Plasma Gasification

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Clean Energy and Water Technologies

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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Syngas as Feedstock for Biofuels

An attractive approach to converting biomass into liquid or gaseous fuels is direct gasification, followed by conversion of the gas to final fuel. NKGE98YUDMEC Ethanol can be produced this way, but other fuels can be produced more easily and potentially at lower cost, though none of the approaches is currently inexpensive. The choice of which process to use is influenced by the fact that lignin cannot easily be converted into a gas through biochemical conversion. Lignin can, however, be gasified through a heat process. The lignin components of plants can range from near 0% to 35%. For those plants at the lower end of this range, the chemical conversion approach is better suited. For plants that have more lignin, the heat-dominated approach is more effective. Once the gasification of biomass is complete, the resulting gases can be used in a variety of ways to produce liquid fuels discussed, in brief, below

Fischer-Tropsch (F-T) fuels

The Fischer-Tropsch process converts “syngas” (mainly carbon monoxide and hydrogen) into diesel fuel and naphtha (basic gasoline) by building polymer chains out of these basic building blocks. Typically a variety of co-products (various chemicals) are also produced.  Figure 2.1 shows the production of diesel fuel from bio-syngas by Fisher-Tropsch synthesis (FTS).

The Fisher-Tropsch process is an established technology and has been proven on a large scale but adoption has been limited by high capital and O&M costs. According to Choren Industries, a German based developer of the technology, it takes 5 tons of biomass to produce 1 ton of biodiesel, and 1 hectare generates 4 tons of biodiesel.

Methanol

Syngas can also be converted into methanol through dehydration or other techniques, and in fact methanol is an intermediate product of the F-T process (and is therefore cheaper to produce than F-T gasoline and diesel). Methanol is somewhat out of favour as a transportation fuel due to its relatively low energy content and high toxicity, but might be a preferred fuel if fuel cell vehicles are developed with on-board reforming of hydrogen.

Dimethyl ether

DME also can be produced from syngas, in a manner similar to methanol. It is a promising fuel for diesel engines, due to its good combustion and emissions properties. However, like LPG, it requires special fuel handling and storage equipment and some modifications of diesel engines, and is still at an experimental phase. If diesel vehicles were designed and produced to run on DME, they would become inherently very low pollutant emitting vehicles; with DME produced from biomass, they would also become very low GHG vehicles.

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 850^oC
• 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 75^oC (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 650^oC
• 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 850^oC 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 3000^oC
• 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

Carbon Sequestration and Biochar

Biochar sequestration is considered carbon negative as it results in a net decrease in atmospheric carbon dioxide over centuries or millennia time scales. Instead of allowing the organic matter to decompose and emit CO₂, pyrolysis can be used to sequester the carbon and  remove circulating carbon dioxide from the atmosphere and stores it in virtually permanent soil carbon pools, making it a carbon-negative process.

According to Johannes Lehmann of Cornell University, biochar sequestration could make a big difference in the fossil fuel emissions worldwide and act as a major player in the global carbon market with its robust, clean and simple production technology. The use of pyrolysis also provides an opportunity for the processing of agricultural residues, wood wastes and municipal solid waste into useful clean energy. Although some  organic matter is necessary for agricultural soil to maintain its productivity, much of the agricultural waste can be turned directly into biochar, bio-oil, and syngas. Pyrolysis transforms organic material such as agricultural residues and wood chips into three main components: syngas, bio-oil and biochar (which contain about 60 per cent of the carbon contained in the biomass.

Biomass CHP

Biomass conversion technologies transform a variety of wastes into heat, electricity and biofuels by employing a host of strategies. Biomass fuels are typically used most efficiently and beneficially when generating both power and heat through a Combined Heat and Power (or Cogeneration) system. Combined Heat and Power (CHP) technologies are well suited for sustainable development projects, because they are, in general, 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. Prime movers for CHP units include reciprocating engines, combustion or gas turbines, steam turbines, microturbines, and fuel cells. The success of any biomass-fuelled CHP project is heavily dependent on the availability of a suitable biomass feedstock freely available in urban and rural areas.

Energy Recovery from Tannery Wastes

The conventional leather tanning technology is highly polluting as it produces large amounts of organic and chemical pollutants. Wastes generated by the leather processing industries pose a major challenge to the environment. According to conservative estimates, about 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.

The energy generated by anaerobic digestion or gasification of tannery wastes can be put to beneficial use, in both drying the wastes and as an energy source for the tannery’s own requirements, CHP or electricity export from the site. A large amount of the energy recovered is surplus to the energy conversion process requirements and can be reused by the tannery directly. Infact, implementation of waste-to-energy systems have the potential to make the industry self-sufficient in terms of thermal energy requirements. Tanneries are major energy users, and requires up to 30 kW of energy to produce a single finished hide. Thus, waste-to-energy plant in a tannery promotes the production of electricity from decentralized renewable energy sources, apart from resolving serious environmental issues posed by leather industry wastes.

To read the full article, please visit http://www.altenergymag.com/emagazine.php?art_id=1499

Primary Biomass Conversion Technologies – Thermochemical

A wide range of technologies exists to convert the energy stored in biomass to more useful forms of energy. 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). The three principal methods of thermo-chemical conversion corresponding to each of these energy carriers are combustion in excess air, gasification in reduced air, and pyrolysis in the absence of air.

Conventional combustion technologies raise steam through the combustion of biomass. This steam may then be expanded through a conventional turbo-alternator to produce electricity. A number of combustion technology variants have been developed. Underfeed stokers are suitable for small scale boilers up to 6 MWth. Grate type boilers are widely deployed. They have relatively low investment costs, low operating costs and good operation at partial loads. However, they can have higher NOx emissions and decreased efficiencies due to the requirement of excess air, and they have lower efficiencies.

Fluidized bed combustors (FBC), which use a bed of hot inert material such as sand, are a more recent development. Bubbling FBCs are generally used at 10-30 MWth capacity, while Circulating FBCs are more applicable at larger scales. Advantages of FBCs are that they can tolerate a wider range of poor quality fuel, while emitting lower NOx levels.

Gasification of biomass takes place in a restricted supply of oxygen and occurs through initial devolatilization of the biomass, combustion of the volatile material and char, and further reduction to produce a fuel gas rich in carbon monoxide and hydrogen. This combustible gas has a lower calorific value than natural gas but can still be used as fuel for boilers, for engines, and potentially for combustion turbines after cleaning the gas stream of tars and particulates. If gasifiers are ‘air blown’, atmospheric nitrogen dilutes the fuel gas to a level of 10-14 percent that of the calorific value of natural gas. Oxygen and steam blown gasifiers produce a gas with a somewhat higher calorific value. Pressurized gasifiers are under development to reduce the physical size of major equipment items.

A variety of gasification reactors have been developed over several decades. These include the smaller scale fixed bed updraft, downdraft and cross flow gasifiers, as well as fluidized bed gasifiers for larger applications. At the small scale, downdraft gasifiers are noted for their relatively low tar production, but are not suitable for fuels with low ash melting point (such as straw). They also require fuel moisture levels to be controlled within narrow levels.

Pyrolysis is the term given to the thermal degradation of wood in the absence of oxygen. It enables biomass to be converted to a combination of solid char, gas and a liquid bio-oil. Pyrolysis technologies are generally categorized as “fast” or “slow” according to the time taken for processing the feed into pyrolysis products. These products are generated in roughly equal proportions with slow pyrolysis. 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. A range of bio-oil production processes are under development, including fluid bed reactors, ablative pyrolysis, entrained flow reactors, rotating cone reactors, and vacuum pyrolysis.

The Importance of Bio-Oil

Bio-oil is a dark brown liquid and has a similar composition to biomass. It has a much higher density than woody materials which reduces storage and transport costs. Bio-oil is not suitable for direct use in standard internal combustion engines. Alternatively, the oil can be upgraded to either a special engine fuel or through gasification processes to a syngas and then bio-diesel.

Bio-oil is particularly attractive for co-firing because it can be more readily handled and burned than solid fuel and is cheaper to transport and store.  Co-firing of bio-oil has been demonstrated in 350 MW gas fired power station in Holland, when 1% of the boiler output was successfully replaced. It is in such applications that bio-oil can offer major advantages over solid biomass and gasification due to the ease of handling, storage and combustion in an existing power station when special start-up procedures are not necessary. In addition, bio-oil is also a vital source for a wide range of organic compounds and speciality chemicals.

Woody Biomass Resources

Biomass power is the largest source of renewable energy as well as a vital part of the waste management infrastructure. An increasing global awareness about environmental issues is acting as the driving force behind the use of alternative and renewable sources of energy. A greater emphasis is being laid on the promotion of bioenergy in the industrialized as well as developing world to counter environmental issues.

Biomass may be used for energy production at different scales, including large-scale power generation, CHP, or small-scale thermal heating projects at governmental, educational or other institutions. Biomass comes from both human and natural activities and incorporates by-products from the timber industry, agricultural crops, forestry residues, household wastes, and wood. The resources range from corn kernels to corn stalks, from soybean and canola oils to animal fats, from prairie grasses to hardwoods, and even include algae. The largest source of energy from wood is pulping liquor or black liquor, a waste product from the pulp and paper industry.

Woody biomass is the most important renewable energy source if proper management of vegetation is ensured. The main benefits of woody biomass are as follows:

• Uniform distribution over the world’s surface, in contrast to finite sources of energy.
• Less capital-intensive conversion technologies employed for exploiting the energy potential.
• Attractive opportunity for local, regional and national energy self-sufficiency.
• Techno-economically viable alternative to fast-depleting fossil fuel reserves.
• Reduction in GHGs emissions.
• Provide opportunities to local farmers, entrepreneurs and rural population in making use of its sustainable development potential.

The United States is currently the largest producer of electricity from biomass having more than half of the world’s installed capacity. Biomass represents 1.5% of the total electricity supply compared to 0.1% for wind and solar combined. More than 7800 MW of power is produced in biomass power plants installed at more than 350 locations in the U.S., which represent about 1% of the total electricity generation capacity. According to the International Energy Agency, approximately 11% of the energy is derived from biomass throughout the world.

Biomass Resources

Biomass processing systems constitute a significant portion of the capital investment and operating costs of a biomass conversion facility depending on the type of biomass to be processed as well as the feedstock preparation requirements. Its main constituents are systems for biomass storage, handling, conveying, size reduction, cleaning, drying, and feeding. Harvesting biomass crops, collecting biomass residues, and storing and transporting biomass resources are critical elements in the biomass resource supply chain.

All processing of biomass yields by-products and waste streams collectively called residues, which have significant energy potential. A wide range of biomass resources are available for transformation into energy in natural forests, rural areas and urban centres. Some of the sources have been discussed in the following paragraphs:

A host of natural and human activities contributes to the biomass feedstock

1. Pulp and paper industry residues
The largest source of energy from wood is the waste product from the pulp and paper industry called black liquor. Logging and processing operations generate vast amounts of biomass residues. Wood processing produces sawdust and a collection of bark, branches and leaves/needles. A paper mill, which consumes vast amount of electricity, utilizes the pulp residues to create energy for in-house usage.

2. Forest residues
Forest harvesting is a major source of biomass for energy. 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 bioenergy. 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.

3. Agricultural or crop residues
Agriculture crop residues include corn stover (stalks and leaves), wheat straw, rice straw, nut hulls etc. Corn stover is a major source for bioenergy applications due to the huge areas dedicated to corn cultivation worldwide.

4. Urban wood waste
Such waste consists of lawn and tree trimmings, whole tree trunks, wood pallets and any other construction and demolition wastes made from lumber. The rejected woody material can be collected after a construction or demolition project and turned into mulch, compost or used to fuel bioenergy plants.

5. Energy crops
Dedicated energy crops are another source of woody biomass for energy. These crops are fast-growing plants, trees or other herbaceous biomass which are harvested specifically for energy production. Rapidly-growing, pest-tolerant, site and soil-specific crops have been identified by making use of bioengineering. For example, operational yield in the northern hemisphere is 10-15 tonnes/ha annually. A typical 20 MW steam cycle power station using energy crops would require a land area of around 8,000 ha to supply energy on rotation.

Herbaceous energy crops are harvested annually after taking two to three years to reach full productivity. These include grasses such as switchgrass, elephant grass, bamboo, sweet sorghum, wheatgrass etc.

Short rotation woody crops are fast growing hardwood trees harvested within five to eight years after planting. These include poplar, willow, silver maple, cottonwood, green ash, black walnut, sweetgum, and sycamore.

Industrial crops are grown to produce specific industrial chemicals or materials, e.g. kenaf and straws for fiber, and castor for ricinoleic acid. Agricultural crops include cornstarch and corn oil? soybean oil and meal? wheat starch, other vegetable oils etc. Aquatic resources such as algae, giant kelp, seaweed, and microflora also contribute to bioenergy feedstock.