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, 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 hightemperature 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. However, 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.
Gasification relies on chemical processes at elevated temperatures above 700°C. This distinguishes it from biological processes such as anaerobic digestion that produce biogas, at ambient or slightly above ambient temperatures.
A variety of waste products can be used as fuel including: crops; tyres; wood-chips; sewagesludge; bio-fuels; industrial waste and Municipal Solid Waste (MSW).
Principal Features:
- Modular (standard module 2.5 tonnes per hour of dried fuel, increasing with new developments)
- Compact size
- No high stacks
- Minimal moving parts
- Low capital cost
- Long life (at least15 years), high reliability
- Unattended (automatic) operation, with minimal labour needs
- 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
- Minimal ash, in contrast with high ash residues from incineration plants, which forms a vitrified slag used in construction.
- Produces up to 1200°C heat with various uses
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 approximately 15mm and is fed into a dryer. The correct sized dry fuel is sent to a dry fuel hopper. The wet air from the dryer is drawn off. This air is then dried through condensation to provide dry air to the forced draft fan, thus burning off any smells. The water collected is recycled within the system.
The plant is designed to accommodate fuels with up to 40% moisture; sanitise the waste for safe sorting and the cleaning of recyclable products.
Design Summary
Plant Operation
The plant is capable of continuous operation with a maximum of 6% down time.
The gasifier process takes any carbonaceous material (wood, plastic, etc.) with a size less than 19mm.
The gas produced from the gasifier is fed into a gas engine or gas turbine, which drives an alternator to generate electrical power for export into the local grid network. The exhaust heat from the gas engine or turbine is added to the system to support the process.
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 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
The nature of the process mitigates against dioxin formation, and prevents the pass through of pre-existing dioxin and related compounds.
For dioxin to be formed, much higher levels of oxygen are required, and indeed any dioxin introduced to the feed would be broken down in the pyrolysis stage.
Any chlorine in the feed, for example PVC plastic, breaks down under reducing conditions and is converted to hydrogen chloride (HCl) gas. HCl is highly soluble in water, and is removed and neutralised in the wet scrubbing process.
Acid components, sulphur etc are removed with the gas and are extracted by the wet scrubber through ph correction.
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 scrubber, appearing as a component of the sludge from the scrubber. The levels of these components in the feed are controlled to minimise the levels of solid treatment residue.
Because of the reducing conditions in the retort and the high temperatures therein, the metals of concern are unlikely to partition to the char fraction. However, the char system is a WID compliant combustion process with all the features necessary to control emissions.
Gasification process produces no toxic emissions and are typically well with in WID limits
Typical Gasifier |
WID limit |
|
|---|---|---|
| CO | 80mg/Nm3 | 100mg/Nm3 |
| Hydrocarbons | 10mg/Nm3 | 20mg/Nm3 |
| Particulate | 10mg/Nm3 | 30mg/Nm3 |
| SO2 | 0mg/Nm3 | 300mg/Nm3 |
| HCI | 3mg/Nm3 | 30mg/Nm3 |
| NOx | 200mg/Nm3 | 350mg/Nm3 |
| Dioxins | 0.000ng/Nm3 | 1.0ng/Nm3 |
Combustion efficiency – 99.9%.
Temperature – 1200° C