Bioenergy Production by Anaerobic Digestion

Using Agricultural Biomass and Organic Wastes

Nicholas Korres, Padraig O'Kiely, John A.H. Benzie, Jonathan S. West, Nicholas Korres, Padraig O'Kiely, John A.H. Benzie, Jonathan S. West

• Editore: Routledge
• Anno: 2013
• ISBN:
  9781136489631
• Edizione: 1
• Argomento: Technik & Maschinenbau
• Categoria: Nachhaltige Landwirtschaft

Interest in anaerobic digestion (AD), the process of energy production through the production of biogas, has increased rapidly in recent years. Agricultural and other organic waste are important substrates that can be treated by AD.

This book is one of the first to provide a broad introduction to anaerobic digestion and its potential to turn agricultural crops or crop residues, animal and other organic waste, into biomethane. The substrates used can include any non-woody materials, including grass and maize silage, seaweeds, municipal and industrial wastes. These are all systematically reviewed in terms of their suitability from a biological, technical and economic perspective. In the past the technical competence and high capital investment required for industrial-scale anaerobic digesters has limited their uptake, but the authors show that recent advances have made smaller-scale systems more viable through a greater understanding of optimising bacterial metabolism and productivity. Broader issues such as life cycle assessment and energy policies to promote AD are also discussed.

Part I

Legislation and energy policy

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Chapter 1

Sustainable agriculture and greenhouse gas emissions

Jonathan S. West

Bawden Plant Pathology Lab, Plant Biology and Crop Science Department Rothamsted Research, Harpenden AL5 2JQ, UK
Email: [email protected]

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Introduction

At a time when oil reserves are running out and carbon emissions from human activities are widely acknowledged to be causing environmental change, there has been an increasing emphasis on alternative ‘clean’ energy sources. Coupled with this is the challenge to increase food production to feed the world's population. Currently over 1 billion people do not have enough to eat (www.fao.org), but the population is predicted to increase by over 35 per cent in the next 40 years, from 7 billion now to over 9 billion by 2050 (Beddington, 2010; Anon., 2011). Already, a reduction in global food stocks has occurred in recent years due to increased demand and decreased yields as a result of environment change (principally insufficient rainfall) and changes to diet (increased meat consumption, associated with increasing affluence) (Anon., 2011). Additionally, the use of some potential food materials such as maize grain for bioenergy production has led to instability in food prices with food price spikes in 2007–8 associated with food export bans in some countries and even riots (Anon., 2012). Without new developments in science and technology, the problem can only worsen as the world's population increases, water sources continue to be overexploited (currently 70 per cent of water is used for agriculture, much extracted from rivers and aquifers) and particularly if a sub-set of food-crops such as oilseeds, maize or wheat grain is used for biofuel production, since there will then be less food available. This may not seem a problem where food is plentiful or for countries with enough wealth to import food, but the problem is passed on to other regions, usually in tropical climates. As a result, more land in tropical countries is being converted from forest to agriculture, often by burning areas of forest. This land use change causes a release of carbon from the burnt vegetation and also from carbon that was stored in the soil. This, together with the loss of productive forest area (which efficiently sequesters carbon) more than cancels out the benefits of biofuel production in the temperate areas. As such, when considered globally, certain biofuels are not ‘carbon neutral’ as claimed.

Fortunately, new scientific advances show great promise in delivering sustainable production of both food (through genetic improvement of crops such as wheat, rice and oilseed rape) and bioenergy. Of course renewable forms of energy are available from wind, wave or tidal action, hydroelectric or geothermal sources but these each produce electricity, rather than a liquid or gas that can be used in conventional engines for transport. Liquid bioethanol, produced from sugarcane (effectively a non-food crop) in Brazil is available for this purpose. However, other liquid fuels such as bio-diesel (produced from oilseed rape/canola) and ethanol (from cereals and particularly maize grain) have the disadvantage of using a potential food source, vegetable oil or carbohydrates, as their respective starting materials (Parry and Hawkesford, 2010; Parry and Jing, 2011). Therefore it is desirable to use non-food crops or waste materials from crops as a feedstock for production of biofuels such as biomethane and many new biofuel crop species are currently being investigated and genetic improvements are being made both for production of liquid and solid biofuels (Karp et al., 2011; Mariani et al., 2010). The EU 2003 biofuels directive targets an increase in biofuel transport energy from 5.75 per cent in 2010 to 10 per cent by 2020 (Anon., 2007). In addition to this, biomass derived liquid or gaseous fuels could substitute current transport fuels and natural gas used for domestic and industrial purposes. Conversion of lignin, cellulose and other carbohydrates in plant cell walls is a potential approach to produce biofuel from non-food and perennial crops or waste-products.

Agriculture and carbon emissions

In addition to producing fuels from renewable biological sources, it is also desirable to reduce the carbon footprint of all agricultural activities associated with food production. Agriculture currently contributes a significant proportion of global carbon emissions. Globally, greenhouse gas (GHG) emissions from agriculture are estimated to amount to 10–12 per cent of all emissions (Smith et al., 2007). For example, GHG emissions from the UK agricultural sector amounted to 7 per cent of the UK total in 2007 (43.3 Mt CO₂ eq out of 618.6 Mt CO₂eq) (National Atmospheric Emissions Inventory; www.naei.org.uk). This is similar to other parts of Western Europe and the UK is committed to reducing agricultural GHG emissions in England by 3 Mt CO₂eq by 2020 (UK Committee on Climate Change; www.theccc.org.uk/sectors/non-co2-gases/agriculture). Much of the agricultural GHG emissions in northwestern Europe are associated with animal production (particularly as methane) and new research on diets, breeds and species of animals is in progress to produce animal products with much lower GHG emissions (Smith et al., 2007). For arable crops, the largest contribution to GHG emissions is by the manufacture and use of fertilisers; for example over 79 per cent of emissions associated with the production of a typical hectare of winter oilseed rape is associated with the manufacture of nitrogen-containing fertiliser (1433 kg CO₂ eq/ha) and a further 1242 kg CO₂ eq/ha is associated with the breakdown of a proportion of the applied nitrogen-containing fertiliser into N₂O, which is a powerful greenhouse gas (Figure 1.1; Mahmuti et al. 2009).

In comparison, only 9.41 kg CO₂ eq/ha or 0.3 per cent of emissions were associated with the manufacture of the pesticides (herbicides, insecticides and fungicides) typically used. Yet fungicides alone were found to increase yields of winter oilseed rape by an average of 12.7 per cent having contributed to 0.04 per cent of GHG emissions in their use (Mahmuti et al. 2009). In the UK, fungicide treatment is estimated to have reduced GHG emissions by 1.64 Mt CO₂ for four major UK arable crops (winter barley, spring barley, winter wheat, and winter oilseed rape) in 2009 compared with releases calculated to have occurred by producing the same yield on the necessarily increased land area but without fungicide-based crop protection (Hughes et al. 2011). Globally, diseases are associated with losses of 16 per cent of crops and more generally losses to pests, weeds and diseases amount to 40 per cent of annual yields (Oerke, 2006). Climate change may itself alter the severity of crop disease epidemics (Evans et al. 2008; Madgwick et al. 2011). Recent studies by Berry et al. (2008), Mahmuti et al. (2009) and Hughes et al. (2011) illustrate that disease control measures

Figure 1.1 GHG emissions (CO₂ equivalents per hectare) associated with the production of a typical winter oilseed rape crop in the UK (data from Mahmuti et al., 2009). ‘Field ops.’ describes GHG emissions associated with mechanical equipment such as tractors and combine harvesters.

can not only reduce crop losses but also reduce the carbon footprint of crop production per tonne of grain produced and play a substantial part as a strategy to reduce agricultural GHG emissions by producing food efficiently on a smaller land area. A substantial reduction in GHG emissions is therefore possible by optimising and even increasing crop protection and by breeding crops that use nutrients more efficiently so that less nitrogen and other fertilisers need be applied. Good crop protection alongside effective application of nutrients and improved plant varieties has delivered substantial increases in yields over the last 60 years in particular (Figure 1.2).

Land use and carbon sequestration

An additional benefit to crop protection and GHG emissions has been realised recently by Berry et al. (2010) in research that has shown that growing arable crops efficiently using good crop protection products, elite cultivars and optimised fertiliser inputs not only increases yield per hectare and reduces the carbon footprint per tonne of grain produced but also means that less land area is required for this food production. This releases land for additional food production and/or for perennial biofuel crops, permanent grassland or woodland, which each sequester CO₂ into their soils to reach a steady state in which a larger amount of CO₂ is stored than in soils of arable crops. Less efficient crop production would require a larger land area to be cropped and Berry et al. (2010) show that this land use change (from pasture to arable crops) will lead to the release of CO₂ stored in converted grassland soils. In terms of GHG emissions associated not only with food production but also with land use, sustainable intensive arable crop production can therefore be considered as a climate-smart, environmentally conscious form of farming, using integrated pest management to reduce the carbon footprint of food production.

Figure 1.2 Effects of changes in farming practices on yields of winter wheat grown in the Broadbalk wheat experiment at Rothamsted Research, UK, since 1843. This dataset is part of the Long-Term Experiments National Capability at Rothamsted Research, funded by the UK Biotechnology and Biological Sciences Research Council and the Lawes Agricultural Trust. NPK = nutrients: Nitrogen, Phosphorus and Potassium, respectively.

Conclusions

To quote Sir John Beddington, Chief Scientific Advisor to the UK Government, “Food production must increase through climate-smart sustainable intensive arable crop production and this will need new scientific advancements, including use of some biotechnology approaches, improved crop varieties and species, and enhanced crop protection to produce more food with decreased associated GHG emissions”. Simultaneously, the policy of the EU and some national governments towards the choice of biofuels must place a strong emphasis on the use of grasslands and (non-food) waste products, rather than grains and oilseeds, since grasslands serve a dual purpose in carbon sequestration in soil and production of a clean form of energy — biomethane — without decreasing food production. Biomethane uses the principle of anaerobic digestion for its production and this is discussed in more detail in later chapters. Financial incentives must be made available to encourage the upta...