Growing bigger

Bioenergy from waste as a source of power Bioenergy is beginning to gain importance in the global fight to prevent climate change. Three high-profile announcements in 2006 greatly raised the public’s awareness of this potential source of energy: First, the UK Government’s Stern Review has highlighted the environmental and economic threats posed by global warming and the opportunities environmental technologies and biomass offer for tackling climate change. Second, Richard Branson and the Virgin Group have declared that they will ‘invest up to US$400 million dollars in renewable energy initiatives over the next three years’. Third, on 27 September 2006, Governor Schwarzenegger of California signed the Global Warming Solutions Act, declaring ‘we simply must do everything in our power to slow down global warming before it’s too late’. A landfill gas-to-energy project in Norfolk, UK. Landfill biogas is amongst the many forms of bioenergy source derived from waste materials. PHOTO: CLP. ENVIROGAS Click here to enlarge image In addition, earlier this year the European Commission announced it is considering strict plans to limit climate change to no more than 2°C above the temperature in pre-industrial times by committing autonomously to reduce its own emissions by at least 20% by 2020. With this context, it is appropriate to examine how bioenergy derived from solid waste is contributing to the development of renewable energy capacity across the globe. Bioenergy: the basics Bioenergy is energy from the sun stored in materials of biological origin. This includes plant matter and animal waste, known as biomass. Plants store solar energy through photosynthesis in cellulose and lignin, whereas animals store energy as fats. When burned, these sugars break down and release energy exothermically, releasing carbon dioxide, heat and steam. The by-products of this reaction can be captured and manipulated to create power, commonly called bioenergy. Biomass is considered renewable because the carbon is taken out of the atmosphere and replenished more quickly than the millions of years required for fossil fuels to form. The use of biofuels to replace fossil fuels contributes to a reduction in the overall release of carbon dioxide into the atmosphere and hence helps to tackle global warming. Municipal solid waste contains a significant biodegradable component. When in landfill, it breaks down, releasing landfill gas. If it is not captured and harnessed, it can be a potent source of greenhouse gas and a major contributor to climate change. This led the European Union to implement the Landfill Directive, which required the landfilling of untreated biodegradable waste to be reduced, and has subsequently banned it in a number of EU countries since 2005. A recent report, commissioned by the European Environment Agency, investigated how much biomass could be technically available for the production of bioenergy without increasing environmental pressure. It concluded that 19 million tonnes of oil equivalent (toe) is available from biomass by 2020,1 46% of which would be derived from biowaste (MSW, agricultural residues, farm wastes and other biodegradable waste streams). This has a huge impact on the waste management industry as a whole and goes hand in hand with meeting landfill diversion targets. Bioenergy from waste: the technologies The range of waste treatment technologies that are tailored to produce bioenergy is growing. There are a number of key areas of bioenergy from wastes including (but not limited to) biogas, biofuels and bioheat. When considering using bioenergy, it is important to take into account the overall emission of carbon in the process of electricity production. It does not make logical sense to put more carbon into the atmosphere from the creation of a biofuel than it offsets via its use as an energy source. As such, when evaluating the use of a bioenergy source, a full carbon and energy balance needs to be conducted for the specific scenario. Biogas Biogas typically comprises of 50%-75% methane and carbon dioxide along with other minor gases. It is the methane that is used for the generation of electricity or use as a fuel for transportation. Biogas is produced by anaerobic digestion where complex carbon molecules in organic material are broken down into simpler structures including CH4 and CO2. Biogas can be produced from a variety of biodegradable waste feedstocks including sewage sludge, biodegradable waste and mixed municipal waste or as a natural process of decomposition in landfills. Typically different variants of anaerobic digesters need to be used to treat each different feedstock optimally. The UK is the leading European country for crude biogas production. UK production was estimated at 1.473 million toe in 2004.2 The majority of this gas is valorized in the form of electricity. Landfill gas is a form of biogas and makes up a large proportion of the UK’s biogas production. The Landfill Directive as a driver and the green power support mechanisms, the Non-Fossil Fuels Obligation (NFFOs) and their successor Renewables Obligation Certificates (ROCs), were major spurs in developing the UK’s biogas capacity. Anaerobic digesters enable the controlled production of biogas. Digesters vary in performance depending upon the specific type used and their application to different waste streams. The wastewater industry has had a long history in the use of digestion systems for sewage sludge. The liquid nature of sewage particularly lends itself to anaerobic digesters; however, the fact that the biodegradable matter has already been digested once results in lower biogas yields. Biofuels Organic waste can be converted into several different types of fuel, as detailed below. Digestate as biofuel The ‘digestate’ residue from anaerobic digestion is most commonly used as a soil improver. The digestate consists largely of lignin and cellulose fibres that cannot be fully broken down by naturally occurring micro-organisms. The digestate can also be used as a biofuel when dried. Depending upon the processing method, digestate can have calorific values that are suitable for energy generation. There is some debate however, over whether the digestate is better used as a stabilized carbon sink, fixing a proportion of the waste carbon in the ground. Biodiesel from waste oil Biodiesel refers to the diesel equivalent of fossil fuel-derived diesel and originates from recent biological sources. Biodiesel can be created from a number of vegetable oils or animal fats. Specially grown crops producing straight vegetable oil (SVO) or waste vegetable oils (WVO) can be refined into biodiesel that can be used directly in standard diesel engines. The production process for biodiesel from waste oils is relatively simple and can be converted at a wide range of scales. The Biofuels Corporation biodiesel plant at Seal Sands is a major stepping stone to increasing the biodiesel infrastructure in the UK. The plant has an initial capacity of 284 million litres of biodiesel per year. It will mainly use SVOs such as palm, rape and canola oils but it can also accept tallow oil and yellow grease waste cooking oils. Bioethanol from sugar wastes Bioethanol is ethanol derived from biological origins. Ethanol can be used as a fuel for vehicles directly or blended with petrol to different standards. When used alone or mixed, bioethanol helps to reduce carbon emissions from vehicles. Bioethanol and its potential is nothing new. Brazil, following the 1973 oil crisis, began to drive a national programme for the use of bioethanol, produced from refining sugar cane, as a local vehicle fuel. This industry was driven by increasing taxes on petrol and providing subsidies for ethanol production. There has been a renewed global focus on bioethanol as a fuel source in recent years. The EU Biofuel Directive has set a target of 5.75% biofuel in all transportation fuels. And George W. Bush’s recent State of the Union address highlighted support for the growth a home-based bioethanol industry supplying the needs of the US. There is opportunity to produce bioethanol from waste products. The first bioethanol plant planned in the UK is with British Sugar and CEL International Ltd. British Sugar’s Wissington Factory in Norfolk is the site of this venture, which is due to start operations in 2008. British Sugar has reached agreement with the Farmers’ Union to supply low-grade ‘C’ sugar that will be used to produce 70 million litres of bioethanol per year. Bioheat Woody biomass is usually converted into power through combustion or gasification. Biomass can be specially grown in the case of energy crops such as Miscanthus, a tall perennial quick-growing grass that has good energy potential, or it can be reclaimed from waste wood products. Waste wood makes up a significant proportion of a variety of municipal, commercial and industrial waste streams. It is common practice to dispose of this waste wood in landfills where it slowly degrades and takes up valuable void space. This wood is a good source of energy and is an alternative to energy crops. The wood is typically chipped up into finer, transportable pieces that can be easily used as a fuel. Although most attention is paid to generating electricity from renewable energy sources, the UK uses far more heat energy than electricity, as reflected in the conclusions of the biomass task force.3 Biomass boiler technology is mature and is available from small-scale domestic boiler size to industrial facilities able to produce many megawatts of power, with efficiencies over 90%. The technology offers potential for CHP facilities that can supply both heat and power on a modular scale, further reducing the need for fossil fuel-derived energy. Tapping other sources of waste heat In addition to burning waste wood, it is possible to tap into the heat-generating potential of this resource in other ways. Bioenergy generation can be boosted further with Rankine cycle engines such as the Freepower organic Rankine cycle turbine generator; a test site is planned for a 120 kW engine in the Netherlands in the near future. These systems, which are complementary to existing infrastructure, use the waste heat from gas engines (such as those found on landfill sites), to boost bioelectricity production by up to 10%. The waste heat, which is ultimately derived from biological origins, is used to heat a closed-cycle electrical power generation system. Increasing the efficiency of gas conversion into electricity will maximize the potential energy yields from landfill gas. Conclusions Bioenergy is a growing source of power that is playing an ever-increasing role in the provision of electricity. The potential contribution of the waste industry to bioenergy is huge and has the ability to account for a source of large amount of total bioenergy production in the EU. Government subsidies and penalties for the disposal of biodegradable waste and the production of greenhouse gases are driving the use of bioenergy. Fossil fuels are unsustainable sources of energy and biofuels offer to contribute to a large part of initial efforts to change the world’s energy production mix. The range of technologies for the production of energy from biologically derived wastes is growing and covers gases, liquids and solid fuels with a wide variety of uses. As politicians are waking up to the threats of climate change and the problems with securing reliable sources of electricity, the role of the waste industry and bioenergy production will continue to grow. Alex Marshall is Senior Consultant at RPS Group, UK.e-mail: marshallal@rpsgroup.com Notes European Environment Agency (2006) How much bioenergy can Europe produce without harming the environment? EEA Report no. 7 http://reports.eea.europa.eu/eea_report_2006_7/en European Commission (2005) Bioenergy: Objectives-Technology. Bent, E. (2006) Fast Growth of Wood Heat, British Bioenergy News, Issue 4, p. 6-7. ArrowBio produces electricity from biogas In Israel and Australia hybrid anaerobic digestion systems have been developed, which combine the efficiency of wastewater digestion with the energy content of a mixed solid waste. The ArrowBio process is designed on treating over 70,000 tonnes per annum mixed MSW and produces biogas with 75% methane concentration. The first ArrowBio facility has been operating outside Tel Aviv at the Hiriya waste park since 2003. A second ArrowBio facility is being built at the Jack’s Gulley Landfill near Sydney, which will process 90,000 tonnes per annum MSW and generate over 2 MW of electricity from biogas. ArrowBio's two-stage anaerobic digesters. PHOTO: OAKTECH 2006. Click here to enlarge image ArrowBio’s two-stage anaerobic digesters. Photo: Oaktech 2006 MixAlco pilots production of mixed alcohols The MixAlco Process produces a range of mixed alcohols from biodegradable feedstocks. A pilot plant is in operation at College Station in Texas. The process is a system similar to the initial stages of anaerobic digestion. Anaerobic bacteria, harvested from the rumen of cattle, are used to encourage the production of carboxylic acids. The natural pathways of methane production are prevented by the addition of iodoform, a chemical inhibitor. The resulting feedstock goes on to yeast fermentation and yields a variety of useful precursors and then alcohols such as ethanol, propanol and butanol. The MixAlco pilot plant in College Station, Texas. PHOTO: TEXAS A&M UNIVERSITY 2006. Click here to enlarge image The MixAlco pilot plant in College Station, Texas. photo: texas a&m university 2006