The Technology

Gasification is a process that converts carbonaceous materials, e.g. organic materials, biomass or petroleum products, into a synthetic gas by reacting the raw material at high temperatures. The resulting gas mixture is called synthesis gas or syngas and is itself a fuel. Gasification is a very efficient method for extracting energy from many different organic materials.

The advantage of gasification is that creating syngas is more efficient than direct combustion of the original fuel because more of the fuel’s energy is extracted. Syngas may be burned directly in internal combustion engines which are used to drive a generator, oxidised in a thermal reactor and the heat used to raise steam, used to produce methanol and hydrogen or converted into synthetic liquid fuel, like bio-diesel.

Gasification can also begin with materials that are not otherwise useful fuels, such as biomass or organic waste. In addition, the high temperature conversion refines out corrosive ash elements such as chloride and potassium, allowing clean gas production. Gasification of fossil fuels is currently widely used on industrial scales to generate electricity. Almost any type of organic material, such as wood, biomass, or even plastic waste, can be gasified.

Importantly biomass gasification is carbon neutral, as carbon dioxide will have been extracted in the process of growing biomass.

A variety of waste products can be used as fuel including: crops; tyres; wood-chips; sewage sludge; bio-fuels; industrial waste and Municipal Solid Waste (MSW).

Principal Features:

  • Modular (standard module 5 tonnes per hour of fuel)
  • Compact size
  • No high stacks
  • Minimal moving parts
  • Low capital cost
  • Long life (at least 20 years), high reliability
  • Unattended (automatic) operation, with minimal labour needs, though typically plants are manned 24/7
  • Low emissions (even composting, like landfill, produces methane)
    • Predominantly clean emissions comprising nitrogen gas and small amounts of water vapour, oxygen and C02
    • SOX, NOX, CO and VOCs (volatile organic compounds) easily comply with strict environmental standards worldwide
  • High efficiency of energy into electrical power
  • Heat from the gasifier and engine is recovered through a heat recovery steam generator and used for district heating or industry
  • Minimal ash, in contrast with high ash residues from incineration plants which

Brief Process Overview

The process takes place entirely within a closed building and processes any carbonaceous material; household waste, sewage, offal, wood, plastic etc and converts it to a good quality gas to fuel a reciprocating gas engine or gas turbine to generate electricity.

Untreated fuel (biomass and waste) is shredded to maximum 50mm. If the moisture content is high, above 40% the waste is dried in a conventional drier using surplus heat from the plant. Metals and inerts are removed from the fuel as far as possible but if contaminants do go into the plant they do not affect the process, just add to the residue. The correct sized dry fuel is sent to a dry fuel hopper.

The fuel is fed through a gate or progressive augur, to eliminate air, into the pyrolyser where the volatile hydrocarbons are driven off. The gases and residual solids, mainly carbon are then passed to the gasifier, where the carbon is reacted with water to produce carbon mon-oxide and hydrogen and long chain hydrocarbons are reduced to short chain hydrocarbons.

The gases are cleaned and then fed to an internal engine or gas turbine which in turn drives a generator. The ash is inert and carbon free.

Heat is recovered from the gasifier and the engine exhaust through a heat recovery steam generator (HRSG) and this heat can be used for district heating or industrial purposes.

Design Summary

Plant Operation

The plant is capable of continuous operation with a target operational time of 8000hours per year. The operation of the plant is very automated but will be manned continuously.

Odour Control

The areas of the plant/building are designed to eliminate any odours and all operations are contained within the building which is held under negative pressure to further reduce the possibility of odours.

Noise

The major noise sources from the site will be delivery vehicles and front loaders, which will comply with relevant regulations for noise levels.

The plant inside the building will be fitted with appropriate equipment or enclosed to ensure noise attenuation.

Visual Impact

The plant and equipment are small and so the building has a low impact. Of the operational plant the combined gasifier and gas engine exhausts are minimal and the only sources of emissions during normal operation and are catered for with a short chimney on the flare stack. Normally there will be no visual emissions but there may be some condensation of steam on cold days.

Process Monitoring

The products discharged to the atmosphere from the engines exhausts have to be measured to ensure that the emissions stay within the statutory regulations. The emission data will be published on line and full compliance will be ensured.

Any water discharges will be fully compliant with local regulations and will be monitored to ensure this. Again the data will be published.

Typical Emissions

Sophisticated gas clean up technology is used so the plant will operate well with permitted levels.

The nature of the process mitigates against dioxin formation, and prevents the pass through of pre-existing dioxin and related compounds. Any dioxin introduced to the feed would be broken down in the gasification stage.

Heavy metals and volatile metals such as lead, mercury, antimony, bismuth, that have significant vapour pressure at the pyrolysis temperature will be reduced to metallic form and will be trapped in the gas scrubbers. The levels of these components in the feed are controlled to minimise the levels of solid treatment residue.