Pyrolysis of Municipal Wastes

Pyrolysis is rapidly developing biomass thermal conversion technology and has been garnering much attention worldwide due to its high efficiency and good eco-friendly performance characteristics. Pyrolysis technology provides an opportunity for the conversion of municipal solid wastes, agricultural residues, scrap tires, non-recyclable plastics etc into clean energy. It offers an attractive way of converting urban wastes into products which can be effectively used for the production of heat, electricity and chemicals.

Pyrolysis-MSW

Pyrolysis of Municipal Wastes

Pyrolysis process consists of both simultaneous and successive reactions when carbon-rich organic material is heated in a non-reactive atmosphere. Simply speaking, pyrolysis is the thermal degradation of organic materials in the absence of oxygen. Thermal decomposition of organic components in the waste stream starts at 350°C–550°C and goes up to 700°C–800°C in the absence of air/oxygen.

Pyrolysis of municipal wastes begins with mechanical preparation and separation of glass, metals and inert materials prior to processing the remaining waste in a pyrolysis reactor. The commonly used pyrolysis reactors are rotary kilns, rotary hearth furnaces, and fluidized bed furnaces. The process requires an external heat source to maintain the high temperature required.

Pyrolysis can be performed at relatively small-scale which may help in reducing transport and handling costs.  In pyrolysis of MSW, heat transfer is a critical area as the process is endothermic and sufficient heat transfer surface has to be provided to meet process heat requirements.

The main products obtained from pyrolysis of municipal wastes are a high calorific value gas (synthesis gas or syngas), a biofuel (bio oil or pyrolysis oil) and a solid residue (char). Depending on the final temperature, MSW pyrolysis will yield mainly solid residues at low temperatures, less than 4500C, when the heating rate is quite slow, and mainly gases at high temperatures, greater than 8000C, with rapid heating rates. At an intermediate temperature and under relatively high heating rates, the main product is a liquid fuel popularly known as bio oil.

Wide Range of Products

Bio oil is a dark brown liquid and can be upgraded to either engine fuel or through gasification processes to a syngas and then biodiesel. Pyrolysis oil may also be used as liquid fuel for diesel engines and gas turbines to generate electricity.

Bio oil is particularly attractive for co-firing because it can be relatively easy to handle and burn than solid fuel and is cheaper to transport and store. In addition, bio oil is also a vital source for a wide range of organic compounds and specialty chemicals.

Syngas is a mixture of energy-rich gases (combustible constituents include carbon monoxide, hydrogen, methane and a broad range of other VOCs). The net calorific value (NCV) of syngas is between 10 and 20MJ/Nm3. Syngas is cleaned to remove particulates, hydrocarbons, and soluble matter, and then combusted to generate electricity.

Diesel engines, gas turbines, steam turbines and boilers can be used directly to generate electricity and heat in CHP systems using syngas and pyrolysis oil. Syngas may also be used as a basic chemical in petrochemical and refining industries.

The solid residue from MSW pyrolysis, called char, is a combination of non-combustible materials and carbon. Char is almost pure carbon and can be used in the manufacture of activated carbon filtration media (for water treatment applications) or as an agricultural soil amendment.

Cogeneration of Bagasse

Cogeneration of bagasse is one of the most attractive and successful biomass energy projects that have already been demonstrated in many sugarcane producing countries such as Mauritius, Reunion Island, India and Brazil. Combined heat and power from sugarcane in the form of power generation offers renewable energy options that promote sustainable development, take advantage of domestic resources, increase profitability and competitiveness in the industry, and cost-effectively address climate mitigation and other environmental goals.

bagasse_cogeneration

According to World Alliance for Decentralized Energy (WADE) report on Bagasse Cogeneration, bagasse-based cogeneration could deliver up to 25% of current power demand requirements in the world’s main cane producing countries. The overall potential share in the world’s major developing country producers exceeds 7%.

There is abundant opportunity for the wider use of bagasse-based cogeneration in sugarcane-producing countries. It is especially great in the world’s main cane producing countries like Brazil, India, Thailand, Pakistan, Mexico, Cuba, Colombia, Philippines and Vietnam. Yet this potential remains by and large unexploited.

Using bagasse to generate power represents an opportunity to generate significant revenue through the sale of electricity and carbon credits. Additionally, cogeneration of heat and power allows sugar producers to meet their internal energy requirements and drastically reduce their operational costs, in many cases by as much as 25%. Burning bagasse also removes a waste product through its use as a feedstock for the electrical generators and steam turbines.

Most sugarcane mills around the globe have achieved energy self-sufficiency for the manufacture of raw sugar and can also generate a small amount of exportable electricity. However, using traditional equipment such as low-pressure boilers and counter-pressure turbo alternators, the level and reliability of electricity production is not sufficient to change the energy balance and attract interest for export to the electric power grid.

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On the other hand, revamping the boiler house of sugar mills with high pressure boilers and condensing extraction steam turbine can substantially increase the level of exportable electricity. This experience has been witnessed in Mauritius, where, following major changes in the processing configurations, the exportable electricity from its sugar factory increased from around 30-40 kWh to around 100–140 kWh per ton cane crushed.

In Brazil, the world’s largest cane producer, most of the sugar mills are upgrading their boiler configurations to 42 bars or even higher pressure of up to 67 bars.

Technology Options

The prime technology for sugar mill cogeneration is the conventional steam-Rankine cycle design for conversion of fuel into electricity. A combination of stored and fresh bagasse is usually fed to a specially designed furnace to generate steam in a boiler at typical pressures and temperatures of usually more than 40 bars and 440°C respectively.

The high pressure steam is then expanded either in a back pressure or single extraction back pressure or single extraction condensing or double extraction cum condensing type turbo generator operating at similar inlet steam conditions.

35MW-bagasse-coal-chp-plant-mauritius

35MW Bagasse and Coal CHP Plant in Mauritius

 

Due to high pressure and temperature, as well as extraction and condensing modes of the turbine, higher quantum of power gets generated in the turbine–generator set, over and above the power required for sugar process, other by-products, and cogeneration plant auxiliaries. The excess power generated in the turbine generator set is then stepped up to extra high voltage of 66/110/220 kV, depending on the nearby substation configuration and fed into the nearby utility grid.

As the sugar industry operates seasonally, the boilers are normally designed for multi-fuel operations, so as to utilize mill bagasse, sugarcane trash, crop residues, coal and other fossil fuel, so as to ensure year round operation of the power plant for export to the grid.

Latest Trends

Modern power plants use higher pressures, up to 87 bars or more. The higher pressure normally generates more power with the same quantity of Bagasse or biomass fuel. Thus, a higher pressure and temperature configuration is a key in increasing exportable surplus electricity.

In general, 67 bars pressure and 495°C temperature configurations for sugar mill cogeneration plants are well-established in many sugar mills in India. Extra high pressure at 87 bars and 510°C, configuration comparable to those in Mauritius, is the current trend and there are about several projects commissioned and operating in India and Brazil. The average increase of power export from 40 bars to 60 bars to 80 bars stages is usually in the range of 7-10%.

A promising alternative to steam turbines are gas turbines fuelled by gas produced by thermochemical conversion of biomass. The exhaust is used to raise steam in heat recovery systems used in any of the following ways: heating process needs in a cogeneration system, for injecting back into gas turbine to raise power output and efficiency in a steam-injected gas turbine cycle (STIG) or expanding through a steam turbine to boost power output and efficiency in a gas turbine/steam turbine combined cycle (GTCC).

Gas turbines, unlike steam turbines, are characterized by lower unit capital costs at modest scale, and the most efficient cycles are considerably more efficient than comparably sized steam turbines.

Renewable Energy from Food Residuals

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.

food-waste

In United States, food scraps is the third largest waste stream after paper and yard waste. Around 12.7 percent of the total municipal solid waste (MSW) generated in the year 2008 was food scraps that amounted to about 32 million tons. According to EPA, about 31 million tons of food waste was thrown away into landfills or incinerators in 2008. As far as United Kingdom is concerned, households throw away 8.3 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. The use of anaerobic digestion technology generates 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.

A Typical Food Waste-to-Energy Plant

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.

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.

Unending Benefits of Biomass Energy

Biomass is material originating from plant and animal matter. Biomass energy uses biomass to create energy by burning organic materials. The heat energy released through burning these materials can heat homes or water. Heated water produces steam, which in turn can generate electricity. Using organic materials to create heat and power is an eco-friendlier alternative compared to using fossil fuels.

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Indefinitely Renewable

The majority of the world’s energy comes from burning fossil fuels. Fossil fuels are a finite resource. Once fossil fuel resources run out, new fuel sources will be needed to meet global energy demands. Biomass offers a solution to meet this need.

Organic waste material from agriculture and logging operations, animal manure, and sludge from wastewater treatment are all viable fuels for generating biomass energy. As long as the earth is inhabited, these materials will be readily available.

Reduce, Reuse, Recycle

Waste organic material that would typically be disposed of in landfills could be redirected for biomass energy use. This reduces the amount of material in landfills and slows the rate at which landfills are filled. Some of the most common waste products used for biomass energy are wood chips and agricultural waste products. Wood materials can easily be converted from already existing wood structures that will be destroyed, such as wooden furniture and log cabins, preferably both would also come from responsible logging and practices as well.

As more organic material is diverted from landfills, the number of new landfills needed would be reduced. Older landfills are at risk for leaking leachate. Leachate contains many environmental pollutants that can contaminate groundwater sources.

Burning fossil fuel releases carbon into the atmosphere which was previously trapped below ground. Trapped carbon isn’t at risk for contributing to global climate change since it can’t interact with air. Each time fossil fuels are burned, they allow previously trapped carbon to enter the atmosphere and contribute to global climate change. In comparison, biofuel is carbon-neutral.

The materials used to create biomass energy naturally release carbon into the environment as they decompose. Living plants and trees use carbon dioxide to grow and release oxygen into the atmosphere. Carbon dioxide released by burning organic material will be absorbed by existing plants and trees. The biomass cycle is carbon-neutral as no new carbon is introduced to the system.

Smaller Carbon Footprint

The amount of unused farmland is increasing as agriculture becomes more efficient. Maintaining open land is expensive. As a result, farmers are selling off their property for new developments. Unused open agricultural land could be used to grow organic material for biofuels.

Converting open tracts of land to developed areas increases the amount of storm-water runoff. Storm-water runoff from developed areas contains more pollutants than storm-water runoff from undeveloped areas. Using open areas to grow biomass sources instead of creating new developments would reduce water pollution.

Biomass-Resources

A quick glance at popular biomass resources

Forested areas also provide sources of biofuel material. Open land converted to sustainable forestry would create new animal habitats and offset carbon emissions from existing fossil fuel sources as more plants and trees would be available to absorb carbon dioxide.

Societal Benefits

Burning fossil fuels releases sulfur dioxide, mercury and particulate matter into the atmosphere which can cause asthma, cancer and respiratory problems. Biomass energy emits less harmful byproducts compared to fossil fuels, which means cleaner air and healthier people.

Biofuel can improve rural economies by providing more people with unused land the opportunity to grown biomass material for energy use. Workers would be needed to harvest and process the materials needed to generate biofuel.

Since biomass is a renewable energy source, energy providers can receive tax credits and incentives. Countries with land resources will be less reliant on foreign fossil fuel providers and can improve their local economies.

Increasing biomass energy usage can reduce forest fires. Selectively reducing brush can still reduce the risk of wildfires spreading. Exposing underbrush and groundcover to rainfall decreases the change of it drying out and creating optimal, fire spreading conditions.

Denmark and Biomass Energy

Denmark is an example of how effective biomass energy can be in improving energy efficiency. Approximately 70 percent of renewable-energy consumption in Denmark comes from biomass.

Woody biomass creates an increasing percentage of heating from combined heat and power (CHP) plants with a goal to for 100 percent of hearing to be derived from woody biomass by 2035. Another form of biomass is agricultural biomass. This form utilizes materials such as straw and corn to create end-products like electricity, heating and biofuels.

The Danish Energy Agency has developed a plan including four scenarios that will help Denmark become fossil fuel free by 2050. The biomass scenario involves CHP for electricity and district heating, indicating that biomass energy is important in Denmark’s energy sector today and will play an increasingly important role in the future.

Biomass offers an eco-friendly and renewable method of reducing pollution and the effects of global climate change. And, like other forms of renewable energy, the products needed to develop biomass energy are readily available.

Description of a Biogas Power Plant

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.

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Working of a Biogas Plant

The fresh animal manure 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 digesters 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 Cleanup

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.

Control System

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 Conversion Routes

Waste-to-energy is the use of modern 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.

Teesside-WTE-plant

Thermochemical Conversion

The three principal methods of thermochemical conversion 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.

WTE_Pathways

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

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

The physico-chemical technology 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.

Trends in Utilization of Biogas

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.

DaishThe 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.

CHP Applications

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.

New Trends

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.

Typical layout of a modern biogas facility

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.

Overview of Biomass Energy Systems

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 waste-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 Concept of Biorefinery

A biorefinery is a facility that integrates biomass conversion processes and equipment to produce fuels, power, and value-added chemicals from biomass. Biorefinery is analogous to today’s petroleum refinery, which produces multiple fuels and products from petroleum. By producing several products, a biorefinery takes advantage of the various components in biomass and their intermediates, therefore maximizing the value derived from the biomass feedstock.

A biorefinery could, for example, produce one or several low-volume, but high-value, chemical products and a low-value, but high-volume liquid transportation fuel such as biodiesel or bioethanol. At the same time, it can generate electricity and process heat, through CHP technology, for its own use and perhaps enough for sale of electricity to the local utility. The high value products increase profitability, the high-volume fuel helps meet energy needs, and the power production helps to lower energy costs and reduce GHG emissions from traditional power plant facilities.

Biorefinery Platforms

There are several platforms which can be employed in a biorefinery with the major ones being the sugar platform and the thermochemical platform (also known as syngas platform).

Sugar platform biorefineries breaks down biomass into different types of component sugars for fermentation or other biological processing into various fuels and chemicals. On the other hand, thermochemical biorefineries transform biomass into synthesis gas (hydrogen and carbon monoxide) or pyrolysis oil.

The thermochemical biomass conversion process is complex, and uses components, configurations, and operating conditions that are more typical of petroleum refining. Biomass is converted into syngas, and syngas is converted into an ethanol-rich mixture.

However, syngas created from biomass contains contaminants such as tar and sulphur that interfere with the conversion of the syngas into products. These contaminants can be removed by tar-reforming catalysts and catalytic reforming processes. This not only cleans the syngas, it also creates more of it, improving process economics and ultimately cutting the cost of the resulting ethanol.

Plus Points

Biorefineries can help in utilizing the optimum energy potential of organic wastes and may also resolve the problems of waste management and GHGs emissions. Biomass wastes can be converted, through appropriate enzymatic/chemical treatment, into either gaseous or liquid fuels.

The pre-treatment processes involved in biorefining generate products like paper-pulp, HFCS, solvents, acetate, resins, laminates, adhesives, flavour chemicals, activated carbon, fuel enhancers, undigested sugars etc. which generally remain untapped in the traditional processes. The suitability of this process is further enhanced from the fact that it can utilize a variety of biomass resources, whether plant-derived or animal-derived.

Future Perspectives

The concept of biomass-based refinery is still in early stages at most places in the world. Problems like raw material availability, feasibility in product supply chain, scalability of the model are hampering its development at commercial-scales. The National Renewable Energy Laboratory (NREL) of USA is leading the front in biorefinery research with path-breaking discoveries and inventions. Although the technology is still in nascent stages, but it holds the key to the optimum utilization of wastes and natural resources that humans have always tried to achieve. The onus now lies on governments and corporate sector to incentivize or finance the research and development in this highly promising field.

Anaerobic Digestion of Tannery Wastes

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 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.