Biomass Gasfication Power Systems


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Biomass power systems using biomass gasification has followed two divergent pathways, which are a function of the scale of operations.

Salman Zafar‘s insight:

The most attractive means of utilising a biomass gasifier for power generation is to integrate the gasification process into a gas turbine combined cycle power plant. This will normally require a gasifier capable of producing a gas with heat content close to 19 MJ/Nm3.

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Emerging Trends in Municipal Solid Waste Gasification


Fluidized bed gasification with FT-pilot in Gü...

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Gasification with pure oxygen or hydrogen

Gasification with pure oxygen or pure hydrogen (or hydrogasification) may provide better alternatives to the air blown or indirectly heated gasification systems. This depends greatly on reducing the costs associated with oxygen and hydrogen production and improvements in refractory linings in order to handle higher temperatures. Pure oxygen could be used to generate higher temperatures, and thus promote thermal catalytic destruction of organics within the fuel gas.  Hydrogasification is an attractive proposition because it effectively cracks tars within the primary gasifying vessel. It also promotes the formation of a methane rich gas that can be piped to utilities without any modifications to existing pipelines or gas turbines, and can be reformed into hydrogen or methanol for use with fuel cells.

Plasma gasification

Plasma gasification or plasma discharge uses extremely high temperatures in an oxygen-starved environment to completely decompose input waste material into very simple molecules in a process similar to pyrolysis. The heat source is a plasma discharge torch, a device that produces a very high temperature plasma gas. Plasma gasification has two variants, depending on whether the plasma torch is within the main waste conversion reactor or external to it. It is carried out under oxygen-starved conditions and the main products are vitrified slag, syngas and molten metal. Vitrified slag may be used as an aggregate in construction; the syngas may be used in energy recovery systems or as a chemical feedstock; and the molten metal may have a commercial value depending on quality and market availability.

Thermal depolymerization

Such processes use high-energy microwaves in a nitrogen atmosphere to decompose waste material. The waste absorbs microwave energy increasing the internal energy of the organic material to a level where chemical decomposition occurs on a molecular level. The nitrogen blanket forms an inert, oxygen free environment to prevent combustion. Temperatures in the chamber range from 150 to 3500C. At these temperatures, metal, ceramics and glass are not chemically affected.

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Comparison of Different Waste-to-Energy Processes


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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Technology Options for Waste-to-Energy Projects


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A wide variety of conversion methods are available for realizing the potential of waste as an energy source, ranging from very simple systems for disposing of dry waste to more complex technologies capable of dealing with large amounts of industrial waste.  These methods can be broadly divided into thermal and biological processes. Some of the emerging technologies are summarized below:

  1. Gasification – Conversion of carbonaceous materials into synthesis gas by reacting waste at high temperatures with a controlled amount of oxygen and/or steam.
  2. Thermal depolymerization – process of reducing complex materials into light crude oil.
  3. Anaerobic digestion (AD) – Making use of microorganisms to break down biodegradable material in absence of oxygen.
  4. Mechanical biological treatment (MBT)– combination technique where recyclable elements are removed from a mixed waste stream and a biological process is used to extract energy from the elements. The types of biological processes utilized encompass anaerobic digestion, composting and bio-drying.
  5. Pyrolysis – Thermal degradation of organic materials through use of indirect, external source of heat. Product is char, bio-oil and syngas
  6. Plasma Gasification – Use of electricity passed through graphite or carbon electrodes, with steam and/or oxygen / air injection to produce electrically conducting gas (plasma). Organic materials are converted to syngas

 Of the various modern energy conversion methods, pyrolysis and plasma gasification are attracting maximum attention these days, and these technologies have the potential to change the face of solid waste management in the coming years. Present trends indicate a move away from single solutions such as mass burn or landfill towards the integration of more advanced WTE technologies, based on setting priorities for waste treatment methods. These include waste minimisation, recycling, materials recovery, composting, biogas production, energy recovery through RDFs, gasification and residual land filling.

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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.

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