Case Story Sweden

The Swedish voluntary system for control of methane emissions

May 2012

Biogas production and use

Biogas is formed during the anaerobic degradation of organic material. It can be produced from any material that is biodegradable and consists mainly of two gases, methane and carbon dioxide. Biogas today is primarily produced from sewage sludge, household waste and energy crops. Biogas production and use has gained increased interest over the last few decades, mainly due to its many environmental benefits. As a result the annual production is increasing. Biogas, or to be more precise, the methane part of biogas, can be used as an important source of renewable energy. It can be used for heat production or heat and power production, as a raw material in industry and as a vehicle fuel. Biogas can also be used to replace natural gas in the gas distribution grid.

Biogas cleaning and upgrading

In order to use biogas, it needs to be cleaned to a greater or lesser extent, depending on the use. Biogas to be used as a vehicle fuel, or to replace natural gas, needs a treatment whereby the carbon dioxide is separated from the methane. Removing carbon dioxide increases the energy content per volume. This is known as biogas upgrading, and several techniques are commercially available. Emissions from three different technologies have been quantified in the Voluntary Agreement, and are described below.

PSA

PSA (Pressure Swing Adsorption) is a technique by which carbon dioxide in the raw biogas is adsorbed on activated carbon or zeolites. In a PSA plant, there are usually several columns with the adsorbing material working in parallel. The material is regenerated by decreasing the pressure which will make the carbon dioxide desorb. Small amounts of methane that may also have been adsorbed are also released; these end up in the off-gas.

Water scrubbing

Water scrubbing is a technique by which the raw biogas meets a counter flow of water in which carbon dioxide dissolves. Water scrubbers can be of flow-through type, but usually the water is regenerated by a decrease in pressure and by bubbling air through the water. Methane slip will occur in air leaving the desorption column where the water is regenerated. If the water is not regenerated, methane slip will occur in the water stream leaving the plant.

Chemical scrubbing

Chemical scrubbing is similar to water scrubbing, the difference being that in chemical scrubbing carbon dioxide is chemically bound to amines present in the counter flowing liquid. The liquid is regenerated by heat and, since methane has a low solubility in the liquid, only small amounts of methane occurs in the off gas.

Environmental benefits and methane emissions

Biogas production and the use of its energy content create several environmental benefits. Replacing fossil energy leads to decreased carbon dioxide emissions. The environmental benefit is dependent on how the biogas is produced and used. Biogas production from manure leads to a “double” positive environmental effect. Not only does the biogas replace fossil energy, thus decreasing carbon dioxide emissions, but also the methane emission that usually occurs during manure storage is avoided. However, while biogas use does lead to decreased emissions of fossil carbon dioxide, methane is, in fact, a strong greenhouse gas and thus emission of methane has to be avoided or minimized during the process. The global warming potential for methane over a 100 year timeframe is 25 times that of carbon dioxide (IPCC, 2005). Overall, the environmental benefits, and in particular the decrease in fossil carbon dioxide emissions, mean that a certain level of methane emissions can be tolerated while still maintaining a positive environmental effect. How high the emissions can be before the total effect becomes negative depends on the raw material used. A study has shown that for production and upgrading of biogas from manure, the methane emissions can be up to 22–26%, while for organic waste the level is 12–17%, and for grass, straw or beet tops 8–16% (Börjesson and Berglund, 2003). In general though, the greatest environmental benefit is obtained when emissions are minimized. Apart from the environmental aspect, there are also other reasons to avoid methane emissions. There is, of course, an economic incentive not to lose the methane produced. A loss of methane is a loss of income. Safety is also an important consideration, as methane can form explosive mixtures with air. Plus, biogas emissions can lead to odour problems in and around the plant. Thus, the Voluntary Agreement was initiated with the aim of quantifying and minimizing methane slip from biogas production plants and biogas upgrading plants.

Background for the Voluntary Agreement

A Swedish study (Persson, 2003) concluded that in some cases methane emissions from biogas upgrading plant were higher than the 2% that was at that time the level generally claimed by the plant’s manufacturers. A different Swedish study (Gunnarsson et.al. 2005) showed small methane emissions at various points in the plant, with a total methane slip of between 0,5% and 4%. In 2007, the Voluntary Agreement was initiated (Persson et.al. 2007) in order to establish a systematic approach to quantification and minimization of methane emissions. The system for the Voluntary Agreement was then revised in 2009 (Holmgren, 2009). A more detailed description of how to measure and calculate the methane slip was also published by Holmgren in 2011. However, one remaining problem is that methane slip from surfaces such as uncovered digestate tanks can be hard to quantify. To be able to improve the system of the Voluntary Agreement, work is on-going in this area. The results of the system of the Voluntary Agreement have been presented at several national and international conferences. The title for the Voluntary Agreement in Swedish is “Frivilligt åtagande”.