A biogas plant is a decentralized energy system, which can lead to self-sufficiency in heat and power needs, and at the same time reduces environmental pollution. The key components of a modern biogas power (or anaerobic digestion) plant include: manure collection, anaerobic digester, effluent treatment, biogas storage, and biogas use/electricity generating equipment.
Working of a Biogas Plant
The fresh organic waste is stored in a collection tank before its processing to the homogenization tank which is equipped with a mixer to facilitate homogenization of the waste stream. The uniformly mixed waste is passed through a macerator to obtain uniform particle size of 5-10 mm and pumped into suitable-capacity anaerobic digester where stabilization of organic waste takes place.
In anaerobic digestion, organic material is converted to biogas by a series of bacteria groups into methane and carbon dioxide. The majority of commercially operating digesters are plug flow and complete-mix reactors operating at mesophilic temperatures. The type of digester used varies with the consistency and solids content of the feedstock, with capital investment factors and with the primary purpose of digestion.
Biogas contain significant amount of hydrogen sulfide (H2S) gas which needs to be stripped off due to its highly corrosive nature. The removal of H2S takes place in a biological desulphurization unit in which a limited quantity of air is added to biogas in the presence of specialized aerobic bacteria which oxidizes H2S into elemental sulfur.
Utilization of Biogas
Biogas is dried and vented into a CHP unit to a generator to produce electricity and heat. The size of the CHP system depends on the amount of biogas produced daily.
Treatment of Digestate
The digested substrate is passed through screw presses for dewatering and then subjected to solar drying and conditioning to give high-quality organic fertilizer. The press water is treated in an effluent treatment plant based on activated sludge process which consists of an aeration tank and a secondary clarifier. The treated wastewater is recycled to meet in-house plant requirements.
Monitoring of Environmental Parameters
A chemical laboratory is necessary to continuously monitor important environmental parameters such as BOD, COD, VFA, pH, ammonia, C:N ratio at different locations for efficient and proper functioning of the process.
The continuous monitoring of the biogas plant is achieved by using a remote control system such as Supervisory Control and Data Acquisition (SCADA) system. This remote system facilitates immediate feedback and adjustment, which can result in energy savings.
Waste-to-energy is the use of combustion and biological technologies to recover energy from urban wastes. There are three major waste to energy conversion routes – thermochemical, biochemical and physico-chemical. Thermochemical conversion, characterized by higher temperature and conversion rates, is best suited for lower moisture feedstock and is generally less selective for products. On the other hand, biochemical technologies are more suitable for wet wastes which are rich in organic matter.
Thermochemical Conversion of Waste
The three principal methods of thermochemical conversion of waste are combustion in excess air, gasification in reduced air, and pyrolysis in the absence of air. The most common technique for producing both heat and electrical energy from household wastes is direct combustion.
Combined heat and power (CHP) or cogeneration systems, ranging from small-scale technology to large grid-connected facilities, provide significantly higher efficiencies than systems that only generate electricity.
Combustion technology is the controlled combustion of waste with the recovery of heat to produce steam which in turn produces power through steam turbines. Pyrolysis and gasification represent refined thermal treatment methods as alternatives to incineration and are characterized by the transformation of the waste into product gas as energy carrier for later combustion in, for example, a boiler or a gas engine. Plasma gasification, which takes place at extremely high temperature, is also hogging limelight nowadays.
Biochemical Conversion of Waste
Biochemical processes, like anaerobic digestion, can also produce clean energy in the form of biogas which can be converted to power and heat using a gas engine. Anaerobic digestion is the natural biological process which stabilizes organic waste in the absence of air and transforms it into biofertilizer and biogas.
Anaerobic digestion is a reliable technology for the treatment of wet, organic waste. Organic waste from various sources is biochemically degraded in highly controlled, oxygen-free conditions circumstances resulting in the production of biogas which can be used to produce both electricity and heat.
In addition, a variety of fuels can be produced from waste resources including liquid fuels, such as ethanol, methanol, biodiesel, Fischer-Tropsch diesel, and gaseous fuels, such as hydrogen and methane. The resource base for biofuel production is composed of a wide variety of forestry and agricultural resources, industrial processing residues, and municipal solid and urban wood residues. Globally, biofuels are most commonly used to power vehicles, heat homes, and for cooking.
Physico-chemical Conversion of Waste
The physico-chemical conversion of waste involves various processes to improve physical and chemical properties of solid waste. The combustible fraction of the waste is converted into high-energy fuel pellets which may be used in steam generation.
The waste is first dried to bring down the high moisture levels. Sand, grit, and other incombustible matter are then mechanically separated before the waste is compacted and converted into fuel pellets or RDF.
Fuel pellets have several distinct advantages over coal and wood because it is cleaner, free from incombustibles, has lower ash and moisture contents, is of uniform size, cost-effective, and eco-friendly.
Combined heat and power systems in the biomass industry means the simultaneous generation of multiple forms of useful energy (usually mechanical and thermal) from biomass resources in a single, integrated system. In a conventional electricity generation systems, about 35% of the energy potential contained in the fuel is converted on average into electricity, whilst the rest is lost as waste heat. CHP systems use both electricity and heat and therefore can achieve an efficiency of up to 90%.
CHP technologies are well suited for sustainable development projects because they are socio-economically attractive and technologically mature and reliable. In developing countries, cogeneration can easily be integrated in many industries, especially agriculture and food processing, taking advantage of the biomass residues of the production process. This has the dual benefits of lowering fuel costs and solving waste disposal issues.
CHP systems consist of a number of individual components—prime mover (heat engine), generator, heat recovery, and electrical interconnection—configured into an integrated whole. Prime movers for CHP units include reciprocating engines, combustion or gas turbines, steam turbines, microturbines, and fuel cells.
A typical CHP system provides:
Distributed generation of electrical and/or mechanical power.
Waste-heat recovery for heating, cooling, or process applications.
Seamless system integration for a variety of technologies, thermal applications, and fuel types.
The success of any biomass-fuelled CHP plant is heavily dependent on the availability of a suitable biomass feedstock freely available in urban and rural areas.
Urban wood waste
Municipal solid wastes
Food processing residues
Reciprocating or internal combustion engines (ICEs) are among the most widely used prime movers to power small electricity generators. Advantages include large variations in the size range available, fast start-up, good efficiencies under partial load efficiency, reliability, and long life.
Steam turbines are the most commonly employed prime movers for large power outputs. Steam at lower pressure is extracted from the steam turbine and used directly or is converted to other forms of thermal energy. System efficiencies can vary between 15 and 35% depending on the steam parameters.
Co-firing of biomass with coal and other fossil fuels can provide a short-term, low-risk, low-cost option for producing renewable energy while simultaneously reducing the use of fossil fuels. Biomass can typically provide between 3 and 15 percent of the input energy into the power plant. Most forms of biomass are suitable for co-firing.
Steam engines are also proven technology but suited mainly for constant speed operation in industrial environments. Steam engines are available in different sizes ranging from a few kW to more than 1 MWe.
A gas turbine system requires landfill gas, biogas, or a biomass gasifier to produce the gas for the turbine. This biogas must be carefully filtered of particulate matter to avoid damaging the blades of the gas turbine.
Stirling engines utilize any source of heat provided that it is of sufficiently high temperature. A wide variety of heat sources can be used but the Stirling engine is particularly well-suited to biomass fuels. Stirling engines are available in the 0.5 to 150 kWe range and a number of companies are working on its further development.
A micro-turbine recovers part of the exhaust heat for preheating the combustion air and hence increases overall efficiency to around 20-30%. Several competing manufacturers are developing units in the 25-250kWe range. Advantages of micro-turbines include compact and light weight design, a fairly wide size range due to modularity, and low noise levels.
Fuel cells are electrochemical devices in which hydrogen-rich fuel produces heat and power. Hydrogen can be produced from a wide range of renewable and non-renewable sources. A future high temperature fuel cell burning biomass might be able to achieve greater than 50% efficiency.
Food residuals are an untapped renewable energy source that mostly ends up rotting in landfills, thereby releasing greenhouse gases into the atmosphere. Food residuals are difficult to treat or recycle since it contains high levels of sodium salt and moisture, and is mixed with other waste during collection. Major generators of food wastes include hotels, restaurants, supermarkets, residential blocks, cafeterias, airline caterers, food processing industries, etc.
According to EPA, about 63.1 million tons of food waste was thrown away into landfills or incinerators the United States in 2018. As far as United Kingdom is concerned, households threw away 6.6 million tons of food each year. These statistics are an indication of tremendous amount of food waste generated all over the world.
The proportion of food residuals in municipal waste stream is gradually increasing and hence a proper food waste management strategy needs to be devised to ensure its eco-friendly and sustainable disposal. Currently, only about 3 percent of food waste is recycled throughout U.S., mainly through composting. Composting provides an alternative to landfill disposal of food waste, however it requires large areas of land, produces volatile organic compounds and consumes energy. Consequently, there is an urgent need to explore better recycling alternatives.
Anaerobic digestion has been successfully used in several European and Asian countries to stabilize food wastes, and to provide beneficial end-products. Sweden, Austria, Denmark, Germany and England have led the way in developing new advanced biogas technologies and setting up new projects for conversion of food waste into energy.
Anaerobic Digestion of Food Waste
Anaerobic digestion is the most important method for the treatment of organic waste, such as food residuals, because of its techno-economic viability and environmental sustainability. Anaerobic digestion generates renewable energy from food waste in the form of biogas and preserves the nutrients which are recycled back to the agricultural land in the form of slurry or solid fertilizer.
The relevance of biogas technology lies in the fact that it makes the best possible use of various organic wastes as a renewable source of clean energy. A biogas plant is a decentralized energy system, which can lead to self-sufficiency in heat and power needs, and at the same time reduces environmental pollution. Thus, anaerobic digestion of food waste can lead to climate change mitigation, economic benefits and landfill diversion opportunities.
Of the different types of organic wastes available, food waste holds the highest potential in terms of economic exploitation as it contains high amount of carbon and can be efficiently converted into biogas and organic fertilizer. Food waste can either be used as a single substrate in a biogas plant, or can be co-digested with organic wastes like cow manure, poultry litter, sewage, crop residues, slaughterhouse wastes, etc.
Renewable Energy from Food Residuals
The feedstock for the food waste-to-energy plant includes leftover food, vegetable refuse, stale cooked and uncooked food, meat, teabags, napkins, extracted tea powder, milk products, etc. Raw waste is shredded to reduce to its particle size to less than 12 mm. The primary aim of shredding is to produce a uniform feed and reduce plant “down-time” due to pipe blockages by large food particles. It also improves mechanical action and digestibility and enables easy removal of any plastic bags or cling-film from waste.
Fresh waste and re-circulated digestate (or digested food waste) are mixed in a mixing tank. The digestate is added to adjust the solids content of the incoming waste stream from 20 to 25 percent (in the incoming waste) to the desired solids content of the waste stream entering the digestion system (10 to 12 percent total solids). The homogenized waste stream is pumped into the feeding tank, from which the anaerobic digestion system is continuously fed. Feeding tank also acts as a pre-digester and subjected to heat at 55º to 60º C to eliminate pathogens and to facilitate the growth of thermophilic microbes for faster degradation of waste.
From the predigestor tank, the slurry enters the main digester where it undergoes anaerobic degradation by a consortium of Archaebacteria belonging to Methanococcus group. The anaerobic digester is a CSTR reactor having average retention time of 15 to 20 days. The digester is operated in the mesophilic temperature range (33º to 38°C), with heating carried out within the digester. Food waste is highly biodegradable and has much higher volatile solids destruction rate (86 to 90 percent) than biosolids or livestock manure. As per conservative estimates, each ton of food waste produces 150 to 200 m3 of biogas, depending on reactor design, process conditions, waste composition, etc.
Biogas contains significant amount of hydrogen sulfide (H2S) gas that needs to be stripped off due to its corrosive nature. The removal of H2S takes place in a biological desulphurization unit in which a limited quantity of air is added to biogas in the presence of specialized aerobic bacteria that oxidizes H2S into elemental sulfur. The biogas produced as a result of anaerobic digestion of waste is sent to a gas holder for temporary storage. Biogas is eventually used in a combined heat and power (CHP) unit for its conversion into thermal and electrical energy in a cogeneration power station of suitable capacity. The exhaust gases from the CHP unit are used for meeting process heat requirements.
The digested substrate leaving the reactor is rich in nutrients like nitrogen, potassium and phosphorus which are beneficial for plants as well as soil. The digested slurry is dewatered in a series of screw presses to remove the moisture from slurry. Solar drying and additives are used to enhance the market value and handling characteristics of the fertilizer.
Diverting Food from Landfills
Food residuals are one of the single largest constituents of municipal solid waste stream. Diversion of food waste from landfills can provide significant contribution towards climate change mitigation, apart from generating revenues and creating employment opportunities. Rising energy prices and increasing environmental pollution makes it more important to harness renewable energy from food scraps and create a sustainable food supply chain.
Anaerobic digestion technology is widely available worldwide and successful projects are already in place in several European as well as Asian countries that makes it imperative on waste generators and environmental agencies to root for a sustainable food waste management system.
Biomass is a versatile energy source that can be used for production of heat, power, transport fuels and biomaterials, apart from making a significant contribution to climate change mitigation. Currently, biomass-driven combined heat and power, co-firing, and combustion plants provide reliable, efficient, and clean power and heat.
Feedstock for biomass energy plants can include residues from agriculture, forestry, wood processing, and food processing industries, municipal solid wastes, industrial wastes and biomass produced from degraded and marginal lands.
The terms biomass energy, bioenergy and biofuels cover any energy products derived from plant or animal or organic material. The increasing interest in biomass energy and biofuels has been the result of the following associated benefits:
Potential to reduce GHG emissions.
Energy security benefits.
Substitution for diminishing global oil supplies.
Potential impacts on waste management strategy.
Capacity to convert a wide variety of wastes into clean energy.
Technological advancement in thermal and biochemical processes for waste-to-energy transformation.
Biomass can play the pivotal role in production of carbon-neutral fuels of high quality as well as providing feedstock for various industries. This is a unique property of biomass compared to other renewable energies and which makes biomass a prime alternative to the use of fossil fuels. Performance of biomass-based systems for heat and power generation has been already proved in many situations on commercial as well as domestic scales.
Biomass energy systems have the potential to address many environmental issues, especially global warming and greenhouse gases emissions, and foster sustainable development among poor communities. Biomass fuel sources are readily available in rural and urban areas of all countries. Biomass-based industries can provide appreciable employment opportunities and promote biomass re-growth through sustainable land management practices.
The negative aspects of traditional biomass utilization in developing countries can be mitigated by promotion of modern biomass-to-energy technologies which provide solid, liquid and gaseous fuels as well as electricity as shown. Biomass wastes can be transformed into clean and efficient energy by biochemical as well as thermochemical technologies.
The most common technique for producing both heat and electrical energy from biomass wastes is direct combustion. Thermal efficiencies as high as 80 – 90% can be achieved by advanced gasification technology with greatly reduced atmospheric emissions. Combined heat and power (CHP) systems, ranging from small-scale technology to large grid-connected facilities, provide significantly higher efficiencies than systems that only generate electricity.
Biochemical processes, like anaerobic digestion and sanitary landfills, can also produce clean energy in the form of biogas and producer gas which can be converted to power and heat using a gas engine.
In addition, biomass wastes can also yield liquid fuels, such as cellulosic ethanol, which can be used to replace petroleum-based fuels. Cellulosic ethanol can be produced from grasses, wood chips and agricultural residues by biochemical route using heat, pressure, chemicals and enzymes to unlock the sugars in lignocellulosic biomass. Algal biomass is also emerging as a good source of energy because it can serve as natural source of oil, which conventional refineries can transform into jet fuel or diesel fuel.
The valuable component of biogas is methane (CH4) which typically makes up 60%, with the balance being carbon dioxide (CO2) and small percentages of other gases. The proportion of methane depends on the feedstock and the efficiency of the process, with the range for methane content being 40% to 70%.
Biogas is saturated and contains H2S, and the simplest use is in a boiler to produce hot water or steam. The gas can also be upgraded and used in gas supply networks. The use of biogas in solid oxide fuel cells is also being researched.
Biogas can be combusted directly to produce heat. In this case, there is no need to scrub the hydrogen sulphide in the biogas. Usually the process utilize dual-fuel burner and the conversion efficiency is 80 to 90%. The main components of the system are anaerobic digester, biogas holder, pressure switch, booster fan, solenoid valve, dual fuel burner and combustion air blower.
The most common method for utilization of biogas in developing countries is for cooking and lighting. Conventional gas burners and gas lamps can easily be adjusted to biogas by changing the air to gas ratio. In more industrialized countries boilers are present only in a small number of plants where biogas is used as fuel only without additional CHP. In a number of industrial applications biogas is used for steam production.
Burning biogas in a boiler is an established and reliable technology. Low demands are set on the biogas quality for this application. Pressure usually has to be around 8 to 25 mbar. Furthermore it is recommended to reduce the level of hydrogen sulphide to below 1 000 ppm, this allows to maintain the dew point around 150 °C.
Biogas is the ideal fuel for generation of electric power or combined heat and power. A number of different technologies are available and applied.
The most common technology for power generation is internal combustion. Engines are available in sizes from a few kilowatts up to several megawatts. Gas engines can either be SI-engines (spark ignition) or dual fuel engines. Dual fuel engines with injection of diesel (10% and up) or sometimes plant oil are very popular in smaller scales because they have good electric efficiencies up to guaranteed 43%.
The biogas pressure is turbo-charged and after-cooled and has a high compression ratio in the gas engines. The cooling tower provides cooling water for the gas engines. The main component of the system required for utilizing the technology are anaerobic digester, moisture remover, flame arrester, waste gas burner, scrubber, compressor, storage, receiver, regulator, pressure switch and switch board.
Gas turbines are an established technology in sizes above 500 kW. In recent years also small scale engines, so called micro-turbines in the range of 25 to 100kW have been successfully introduced in biogas applications. They have efficiencies comparable to small SI-engines with low emissions and allow recovery of low pressure steam which is interesting for industrial applications. Micro turbines are small, high-speed, integrated power plants that include a turbine, compressor, generator and power electronics to produce power.
The benefit of the anaerobic treatment will depend on the improvement of the process regarding a higher biogas yield per m3 of biomass and an increase in the degree of degradation. Furthermore, the benefit of the process can be multiplied by the conversion of the effluent from the process into a valuable product.
In order to improve the economical benefit of biogas production, the future trend will go to integrated concepts of different conversion processes, where biogas production will still be a significant part. In a so-called biorefinery concept, close to 100% of the biomass is converted into energy or valuable by-products, making the whole concept more economically profitable and increasing the value in terms of sustainability.
One example of such biorefinery concept is the Danish Bioethanol Concept that combines the production of bioethanol from lignocellulosic biomass with biogas production of the residue stream. Another example is the combination of biogas production from manure with manure separation into a liquid and a solid fraction for separation of nutrients.
One of the most promising concepts is the treatment of the liquid fraction on the farm-site in a UASB reactor while the solid fraction is transported to the centralized biogas plant where wet-oxidation can be implemented to increase the biogas yield of the fiber fraction. Integration of the wet oxidation pre-treatment of the solid fraction leads to a high degradation efficiency of the lignocellulosic solid fraction.
The conventional leather tanning technology is highly polluting as it produces large amounts of organic and chemical pollutants. Wastes generated by tanneries pose a major challenge to the environment. Anaerobic digestion of tannery wastes is an attractive method to recover energy from tannery wastes.
According to conservative estimates, more than 600,000 tons per year of solid waste are generated worldwide by leather industry and approximately 40–50% of the hides are lost to shavings and trimmings. Everyday a huge quantity of solid waste, including trimmings of finished leather, shaving dusts, hair, fleshing, trimming of raw hides and skins, are being produced from the industries. Chromium, sulphur, oils and noxious gas (methane, ammonia, and hydrogen sulphide) are the elements of liquid, gas and solid waste of tannery industries.
Biogas from Tannery Wastes
Anaerobic digestion (or biomethanation) systems are mature and proven processes that have the potential to convert tannery wastes into energy efficiently, and achieve the goals of pollution prevention/reduction, elimination of uncontrolled methane emissions and odour, recovery of biomass energy potential as biogas, production of stabilized residue for use as low grade fertilizer.
Anaerobic digestion of tannery wastes is an attractive method to recover energy from tannery wastes. This method degrades a substantial part of the organic matter contained in the sludge and tannery solid wastes, generating valuable biogas, contributing to alleviate the environmental problem, giving time to set-up more sustainable treatment and disposal routes. Digested solid waste is biologically stabilized and can be reused in agriculture.
Until now, biogas generation from tannery wastewater was considered that the complexity of the waste water stream originating from tanneries in combination with the presence of chroming would result in the poisoning of the process in a high loaded anaerobic reactor.
When the locally available industrial wastewater treatment plant is not provided by anaerobic digester, a large scale digestion can be planned in regions accommodating a big cluster of tanneries, if there is enough waste to make the facility economically attractive.
In this circumstance, an anaerobic co-digestion plant based on sludge and tanneries may be a recommendable option, which reduces the quantity of landfilled waste and recovers its energy potential. It can also incorporate any other domestic, industrial or agricultural wastes. Chrome-free digested tannery sludge also has a definite value as a fertilizer based on its nutrient content.
Potential Applications of Biogas
Biogas produced in anaerobic digesters consists of methane (50%–80%), carbon dioxide (20%–50%), and trace levels of other gases such as hydrogen, carbon monoxide, nitrogen, oxygen, and hydrogen sulfide. Biogas can be used for producing electricity and heat, as a natural gas substitute and also a transportation fuel. A combined heat and power plant (CHP) not only generates power but also produces heat for in-house requirements to maintain desired temperature level in the digester during cold season.
CHP systems cover a range of technologies but indicative energy outputs per m3 of biogas are approximately 1.7 kWh electricity and 2.5kWh heat. The combined production of electricity and heat is highly desirable because it displaces non-renewable energy demand elsewhere and therefore reduces the amount of carbon dioxide released into the atmosphere.
AD Plant at ECCO’s Tannery (Netherlands)
A highly advanced wastewater treatment plant and biogas system became fully operational in 2012 at ECCO’s tannery in the Netherlands. A large percentage of the waste is piped directly into the wastewater plant to be converted into biogas. This biogas digester provides a source of renewable fuel and also helps to dispose of tannery waste materials by converting waste from both the leather-making processes, and the wastewater treatment plant, into biogas. All excess organic material from the hides is also converted into biogas.
This project enables ECCO Tannery to reduce waste and to substitute virtually all of its consumption of non-renewable natural gas with renewable biogas. The aim is to use more than 40% of the total tannery waste and replace up to 60% of the total natural gas consumption with biogas.
Food waste in the United Kingdom is a matter of serious environmental, economic and social concern that has been attracting widespread attention in recent years. According to ‘Feeding the 5K’ organisation, 13,000 slices of crusts are thrown away every day by a single sandwich factory. More recently, Tesco, one of the largest UK food retailers, has published its sustainability report admitting that the company generated 28,500 tonnes of food waste in the first six months of 2013. TESCO’s report also state that 47% of the bakery produced is wasted. In terms of GHG emissions, DEFRA estimated that food waste is associated with 20 Mt of CO2 equivalent/year, which is equivalent to 3% of the total annual GHG emissions.
Globally, 1.2 to 2 billion tonnes (30%-50%) of food produced is thrown away before it reaches a human stomach. Food waste, if conceived as a state, is responsible for 3.3 Bt-CO2 equivalent/year, which would make it the third biggest carbon emitter after China and USA.
What makes food waste an even more significant issue is the substantially high demand for food which is estimated to grow 70% by 2050 due to the dramatic increase of population which is expected to reach 9.5 billion by 2075. Therefore, there is an urgent need to address food waste as a globally challenging issue which should be considered and tackled by sustainable initiatives.
A War on Food Waste
The overarching consensus to tackle the food waste issue has led to the implementation of various policies. For instance, the European Landfill Directive (1999/31/EC) set targets to reduce organic waste disposed to landfill in 2020 to 35% of that disposed in 1995 (EC 1999).
More recently, the European Parliament discussed a proposal to “apply radical measures” to halve food waste by 2025 and to designate the 2014 year as “the European Year Against Food Waste”. In the light of IMechE’s report (2013), the United Nations Environment Programme (UNEP) in cooperation with FAO has launched the Save Food Initiative in an attempt to reduce food waste generated in the global scale.
In the UK, WRAP declared a war on food waste by expanding its organic waste programme in 2008 which was primarily designed to “establish the most cost-effective and environmentally sustainable ways of diverting household food waste from landfill that leads to the production of a saleable product”. DEFRA has also identified food waste as a “priority waste stream” in order to achieve better waste management performance.
In addition to governmental policies, various voluntary schemes have been introduced by local authorities such as Nottingham Declaration which aims to cut local CO2 emissions 60% by 2050.
Sustainable Food Waste Management
Engineering has introduced numerous technologies to deal with food waste. Many studies have been carried out to examine the environmental and socio-economic impacts of food waste management options. This article covers the two most preferable options; anaerobic digestion and composting.
In-vessel composting (IVC) is a well-established technology which is widely used to treat food waste aerobically and convert it into a valuable fertilizer. IVC is considered a sustainable option because it helps by reducing the amount of food waste landfilled. Hence, complying with the EU regulations, and producing a saleable product avoiding the use of natural resources.
IVC is considered an environmentally favourable technology compared with other conventional options (i.e. landfill and incineration). It contributes less than 0.06% to the national greenhouse gas inventories. However, considering its high energy-intensive collection activities, the overall environmental performance is “relatively poor”.
Anaerobic Digestion (AD) is a leading technology which has had a rapidly growing market over the last few years. AD is a biologically natural process in which micro-organisms anaerobically break down food waste and producing biogas which can be used for both Combined Heat & Power (CHP) and digestate that can be used as soil fertilizers or conditioners. AD has been considered as the “best option” for food waste treatment. Therefore, governmental and financial support has been given to expand AD in the UK.
AD is not only a food waste treatment technology, but also a renewable source of energy. For instance, It is expected that AD would help the UK to meet the target of supplying 15% of its energy from renewable sources by 2020. Furthermore, AD technology has the potential to boost the UK economy by providing 35,000 new jobs if the technology is adopted nationally to process food waste. This economic growth will significantly improve the quality of life among potential beneficiaries and thus all sustainability elements are considered.
Biomass fuels are typically used most efficiently and beneficially when generating both power and heat through biomass cogeneration systems (also known as combined heat and power or CHP system). Biomass conversion technologies transform a variety of wastes into heat, electricity and biofuels by employing a host of strategies. Conversion routes are generally thermochemical or biochemical, but may also include chemical and physical.
The simplest way is to burn the biomass in a furnace, exploiting the heat generated to produce steam in a boiler, which is then used to drive a steam turbine. Advanced biomass conversion technologies include biomass integrated gasification combined cycle (BIGCC) systems, cofiring (with coal or gas), pyrolysis and second generation biofuels.
Biomass Cogeneration Systems
A typical biomass cogeneration (or biomass cogen) system provides:
Distributed generation of electrical and/or mechanical power.
Waste-heat recovery for heating, cooling, or process applications.
Seamless system integration for a variety of technologies, thermal applications, and fuel types into existing building infrastructure.
Biomass cogeneration systems consist of a number of individual components—prime mover (heat engine), generator, heat recovery, and electrical interconnection—configured into an integrated whole. The type of equipment that drives the overall system (i.e., the prime mover) typically identifies the CHP unit.
Prime movers for biomass cogeneration units include reciprocating engines, combustion or gas turbines, steam turbines, microturbines, and fuel cells. These prime movers are capable of burning a variety of fuels, including natural gas, coal, oil, and alternative fuels to produce shaft power or mechanical energy.
A biomass-fueled cogeneration facility is an integrated power system comprised of three major components:
Biomass receiving and feedstock preparation.
Energy conversion – Conversion of the biomass into steam for direct combustion systems or into biogas for the gasification systems.
Power and heat production – Conversion of the steam or syngas or biogas into electric power and process steam or hot water
Feedstock for Biomass Cogeneration Plants
The lowest cost forms of biomass for cogeneration plants are residues. Residues are the organic byproducts of food, fiber, and forest production, such as sawdust, rice husks, wheat straw, corn stalks, and sugarcane bagasse. Forest residues and wood wastes represent a large potential resource for energy production and include forest residues, forest thinnings, and primary mill residues.
Energy crops are perennial grasses and trees grown through traditional agricultural practices that are produced primarily to be used as feedstocks for energy generation, e.g. hybrid poplars, hybrid willows, and switchgrass. Animal manure can be digested anaerobically to produce biogas in large agricultural farms and dairies.
To turn a biomass resource into productive heat and/or electricity requires a number of steps and considerations, most notably evaluating the availability of suitable biomass resources; determining the economics of collection, storage, and transportation; and evaluating available technology options for converting biomass into useful heat or electricity.
Biogas can be used in raw (without removal of CO2) or in upgraded form. The main function of upgrading biogas is the removal of CO2 (to increase the energy content) and H2S (to reduce risk of corrosion). After upgrading, biogas becomes biomethane and possesses identical gas quality properties as natural gas, and can thus be used as natural gas replacement. The main pathways for biomethane utilization are as follows:
Compressed natural gas (CNG) & diesel replacement – (bio-CNG for transport fuel usage)
Liquid natural gas (LNG) replacement – (bio-LNG for transport fuel usage)
Prior to practically all utilization options, the biogas has to be dried (usually through application of a cooling/condensation step). Furthermore, elements such as hydrogen sulphide and other harmful trace elements must be removed (usually trough application of an activated carbon filter) to prevent adverse effects on downstream processing equipment (such as compressors, piping, boilers and CHP systems).
Although biogas is perfectly suitable to be utilized in boilers (as an environmental friendlier source for heat and steam production), this option is rather obsolete due to the abundance of alternative sources from solid waste origin.
Most Palm Oil Mills are already self-reliant with respect to heat and steam production due to the combustion of their solid waste streams (such as EFB and PKS). Consequently, conversion to electricity (by means of a CHP unit) or utilization as natural gas, CNG or LNG replacement, would be a more sensible solution.
The biogas masterplan as drafted by the Asia Pacific Biogas Alliance foresees a distribution in which 30% of the biomethane is used for power generation, 40% for grid injection and 30% as compressed/liquefied fuel for transportation purpose (Asian Pacific Biogas Alliance, 2015).
For each project, the most optimal option has to be evaluated on a case to case basis. Main decision-making factors will be local energy prices and requirements, available infrastructure (for gas and electricity), incentives and funding.
For the locations where local demand is exceeded, and no electricity or gas infrastructure is available within a reasonable distance (<5-10 km, due to investment cost and power loss), production of CNG could offer a good solution.
Moreover, during the utilization of biogas within a CHP unit only 40-50% of the energetic content of the gas is converted into electricity. The rest of the energy is transformed into heat. For those locations where an abundance of heat is available, such as Palm Oil Mills, this effectively means that 50-60% of the energetic content of the biogas is not utilized. Converting the biogas into biomethane (of gas grid or CNG quality) through upgrading, would facilitate the transportation and commercialisation of over 95% of the energetic content of the biogas.
Within the CNG utilization route, the raw biogas will be upgraded to a methane content of >96%, compressed to 250 bar and stored in racks with gas bottles. The buffered gas (bottles) will be suitable for transportation by truck or ship. For transportation over large distances (>200km), it will be advised to further reduce the gas volume by converting the gas to LNG (trough liquefaction).
Enhanced self-sufficiency trough availability of on-site diesel replacement (CNG)
Expansion of economic activities/generation of additional revenues
Sales of surplus electricity (local or to the grid)
Sales of biomethane (injection into the natural gas grid)
Replacement of on-site diesel usage by CNG
Sales of bottled CNG
Reducing global and local environmental impact (through fuel replacement)
Reducing dependence on fossil fuel, and enhances fuel diversity and security of energy supply
Enhancement of local infrastructure and employment
Through electrical and gas supply
Through Fuel (CNG) supply
Co-Authors: H. Dekker and E.H.M. Dirkse (DMT Environmental Technology)
Note:This is the second article in the special series on ‘Sustainable Utilization of POME-based Biomethane’ by Langerak et al of DMT Environmental Technology (Holland). The first article can be viewed at this link
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