Technology Outline

1) Various biomass energy technologies
2) Biomass combined heat and power (CHP)

Biomass energy technologies refer to those technologies using plant matters or animal wastes to produce energy. Biomass energy is considered as renewable energy because the energy content of plant matters originated from the solar energy absorbed during photosynthesis.

Biomass can be derived from a variety of sources, including

  • Forestry residues: thinning residues, wood chips, spurning residues
  • Industrial residues: wood chips and sawdust, residues from furniture industry
  • Agricultural residues: corn stover, rice hulls, sugarcane bagasse, and animal waste
  • Energy crops: willow and switchgrass

Municipal organic waste is in fact also one form of biomass.

Hong Kong does not have forestry industries or sizeable agricultural industries. Topographically, Hong Kong has a mountainous terrain with limited flat land, and is not suitable for energy crop plantation development. There are very limited biomass resources, except for municipal organic waste.

1) Various biomass energy technologies

Thermal treatment processes such as combustion, gasification and pyrolysis can be used to harness the energy stored in biomass. The energy released is often in the form of heat, fuel gases and liquid fuels.

On a larger scale, biomass such as energy crops, fuelwood, forestry residues, and bagasse, can be combusted in furnaces and boilers to produce hot water for process heating, or steam either for direct use in industrial processes or for driving steam-turbines for electricity generation or both (which is called the biomass combined heat and power (CHP) systems). Biomass CHP systems are discussed in this section.

Biomass can also be converted into fuel gases or liquid fuels by biological processes such as fermentation and anaerobic digestion. Ethanol is the final product of fermentation of certain biomass materials that contain sugars, starch or cellulose. Ethanol can be used as an automotive fuel to supplement or substitute petrol in internal combustion engines. This subject of biofuel is treated in another section of this website. Municipal organic wastes can be used to generate biogas. The subject of energy-from-waste is also treated in another section of this website. Furthermore, biomass can be digested under anaerobic conditions to generate methane which can be used for power generation.

2) Biomass combined heat and power (CHP)

CHP (also referred as cogeneration) is the combined generation of heat and electricity in a single, integrated system and from a single primary energy source. When biomass is used as the energy source, the process is regarded as biomass CHP.

Grate combustion and fluidized bed combustion technologies are the major combustion technologies. Grate combustion is the traditional technology for burning solid fuels. Grates are still widely used for both hot water boilers and steam production in small-scale plants. Fluidized bed combustion is increasingly being used for biomass CHP plants.

Usually, the combustion boiler raises steam for power generation in a turbo-generator, while the waste heat is recovered for producing hot water or raising low pressure steam.

New technologies are being developed for biomass CHP systems. Gasification technologies for biomass CHP production have been researched intensively. Stirling engine, organic Rankin cycle, air bottoming cycle, evaporative gas turbine, externally-fired gas turbine, pulverized wood-fired gas turbine, powdered fuel combustion engine are technologies being tried.

Large biomass power generation systems can have comparable efficiencies to fossil fuel systems, but the costs are higher due to the special design of the boilers to facilitate combustion of biomass. CHP systems have much higher overall efficiency than systems generating electricity alone. Conventional power plants usually have an efficiency of around 35%. When the waste heat is recovered in a CHP system, the overall efficiency can reach as higher as 80%.

The economics of a biomass CHP system depends on the availability of fuel, the technology used and the efficiency of the plant, and the demand for heat and power. The cost for collecting and pre-treating biomass and distributing the heat and power generated will add cost to the overall system. Therefore, biomass CHP is viable where there is a sufficient supply of feedstock in the vicinity and there is a local demand for the heat and electricity.

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