Biomass refers to the substances that have grown from animal or vegetable matter and can be used as a renewable fuel. Biomass is regarded as being a carbon neutral fuel as it returns a similar amount of energy to the atmosphere that was taken out by the biomass. Trees for example absorb CO2, they are then chopped and used for energy and the carbon is returned back into the atmosphere. The trees that are cut down are replenished and as a result biomass is part of a closed carbon cycle with no net CO2 emissions that contribute to global warming (REA, 2013).
Biogas
Biogas is the gas that is formed by the natural breakdown of organic waste and matter - biomass - in the absence of oxygen. It makes use of wet wastes and manure and hence plays an important role in reducing greenhouse gases (GHGs). More specifically, in the EU, biogas will play an increasingly important role in the future as new EU policies concerning renewable energy systems (RES) have set the goal of supplying 20% of European energy demands with RES by 2020. A large part of this renewable energy will originate from European farming and forestry and in particular biogas as 'at least 25% of bioenergy in the future can originate from biogas' (Nielson et al., 2009).
Thus let's have a closer look at the process ...
The renewable technology with which biogas is formed is Anaerobic Digestion (AD). AD harnesses natural biological processes, using available biomass to produce renewable methane. This methane in turn can be used to produce electricity and heat or alternatively it can be upgraded for vehicle fuel and injection to the gas grid (Weiland, 2010).
The biomass that is used in biogas production has a wide range - farm manures, crops, stage sludge and catering/food wastes can all be used as feedstock. The only requisite is that the foods have a high moisture content as this makes them more suitable for the AD process (REA, 2013). Many AD technologies involve a process of treatment for the feedstock - this usually takes the form of maceration which reduces the particle size to around 12mm. The process ensures that all of the feedstock is fluid enough to be pumped through the process and also it increases the surface area of the material stimulating bacterial activity.
Some AD plants also include a screening and/or pasteurisation process to ensure all unwanted material are removed and all harmful pathogens such as E.Coli are killed. When animal by products are being used pasteurisation is often however a requirement. Treated organic material is then placed in sealed tanks to allow the break down of naturally occurring micro-organisms and the release of gas (Weiland, 2010). The gas formed by AD is around 60% methane and 40% carbon dioxide. The methane produced can be used to generate renewable electricity and heat. The left over organic material itself, the digestate, is rich in nutrients and as a result it can be used instead of chemical based fertilisers. Methane rich biogas, biomethane, can also replace natural gas as a feedstock for producing materials and chemicals.
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| An illustration of the process of anaerobic digestion (Al Seadi, 2002) |
- Renewable energy production
- Lower greenhouse gas emissions
- Pathogen reduction through sanitation
- Cheap and environmentally healthy organic waste recycling
- Improved fertilization efficiency (Holm-Nielsen et al., 1997)
- Economic advantages for farmers
- Less disruption from odours and flies (Birkmose, 2007)
The energy from biogas can thus be utilizated in various ways -
- Heat production
- Electricity production
- Combined power and heat production
- Biomethane
- Injected into the main gas or electricity grid after conversion
- Converted into transport fuels
A exemplary case study for the conversion of biogas into transport fuel is Sweden. Here the market for the upgrade of biogas to transport fuels has grown rapidly over the last decade. Around 15,000 vehicles now drive on upgraded biogas and this figure was expected to have reached 70,000 by 2012 (Persson et al., 2006)
As animal production units in developed countries are intensifying so is the amount of waste that is being generated by these livestock (Holm-Nielson et al., 2009). This presents a considerable threat to the environment due to the increased air and water pollution and over fertilisation of the land an excess of animal manure entails. Nutrient leaching, mainly nitrogen and phosphorous, ammonia evaporation and pathogen contamination are key threats (Holm-Nielson et al., 2009).
Animal production is responsible for 18% of overall GHG emissions of CO2 and 37% of anthropogenic methane generation which is has 23 times the potential of CO2 in causing global warming. It should also be noted that 65% of anthropogenic nitrous oxide and 64% of anthropogenic ammonia emission originates from the world-wide animal production sector (Steinfeld et al., 2006).
In light of this, the production of biogas presents a viable approach for addressing not only the manure management problems that will result from increased animal production but also for addressing climate change as GHG emissions are reduced and the amount of fossil fuels used can be reduced.
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