to any given demand profile. Secondly, German promotion of renewable energies is focused on electricity production. Because of that, biogas plant operators receive the predominant fraction of revenues from the guaranteed feed-in tariffs for electricity.

However, a great amount of cogenerated heat in local biogas CHP units remains unused. Only a small fraction of it (10%–40% depending on raw material and process conditions) is needed to keep the anaerobic digestion process at operating temperature. An even smaller fraction of this heat energy is usually utilized for heating purposes at the biogas plant.

Therefore, since 2004, the German Renewable Energies Act has included additional revenues to be paid if electricity to be fed into the grid has been produced in ‘real cogeneration mode’ i.e. including the efficient utilization of heat. In spite of this incentive, many biogas plants are still not able to use the processed heat for a simple reason: they are located in rural regions, usually far away from industrial heat demand and even further away from existing district heating (DH) networks in larger cities.

Nevertheless, the rising awareness of the necessity to utilize renewable resources efficiently has led to different approaches in innovative biogas utilization pathways.

Three Utilization Models

The first possibility for increasing heat utilization from biogas production is the erection of new, rural DH networks. This is usually the core of so-called ‘bio-energy village’ projects. One example of this type of efficient bio-energy project has been developed in the village of Jühnde in the centre of Germany. Jühnde makes use of a biogas plant on the outskirts of the village – a woodchip-fired boiler and a biodiesel-fired peak load boiler – all connected to a new district heating network. The latter had been erected in the village after more than 70% of the inhabitants agreed to change their energy supply from their own boilers to district heating from renewable resources.

Although recent studies have revealed that in rural regions district heating networks can be used not only for ecological considerations but also for economic benefits, considerable efforts had to be taken to reach such a high willingness from inhabitants to connect to the new heat supply.

Another example of using heat from biogas currently gaining more importance in Germany, is the installation of biogas pipelines. This means that after producing biogas in an agricultural biogas plant, the major portion of the gas can be transported via new gas pipelines to places where CHP engines can be operated for baseload supply, for example, the DH networks.

One of the first examples of this type of project is in the Northrhine-Westphalian city of Steinfurt. In an agricultural region 3 km outside the city limits, a new biogas plant has been erected. A small fraction of the biogas is used locally in a gas engine to produce electricity and heat for the anaerobic digestion process (a gas engine with 347 kWe power), but the majority of the biogas is dried, compressed and sent through a new pipeline (3.6 km long) to the city centre of Steinfurt. See Figure 2, left. There, a district heating network supplies the county administration, schools and a hospital with the heat. The baseload supply of this DH network has been taken over by a new biogas-powered gas engine (536 kWe).

If neither of the examples mentioned are suitable for a specific biogas plant, a new third possibility has received more attention over the last two years – biogas upgrading. Using this method, the biogas can be purified in such a way that it can be fed into the natural gas grid. To this end, aside from water vapour and hydrogen sulphide, the carbon dioxide has to be removed from the biogas, leading to an almost pure methane gas stream.

Projects of this kind are not new; biogas upgrading projects have been pursued in Germany and the Netherlands since the 1980s. Technology development has been further fostered in Sweden and Switzerland, where several biogas upgrading installations went into operation in the 1990s, mainly producing methane for the use as compressed natural gas (CNG) for fuel. Bus fleets in Kristianstadt and Stockholm (Sweden) and the trucks of the Swiss food retailer Migros, are powered by biogas from the anaerobic digestion of sewage sludge, municipal biowaste or food residues.

In Germany, biogas upgrading has become attractive for project developers since last year’s new regulations were set into force – this has allowed easier access for upgraded biogas to go directly into the gas grid. In combination with the German Renewable Energies Act, it is now possible to produce biogas at a rural installation, upgrade it to biomethane, feed it into the gas grid, use it in a heat demand-controlled CHP unit operated on natural gas, and to receive revenues according to the guaranteed feed-in tariff for biogas-based electricity.

Technologically, such projects are based on the carbon dioxide removal from biogas. Currently, two technologies are being used for this purpose in large-scale applications: pressure swing adsorption (PSA) and water scrubbing. The PSA process employs carbon molecular sieves for the removal of carbon dioxide from biogas, while water scrubbing makes use of the different solubilities of carbon dioxide and methane in water. Both technologies yield a gas stream consisting of more than 96% methane which may be fed into the natural gas grid.

Additional options for biogas upgrading currently under development are membrane and absorption processes. These latter methods use physical or chemical absorbents like ethylene glycol or dimethylamine for the removal of carbon dioxide.

Biogas upgrading, of course, leads to an increase in technical complexity and the operational expenses in running a biogas project. So, which of the previously described biogas utilization pathways, summarized in Figure 3, shown on page 105, can be regarded as economically optimal? Unfortunately, this cannot be answered easily because it depends on local conditions, like prices for substrate supply and revenues for electricity and heat.

Ecological Advantages of Biogas Technology

An easier situation can be found when looking at the ecological effects of different biogas utilization pathways; this was investigated in a study for the German Federal Ministry for the Environment. Complete eco-balances, in accordance with ISO 14040/14044, were performed and the key assumptions for the comparison of different biogas utilization processes, are:

• biogas production based on maize silage using a biogas plant with covered storage tank – methane losses were 1% of the biogas produced
• biogas utilization in a local gas engine, installed at the biogas plant with 500 kWe – electrical efficiency of 37.5%, thermal efficiency of 42.5%, and methane losses of 0.5%
• biogas upgrading with a power consumption 0.3 kWhe/m³ biogas – methane losses of 0.5%
• biogas utilization in a heat demand controlled gas engine supplied out of the natural gas grid with 500 kWe – electrical efficiency of 37.5%, thermal efficiency of 42.5%, and a methane loss of 0.01%

For the calculation of benefits or disadvantages of biogas utilization in the different eco-balance categories the following fossil fuel-based standard energy production processes were defined – electricity: 70% hard coal, 30% natural gas; heat: 57% natural gas, 43% light fuel oil; fuel: 100% gasoline.

Figure 4 presents the results of the greenhouse gas (GHG) savings from different biogas utilization options, in comparison to fossil fuel-based standard energy production processes.

The three upper bars on the graph show the results for biogas utilization in a local gas engine. The difference between the results is due to the degree of surplus heat used. The second bar with 20% surplus heat utilization represents an average German biogas plant. The upper bar gives the less favourable result for a gas engine without heat utilization, while the third bar represents a practically optimal case, in which 80% of the heat produced in baseload can be used.

The three lower bars show GHG savings for different utilization options of biogas, following upgrading and feeding into the gas grid. Utilization in a gas engine operated fully in CHP mode is the most favourable of the investigated upgrading cases with the above assumptions. Nevertheless, even this case yields lower GHG savings than biogas with local decentralized heat utilization (80%).

The simple substitution of natural gas for upgraded biogas, yields the least ecological benefits. This is due to the fact that natural gas is already GHG saving, in comparison to coal or crude oil-based fuels. Finally, as the lowest bar reveals, biogas utilization as a fuel cannot compete with electricity production under the underlying assumptions.

A Promising Option

Summarizing the results of the eco-balances it becomes obvious that – not only by using fossil fuels but also by using renewable fuels like biogas – combined heat and power cogeneration is the optimal way for fighting climate change.

From a technical point of view, the production of biogas, i.e. the conversion of renewable resources and biowaste to energy, can be seen as state-of-the-art technology. Because of the low energy density of the source materials, biogas should be produced in an increasingly decentralized fashion. In the case that no heat utilization options exist nearby, biogas upgrading and feeding into the gas grid is to become a promising new option for consideration.

Dr Ing. Stephan Kabasci is Business Unit Manager Renewable Resources at the Fraunhofer-Institute for Environmental, Safety and Energy Technology (UMSICHT), Oberhausen, Germany. (This article also appeared in the Jul–Aug 2009 issue of COSPP.)

e-mail: stephan.kabasci@umsicht.fraunhofer.de