Technology Outline

1) MSW combustion with energy recovery
2) Types of MSW combustion systems
3) MSW gasification
4) Types of gasifiers

Thermal treatment with energy recovery is the application of heat under controlled conditions to extract energy from organic wastes. The primary function of thermal treatment is to reduce the volume of municipal solid wastes (MSW), with the recovered energy being a by-product of the treatment process.

Thermal treatment system can be an integral part of an integrated MSW management facility, with a recycling facility to serve as a pre-treatment system. The recycling facility can increase the material recovery percentage and also enhance the overall efficiency of the thermal treatment process.

In a thermal treatment system with energy recovery, MSWs are used to produce heat and gas using various types of technologies such as MSW combustion and MSW gasification.

1) MSW combustion with energy recovery

MSW combustion produces flue gases with high temperature, and the thermal energy can be used to raise steam to drive a turbo-generator to produce electricity. In a modern waste-to-energy facility, MSW combustion and energy recovery are accomplished in an integrated design.

The diagram below shows the basic process of a MSW combustion facility with integral heat recovery.

MSW combustion facility with integral heat recovery. The text above describes the image.


MSW combustion typically reduces the volume of wastes by 90%. MSW combustion facilities are considered primarily as waste management facilities rather than renewable energy facilities.

2) Types of MSW combustion systems

MSW combustion systems can be divided into mass burning type and refuse-derived-fuel (RDF) burning type, depending on the fuel preparations steps and the way the refuse is combusted.

Mass burning
For this type of system, the refuse is combusted on a hearth or stoker as it is received, without the preparation step. Stokers are designed to include drying, burning and ash burnout sections, with each of these sections requiring different combustion air quantities.

Owing to the heterogeneity of the refuse, the furnace combustion environment must be designed to cater for very different combustion characteristics of the refuse constituents. On the other hand, combusting unprocessed refuse obviates the need for the extra land area for refuse preparation as in the case of the RDF burning systems. Furthermore, mass burning can produce a small total residue.

Refuse-derived-fuel (RDF) burning
For this type of system, a more homogenous fuel is derived from MSW for the combustion process by separating the combustibles with non-combustibles. Hence part of the useful materials can be recycled.

With a more homogenous fuel, the cost of the combustion furnace for RDF burning can be lowered. Besides, flue gas quantities are reduced and hence the cost for air pollution control equipment can be lowered.

However, RDF burning requires additional capital investment and land area for RDF preparation. The mass and volume of residues that need to be landfilled are higher than that of the mass burning process.

3) MSW gasification

Apart from MSW combustion, gasification is another thermal treatment option for MSW to recover energy from the organic portions of the wastes. The gasification process turns carbon-based substances into energy-rich fuel by heating under controlled conditions to produce a syngas which is the mixture of carbon monoxide and hydrogen.

Pretreatment of MSW is required t to ensure better performance of the gasification process, this includes: separation of inorganic ingredients, and controlling the size and moisture of feedstock.

Gasification of MSW involves a number of processes: pyrolysis, oxidation, and reduction.

Pyrolysis process
In the pyrolysis process, organic substances are split into gaseous, liquid and solid fractions through a combination of thermal cracking and condensation reactions under an oxygen-free environment. The pyrolysis process occurs at a temperature between 400 and 600 degrees Celsius. The products of pyrolysis include:
1) a gas stream containing primarily hydrogen, carbon monoxide, methane and other hydrocarbons;
2) a tar and/or oil stream that is liquid at room temperatures;
3) A char consisting of almost pure carbon and any inert material that may have entered the process.

Pyrolysis can be used as a waste management measure by itself, to reduce waste volume and generate different types of fuels as the by-product. One of the applications of pyrolysis is for biomass liquefaction as show below. The technology has been demonstrated in the U.S.A.

biomass liquefaction via pyrolysis. The text above describes the image.
(Source for above: US Department of Energy website)

Oxidation process (combustion process)
In the oxidation process, gaseous and liquid products derived from the pyrolysis process will be further oxidized by oxygen to produce carbon dioxide and carbon monoxide. The energy produced in the combustion process will be used to power the reduction (gasification) process.

Reduction process (gasification process)
In the reduction process, the char derived from the pyrolysis process will be furthered heated up in order to produce a syngas mainly composed of carbon monoxide and hydrogen. The gas can then be cleaned and burned in a gas engine or gas turbine to generate electricity.

4) Types of gasifiers

Gasification is still a developing technology. The fixed-bed gasifiers are mostly small-scale ones and come in two types, down-draft type (<2 MW) or up-draft type (<10 MW). The fluidized-bed gasifiers are in the higher MW range from tenth of MW to a few hundred MW. Another type of gasifiers, the entrained-flow gasifiers, is lagging the other two types in development due to the higher costs.

Fixed-bed gasifier

In fixed-bed gasifiers, the feedstock is fed in at the top of the gasifier in solid form and goes through different stages of the gasification process. Depending on the directions of the flow of fuel and air, fixed-bed gasifiers can be further divided into up-draft type and down-draft type.

  • In up-draft gasification, the feedstock is fed at the top of a gasifier while air is drawn from the bottom. The air passes through the heating zone at a rate that is insufficient to support full combustion but is adequate for a proportion of feedstock to burn and raise the temperature of the remainder to that at which pyrolysis occurs. The gas leaves at the upper part of the gasifier.
  • In down-draft gasification, air is drawn in through the feedstock into the gasification zone, and the resulting gases pass in sequence through the pyrolysis zone, the oxidation zone and the reduction zone in the gasifier. Gases produced leave the gasifer at its lower part.

Fluidized-bed gasifier

In fluidized-bed gasifier, air and feedstock are mixed up in a hot bed of solid materials, e.g. quarry sand. The mixture is kept in suspension above the bed such that it is almost fluid-like. The temperature is the same throughout the bed and there is no clear differentiation of the pyrolysis zone, oxidation zone and reduction zone. This enables the combustion process to occur at stable temperature and ensures optimal combustion.

   
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