The exploitation of excess heat from electricity production is often what is needed to make the business case more attractive for many biogas projects. In addition to traditional space heating, the heat can be used for many other purposes, including the drying of biomass, industrial process heating – or the heat can be converted into chilled water in a cooling plant. Xergi has built biogas plants with all these methods of application.
Excess heat from electricity production based on biogas has typically been used for space heating – often via a district heating network – especially in countries in northern Europe, where the winters are cold. As biogas becomes more widespread across all of Europe and the rest of the world, Xergi is noting that a number of new methods of application are being brought into play:
"We often experience that a commercial exploitation of the excess heat from biogas plants is what is needed to create an interesting business case for the investors," explains Jørgen Ballermann, Xergi's CEO.
When developing biogas projects, the project developers often have several good reasons to look at the potential of exploiting the excess heat.
"Firstly, many countries have funds which offer financial incentives for exploiting the excess heat. And secondly, a commercial exploitation of the excess heat means that the biogas plants are less dependent on the price they can get by selling the electricity. Xergi has a lot of excellent experience with a number of different opportunities for utilising the excess heat," says Jørgen Fink, country manager at Xergi.
Heating is converted into cooling
Some 25-30 years ago, Denmark was a pioneer in the development of biogas technology. Here, the majority of biogas is used in the production of electricity, and the excess heat is distributed to private households, schools and offices via an extensive district heating grid.
"But space heating is only relevant for a limited number of the world's population. On the other hand, there is a great need for cooling, not least in hot countries, where the majority of the world's population lives. This is why it is interesting that excess heat from green electricity production based on biogas can be converted into cold air," says Jørgen Fink.
The chilled water can be distributed directly to the biogas plant's neighbours or via a distribution net for district cooling in the same way as a district heating net, and there converted to cold air for cooling.
Cools down vegetables at Staples Vegetables
In 2009, Xergi built a biogas plant for the UK's largest vegetable producer, Staples Vegetables, who also got a complementary plant to convert the excess heat into cooling. The chilled water is used to keep the temperature down in the company's packaging areas.
"At Staples Vegetables, the focus is on cooling down vegetables. But it could just as well be the cooling of offices, residential buildings or schools," confirms Jørgen Fink.
Drying of products
Another possibility is to use the excess heat for drying different agricultural products.
The digested by-product from biogas plants is fluid, with a great deal of water, a small percentage of dry matter and a considerable amount of nutrients, such as nitrogen and phosphorous.
"One possibility is to perform a mechanical separation of the by-product into a liquid and a solid portion. You then heat up the liquid fraction so that part of the water evaporates. This reduces the volume quite considerably, so you get a portion of fertiliser that can be transported and delivered at significantly lower costs," explains Jørgen Fink.
You can also heat up the solid portion so it can be dried and pelletised, and the product is converted into bio-fertiliser pellets. This can be sold to companies or private customers as a CO2-neutral bio-fertiliser.
Dry manure and biofuel in France
"We have built four biogas plants for drying products in France so far. We are referring here among other things to drying the by-product from biogas production. This makes it financially viable to transport nitrogen out of the area where there is excess nitrogen. Furthermore, biomass from forestry is also dried to use as fuel," says Guillaume Loir, director of Xergi's French subsidiary.
In the UK, two biogas plants are planned for the exploitation of excess heat to dry organic bio-fertiliser generated by biogas production.
Excess heat to industry
One last but obvious possibility is using the excess heat for various industrial processes, where there is a need for either hot water or steam.
"In areas where the necessary biomass resources are at hand close to a company, which uses hot water or steam, then it is obvious to look at the opportunities for establishing a cooperation, where the company takes the waste heat from the biogas plant's electricity production," says Jørgen Fink.
One example is the biogas plant Holton Renewable Power, which Xergi has built for turkey producer Bernard Matthews in East England. All electricity as well as the excess heat from the biogas plant is used in the company's processing plant.
"There is no doubt that there are many different opportunities – and I'm quite sure that there are more than those we have just mentioned here. There are therefore good reasons for project developers to check the opportunities in their local market thoroughly, when they get started on developing new biogas projects. There are quite considerable sums to be made, both for those companies that can hand over waste and by-products to the biogas plants, as well as for those companies that can make use of the energy," concludes Jørgen Fink.
Incentives to exploit your excess heat
UK: The Renewable Heat Incentive fund provides financial support for exploiting excess heat from, for example, biogas production. In addition to space heating, the waste heat can be used for cooling or drying of products and is thereby entitled to support from the Renewable Heat Incentive.
France: Biogas plants with a high level of energy usage are entitled to a higher level of support per kWh electricity produced. There are therefore considerable financial incentives for exploiting excess heat in connection with electricity production at biogas plants.