For some of the applications the biogas may have to be upgraded. Injection of biogas into the natural gas grid will result in an improved security of supply. This is important since dependency on imported natural gas. The benefits of gas as a fuel have resulted in increased use.
There are three basic ways biogas can be utilised:
- Production of heat and steam
- Electricity production/ co-generation
- Vehicle fuel
The main utilisation of biogas is governed by national frameworks like the tax system, subsidies, availability of gas and heat grids. Worldwide, biogas is mainly used for electricity production whereas in Sweden and in Switzerland a growing portion of the biogas is used in the transport sector.
The major driver defining the way of biogas utilisation is the compensation of the energy, i.e. electricity or (upgraded) biogas. Most of the European countries have increased feed-in tariffs. Sweden has a history of low electricity prices compared to the rest of Europe. It has therefore become the market leader for biogas as vehicle fuel. The rate of growth has been rapid, for example 25% between 2004 and 2005. The development has been driven by several factors including tax exemption, government investment programme, reduction on valuation tax and free parking in several cities.
The most common applications for biogas are heating and power generation. Using biogas as vehicle fuel and injecting the gas into the gas grid are applications that reach more and more interest.
Biogas can be injected and distributed through the natural gas grid since biogas like natural gas mainly consists of methane. There are several incentives for using the gas grid for distribution of biogas. One important advantage is that the grid connects the production site with more densely populated areas which enables the gas to reach new customers. It is also possible to increase the production at a specific site and still use 100% of the gas. Furthermore injecting biogas into the gas grid improves the local security of supply. This is an important factor since most of the countries consume more gas than they produce.
As previously listed some countries like Sweden, Switzerland, Germany and France have a standard for injecting biogas into the natural gas grid. The standards have been set to avoid contamination of the gas grid or end use.
There are three major reasons for gas cleaning:
- fulfil the requirements of gas appliances (gas engines, boilers, fuel cells, vehicles etc.)
- increase the heating value of the gas
- standardization of the gas
When using biogas for vehicle fuel all contaminants as well as carbon dioxide need to be removed to reach a sufficient gas quality. Removing carbon dioxide increases the heating value of the gas and the required Wobbe index of the gas is reached.
There are several different commercial methods for reducing carbon dioxide. Most common are absorption or adsorption processes. Other techniques that are used are membrane separation and cryogenic separation. One interesting method under development is process internal upgrading.