Plasma Gasification

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Clean Energy Diary

The World Bank development indicators 2008 shows that the wealthiest 20% of the world accounts for 76.6% of total private consumption. The poorest fifth just 1.5%.The report further states,

“Today’s consumption is undermining the environmental resource base. It is exacerbating inequalities. And the dynamics of the consumption-poverty-inequality-environment nexus are accelerating. If the trends continue without change — not redistributing from high-income to low-income consumers, not shifting from polluting to cleaner goods and production technologies, not promoting goods that empower poor producers, not shifting priority from consumption for conspicuous display to meeting basic needs — today’s problems of consumption and human development will worsen. The real issue is not consumption itself but its patterns and effects. Inequalities in consumption are stark. Globally, the 20% of the world’s people in the highest-income countries account for 86% of total private consumption expenditures — the poorest 20% a minuscule 1.3%. More specifically, the richest…

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Waste-to-Energy Initiative in Vietnam

Vietnam Environment

Tu Ngoc An, board chairman of Kien Giang Composite Co. (KGC), has paid a visit to Australia to finalize the whole plan of building a plant generating power from garbage in HCMC with Australian partner Trisun International Development Co.

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Thermal Conversion of Wastes

Thermal (or thermochemical) conversion systems consist of primary conversion technologies which convert the waste into heat or gaseous and liquid products. These technologies can be classified according to the principal energy carrier produced in the conversion process. Carriers are in the form of heat, gas, liquid and/or solid products, depending on the extent to which oxygen is admitted to the conversion process (usually as air).

Combustion

Direct combustion is the best established and most commonly used technology for converting wastes to heat. During combustion, waste is burnt in excess air to produce heat. The first stage of combustion involves the evolution of combustible vapours from wastes, which burn as flames. Steam is expanded through a conventional turbo-alternator to produce electricity. The residual material, in the form of charcoal, is burnt in a forced air supply to give more heat. The main products of efficient combustion are carbon dioxide and water vapor, however tars, smoke and alkaline ash particles are also emitted. Minimization of these emissions and accommodation of their possible effects are important concerns in the design of environmentally acceptable waste combustion systems.

Co-Firing

Co-firing or co-combustion of biomass wastes 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. Co-firing involves utilizing existing power generating plants that are fired with fossil fuel (generally coal), and displacing a small proportion of the fossil fuel with renewable biomass fuels. Co-firing has the major advantage of avoiding the construction of new, dedicated, waste-to-energy power plant. An existing power station is modified to accept the waste resource and utilize it to produce a minor proportion of its electricity. Co-firing may be implemented using different types and percentages of wastes in a range of combustion and gasification technologies. Most forms of biomass wastes are suitable for co-firing. These include dedicated municipal solid wastes, wood waste and agricultural residues such as straw and husk.

Gasification

Gasification systems operate by heating wastes in an environment where the solid waste breaks down to form a flammable gas. The gasification of biomass takes place in a restricted supply of air or oxygen at temperatures up to 1200–1300°C. The gas produced—synthesis gas, or syngas—consists of carbon monoxide, hydrogen and methane with small amounts of higher hydrocarbons.  Syngas may be burnt to generate heat; alternatively it may be processed and then used as fuel for gas-fired engines or gas turbines to drive generators. In smaller systems, the syngas can be fired in reciprocating engines, micro-turbines, Stirling engines, or fuel cells. There are also small amounts of unwanted by-products such as char particles, tars, oils and ash, which tend to be damaging to engines, turbines or fuel cells and which must therefore first be removed or processed into additional fuel gas. This implies that gasifier operation is significantly more demanding than the operation of combustion systems.

Pyrolysis

Pyrolysis is thermal decomposition occurring in the absence of oxygen. During pyrolysis process, waste is heated either in the absence of air (i.e. indirectly), or by the partial combustion of some of the waste in a restricted air or oxygen supply. This results in the thermal decomposition of the waste to form a combination of a solid char, gas, and liquid bio-oil, which can be used as a liquid fuel or upgraded and further processed to value-added products. High temperature and longer residence time increase the waste conversion to gas and moderate temperature and short vapour residence time are optimum for producing liquids. Pyrolysis technologies are generally categorized as “fast” or “slow” according to the time taken for processing the feed into pyrolysis products. Using fast pyrolysis, bio-oil yield can be as high as 80 percent of the product on a dry fuel basis. Bio-oil can act as a liquid fuel or as a feedstock for chemical production.

Analyzing Different Waste-to-Energy Technologies

Major components of Waste-to-Energy Processes

  1. Front end MSW pre-processing is used to prepare MSW for treatment and separate any recyclables
  2. Conversion unit (reactor)
  3. Gas and residue treatment plant (optional)
  4. Energy recovery plant (optional): Energy / chemicals production system includes gas turbine, boiler, internal combustion engines for power production. Alternatively, ethanol or other organic chemicals can be produced
  5. Emissions clean up

Incineration

  • Combustion of raw MSW, moisture less than 50%
  • Sufficient amount of oxygen is required to fully oxidize the fuel
  • Combustion temperatures are in excess of 850oC
  • Waste is converted into CO2 and water concern about toxics (dioxin, furans)
  • Any non-combustible materials (inorganic such as metals, glass) remain as a solid, known as bottom ash (used as feedstock in cement and brick manufacturing)
  • Fly ash APC (air pollution control residue) particulates, etc
  • Needs high calorific value waste to keep combustion process going, otherwise requires high energy for maintaining high temperatures

Anaerobic Digestion

  •  Well-known technology for domestic sewage and organic wastes treatment, but not for unsorted MSW
  • Biological conversion of biodegradable organic materials in the absence of oxygen at temperatures 55 to 75oC (thermophilic digestion – most effective temperature range)
  • Residue is stabilized organic matter that can be used as soil amendment after proper dewatering
  • Digestion is used primarily to reduce quantity of sludge for disposal / reuse
  • Methane gas generated used for electricity / energy generation or flared

Gasification

  • Can be seen as between pyrolysis and combustion (incineration) as it involves partial oxidation.
  • Exothermic process (some heat is required to initialize and sustain the gasification process).
  • Oxygen is added but at low amounts not sufficient for full oxidation and full combustion.
  • Temperatures are above 650oC
  • Main product is syngas, typically has net calorific value of 4 to 10 MJ/Nm3
  • Other product is solid residue of non-combustible materials (ash) which contains low level of carbon

Pyrolysis

  • Thermal degradation of organic materials through use of indirect, external source of heat
  • Temperatures between 300 to 850oC are maintained for several seconds in the absence of oxygen.
  • Product is char, oil and syngas composed primarily of O2, CO, CO2, CH4 and complex hydrocarbons.
  • Syngas can be utilized for energy production or proportions can be condensed to produce oils and waxes
  • Syngas typically has net calorific value (NCV) of 10 to 20 MJ/Nm

Plasma Gasification

  • Use of electricity passed through graphite or carbon electrodes, with steam and/or oxygen / air injection to produce electrically conducting gas (plasma)
  • Temperatures are above 3000oC
  • Organic materials are converted to syngas composed of H2, CO
  • Inorganic materials are converted to solid slag
  • Syngas can be utilized for energy production or proportions can be condensed to produce oils and waxes

 

        Net Energy Generation Potential Per Ton MSW

Waste Management Method

Energy Potential*

(kWh per ton MSW)

Recycling

2,250

Landfilling

   105

WTE Incineration

   585

Gasification

   660

Pyrolysis

   660

Anaerobic Digestion

   250

Cost Economics of WTE Processes

Technology

Plant capacity

(tons/day)

Capital cost

(M US$)

O&M cost

(US$/ton)

Planning to commissioning

(months)

Pyrolysis

70-270

16 – 90

80 – 150

12 – 30

Gasification

900

15 – 170

80 – 150

12 – 30

Incineration

1300

30 – 180

80 – 120

54 – 96

Plasma gasification

900

50 – 80

80 – 150

12 – 30

Anaerobic digestion

300

20 – 80

60 – 100

12 – 24

In vessel composting

500

50 – 80

30 – 60

9 – 15

Sanitary landfill

500

5 – 10

10 – 20

9 – 15

Bioreactor landfill

500

10 – 15

15 – 30

12 – 18

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Production of Cellulosic Ethanol

The production of biofuels from lignocellulosic feedstocks can be achieved through two very different processing routes. They are:

  • Biochemical – in which enzymes and other micro-organisms are used to convert cellulose and hemicellulose components of the feedstocks to sugars prior to their fermentation to produce ethanol;
  • Thermo-chemical – where pyrolysis/gasification technologies produce a synthesis gas (CO + H2) from which a wide range of long carbon chain biofuels, such as synthetic diesel or aviation fuel, can be reformed.

Lignocellulosic biomass consists mainly of lignin and the polysaccharides cellulose and hemicellulose. Compared with the production of ethanol from first-generation feedstocks, the use of lignocellulosic biomass is more complicated because the polysaccharides are more stable and the pentose sugars are not readily fermentable by Saccharomyces cerevisiae. In order to convert lignocellulosic biomass to biofuels the polysaccharides must first be hydrolysed, or broken down, into simple sugars using either acid or enzymes. Several biotechnology-based approaches are being used to overcome such problems, including the development of strains of Saccharomyces cerevisiae that can ferment pentose sugars, the use of alternative yeast species that naturally ferment pentose sugars, and the engineering of enzymes that are able to break down cellulose and hemicellulose into simple sugars.

Lignocellulosic processing pilot plants have been established in the EU, in Denmark, Spain and Sweden. The world’s largest demonstration facility of lignocellulose ethanol (from wheat, barley straw and corn stover), with a capacity of 2.5 Ml, was first established by Iogen Corporation in Ottawa, Canada. Many other processing facilities are now in operation or planning throughout the world.

Ethanol from lignocellulosic biomass is produced mainly via biochemical routes. The three major steps involved are pretreatment, enzymatic hydrolysis, and fermentation as shown in Figure. Biomass is pretreated to improve the accessibility of enzymes. After pretreatment, biomass undergoes enzymatic hydrolysis for conversion of polysaccharides into monomer sugars, such as glucose and xylose. Subsequently, sugars are fermented to ethanol by the use of different microorganisms.

Pretreated biomass can directly be converted to ethanol by using the process called simultaneous saccharification and cofermentation (SSCF).  Pretreatment is a critical step which enhances the enzymatic hydrolysis of biomass. Basically, it alters the physical and chemical properties of biomass and improves the enzyme access and effectiveness which may also lead to a change in crystallinity and degree of polymerization of cellulose. The internal surface area and pore volume of pretreated biomass are increased which facilitates substantial improvement in accessibility of enzymes. The process also helps in enhancing the rate and yield of monomeric sugars during enzymatic hydrolysis steps.

Pretreatment methods can be broadly classified into four groups – physical, chemical, physio-chemical and biological. Physical pretreatment processes employ the mechanical comminution or irradiation processes to change only the physical characteristics of biomass. The physio-chemical process utilizes steam or steam and gases, like SO2 and CO2. The chemical processes employs acids (H2SO4, HCl, organic acids etc) or alkalis (NaOH, Na2CO3, Ca(OH)2, NH3 etc). The acid treatment typically shows the selectivity towards hydrolyzing the hemicelluloses components, whereas alkalis have better selectivity for the lignin. The fractionation of biomass components after such processes help in improving the enzymes accessibility which is also important to the efficient utilization of enzymes.

Presently, a ton of dry biomass typically yields 60-70 gallons of bioethanol. The major cost components in bioethanol production from lignocellulosic biomass are the pretreatment and the enzymatic hydrolysis steps. In fact, these two process are someway interrelated too where an efficient pretreatment strategy can save substantial enzyme consumption. Pretreatment step can also affect the cost of other operations such as size reduction prior to pretreatment. Therefore, optimization of these two important steps, which collectively contributes about 70% of the total processing cost, are the major challenges in the commercialization of bioethanol from 2nd generation feedstock.

A Glance at Biomass Resources

Manure, a field in Randers in Denmark
Image via Wikipedia

Biomass energy projects provide major business opportunities, environmental benefits, and rural development.  Feedstocks can be obtained from a wide array of sources without jeopardizing the food and feed supply, forests, and biodiversity in the world.

Agricultural Residues

Crop residues encompasses all agricultural wastes such as bagasse, straw, stem, stalk, leaves, husk, shell, peel, pulp, stubble, etc. Large quantities of crop residues are produced annually worldwide, and are vastly underutilised. Rice produces both straw and rice husks at the processing plant which can be conveniently and easily converted into energy. Significant quantities of biomass remain in the fields in the form of cob when maize is harvested which can be converted into energy. Sugar cane harvesting leads to harvest residues in the fields while processing produces fibrous bagasse, both of which are good sources of energy. Harvesting and processing of coconuts produces quantities of shell and fibre that can be utilized.

Current farming practice is usually to plough these residues back into the soil, or they are burnt, left to decompose, or grazed by cattle. These residues could be processed into liquid fuels or thermochemical processed to produce electricity and heat. Agricultural residues are characterized by seasonal availability and have characteristics that differ from other solid fuels such as wood, charcoal, char briquette. The main differences are the high content of volatile matter and lower density and burning time.

Animal Waste

There are a wide range of animal wastes that can be used as sources of biomass energy. The most common sources are animal and poultry manures. In the past this waste was recovered and sold as a fertilizer or simply spread onto agricultural land, but the introduction of tighter environmental controls on odour and water pollution means that some form of waste management is now required, which provides further incentives for waste-to-energy conversion.

The most attractive method of converting these waste materials to useful form is anaerobic digestion which gives biogas that can be used as a fuel for internal combustion engines, to generate electricity from small gas turbines, burnt directly for cooking, or for space and water heating.

Forestry Residues

Forestry residues are generated by operations such as thinning of plantations, clearing for logging roads, extracting stem-wood for pulp and timber, and natural attrition. Harvesting may occur as thinning in young stands, or cutting in older stands for timber or pulp that also yields tops and branches usable for biomass energy. Harvesting operations usually remove only 25 to 50 percent of the volume, leaving the residues available as biomass for energy.

Stands damaged by insects, disease or fire are additional sources of biomass. Forest residues normally have low density and fuel values that keep transport costs high, and so it is economical to reduce the biomass density in the forest itself.

Wood Wastes

Wood processing industries primarily include sawmilling, plywood, wood panel, furniture, building component, flooring, particle board, moulding, jointing and craft industries. Wood wastes generally are concentrated at the processing factories, e.g. plywood mills and sawmills. The amount of waste generated from wood processing industries varies from one type industry to another depending on the form of raw material and finished product.

Generally, the waste from wood industries such as saw millings and plywood, veneer and others are sawdust, off-cuts, trims and shavings. Sawdust arise from cutting, sizing, re-sawing, edging, while trims and shaving are the consequence of trimming and smoothing of wood. In general, processing of 1,000 kg of wood in the furniture industries will lead to waste generation of almost half (45 %), i.e. 450 kg of wood. Similarly, when processing 1,000 kg of wood in sawmill, the waste will amount to more than half (52 %), i.e. 520 kg wood.

Industrial Wastes

The food industry produces a large number of residues and by-products that can be used as biomass energy sources. These waste materials are generated from all sectors of the food industry with everything from meat production to confectionery producing waste that can be utilised as an energy source.

Solid wastes include peelings and scraps from fruit and vegetables, food that does not meet quality control standards, pulp and fibre from sugar and starch extraction, filter sludges and coffee grounds. These wastes are usually disposed of in landfill dumps.

Liquid wastes are generated by washing meat, fruit and vegetables, blanching fruit and vegetables, pre-cooking meats, poultry and fish, cleaning and processing operations as well as wine making.

These waste waters contain sugars, starches and other dissolved and solid organic matter. The potential exists for these industrial wastes to be anaerobically digested to produce biogas, or fermented to produce ethanol, and several commercial examples of waste-to-energy conversion already exist.

Pulp and paper industry is considered to be one of the highly polluting industries and consumes large amount of energy and water in various unit operations. The wastewater discharged by this industry is highly heterogeneous as it contains compounds from wood or other raw materials, processed chemicals as well as compound formed during processing.  Black liquor can be judiciously utilized for production of biogas using anaerobic UASB technology.

Municipal Solid Wastes and Sewage

Millions of tonnes of household waste are collected each year with the vast majority disposed of in open fields. The biomass resource in MSW comprises the putrescibles, paper and plastic and averages 80% of the total MSW collected. Municipal solid waste can be converted into energy by direct combustion, or by natural anaerobic digestion in the engineered landfill. At the landfill sites the gas produced by the natural decomposition of MSW (approximately 50% methane and 50% carbon dioxide) is collected from the stored material and scrubbed and cleaned before feeding into internal combustion engines or gas turbines to generate heat and power. The organic fraction of MSW can be anaerobically stabilized in a high-rate digester to obtain biogas for electricity or steam generation.

Sewage is a source of biomass energy that is very similar to the other animal wastes. Energy can be extracted from sewage using anaerobic digestion to produce biogas. The sewage sludge that remains can be incinerated or undergo pyrolysis to produce more biogas.

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Woody Biomass and Energy Conversion Efficiency

Every energy conversion system wastes a portion of its input energy. For biomass to electricity conversion systems, 50% or more of the energy input can be lost – even up to 90% for some small-scale and alternative technologies. However, the energy rejected from a conversion system can often be used productively for industrial or residential heating purposes in place of burning fuels separately for that purpose. When this is done the overall efficiency can jump to 75-80%. Most systems must reduce their electricity production somewhat to make cogeneration feasible.

Thermal applications are the most efficient conversion technology for turning woody biomass into energy and should be considered in the development of a national Renewable Portfolio Standard (RPS). Thermal applications for woody biomass can be up to 90% efficient, compared to 20% for electricity and 50-70% for bio-fuels. Thermal systems can be applied at multiple scales, and are often more economically viable, particularly in rural and remote areas, than electrical generation.

By not including thermal energy, one of the most efficient uses of woody biomass energy is put at a disadvantage to generating electricity and processing liquid bio-fuels. This runs counter to the goals of displacing fossil fuels, promoting energy efficiency, and minimizing carbon emissions.

Waste-to-Energy Conversion Pathways

The conversion of organic waste material to energy can proceed along three main pathways – thermochemical, biochemical and physicochemical. Thermochemical conversion, characterized by higher temperature and conversion rates, is best suited for lower moisture feedstock and is generally less selective for products.

Thermochemical conversion includes incineration, pyrolysis and gasification. The incineration 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.

Bio-chemical conversion processes, which include anaerobic digestion and fermentation, are preferred for wastes having high percentage of organic biodegradable (putrescible) matter and high moisture content. Anaerobic digestion is a biological treatment method that can be used to recover both nutrients and energy contained in organic wastes such as animal manure. The process generates gases with a high content of methane (55–70 %) as well as biofertilizer. Alcohol fermentation is the transformation of organic fraction of waste to ethanol by a series of biochemical reactions using specialized microorganisms.

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 were then mechanically separated before the waste is compacted and converted into pellets. 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.

Importance of Waste-to-Energy Plants

Waste-to-energy plants offer two important benefits of environmentally safe waste management and disposal, as well as the generation of clean electric power. Waste-to-energy facilities produce clean, renewable energy through thermal, biochemical and physicochemical methods. The growing use of waste-to-energy as a method to dispose off solid and liquid wastes and generate power has greatly reduced environmental impacts of municipal solid waste management, including emissions of greenhouse gases.

Waste-to-energy conversion reduces greenhouse gas emissions in two ways. Electricity is generated which reduces the dependence on electrical production from power plants based on fossil fuels. The greenhouse gas emissions are significantly reduced by preventing methane emissions from landfills. Moreover, waste-to-energy plants are highly efficient in harnessing the untapped sources of energy from a variety of wastes.

An environmentally sound and techno-economically viable methodology to treat biodegradable waste is highly crucial for the sustainability of\ modern societies. A transition from conventional energy systems to one based on renewable resources is necessary to meet the ever-increasing demand for energy and to address environmental concerns.