Q How does a gasification facility work?
Gasification describes the process by which material (RDF in this instance) is converted into a synthetic gas (and ash) by using an external heat source in a low oxygen environment. The process is similar to that used for making town gas from coal, which has been done for decades. The syngas is combusted in a high efficiency boiler and the heat generated is used to raise steam for a turbine, where electricity is generated. In addition, a proportion of the heat generated can be supplied for use in external applications, either as steam or very hot water. Heat is recirculated from the gas combustion process to heat up the incoming RDF to create more syngas so only a small amount of fossil fuel (usually natural gas) is required to kick-start the process. Gasification is classed as an Advanced Conversion Technology (ACT).
Q Where else is this technology used?
Gasification technology has been used for over 100 years, and it was the basis of town gasworks using coal before being replaced by North Sea gas. The technology has also been used with various types of waste for some decades. Its application to mixed wastes, such as RDF, is more recent, although many commercial scale plants have been constructed in the last 20 years.
There are numerous working gasification plants successfully using RDF, particularly in Scandinavia and Japan. The technology proposed for this application has a successful track record of dealing with RDF.
There are a number of similar gasification plants being built in the UK at present, for example in Milton Keynes, Derby, Hull and Levenseat in Scotland.
Q What are the benefits of gasification?
Gasification is a highly efficient process with very low emissions. It is a naturally low Nitrogen Oxide (NOx) process - NOx is one of the main road traffic pollutants. Gasification plants may also be more adaptable and flexible in the long term to be converted to an even higher efficiency energy conversion process, where the synthetic gas is burnt in an internal combustion engine.
Q Isn’t this just an incinerator?
No. Incineration purely on its own is classified as a waste disposal technology on the waste hierarchy, whereas gasification with an efficient energy recovery system is a recovery process, which means energy is recovered for use. In addition, waste incineration works as open combustion in an oxygen rich environment; whereas gasification creates a synthetic gas in the absence of oxygen.
When incineration plants are equipped with energy recovery equipment, the term ‘energy from waste’ or EfW plant is usually used; this is sometimes also applied to gasification plants with energy recovery systems.
Q Could the plant explode?
No. The gasification plant works slightly below atmospheric pressure the whole time, because large fans pull air through the system constantly. There is nothing inside the plant to cause an explosion and there is no pressure to release.
Q Is this technology the same as plasma gasification technology?
No. Plasma gasification uses a very high temperature plasma ‘torch’ to achieve
the conversion process, whereas normal gasification technology uses recycled
heat from the combustion of the gas in order to create the syngas. This is one reason why the process can be so efficient.
Q How efficient is the process?
Gasification is a very efficient method of converting waste fuel into electricity, which can be made even more efficient by also utilising the heat generated by the facility in the local area. The more heat the facility exports, the higher the overall efficiency achieved. The efficiency of the facility will therefore increase over time, as a heat export network is developed, established and expanded.
Q What is the ‘R1’ efficiency measure that is sometimes talked about?
The term ‘R1 Energy Efficiency Formula’, as defined by the EU Waste Framework Directive (WFD) can be used to qualify an incinerator as a ‘recovery operation’; however this measure is intended to be applied to incinerators which are dedicated to municipal waste, not RDF as this facility is proposing. In the past, it has most commonly been used by plants on the continent that wish to import waste from other countries in the EU.
The recently revised Waste Framework Directive (WFD) now specifies that incineration facilities dedicated to the processing of municipal solid waste can only be classified as R1 where its energy efficiency is equal to or above an R1 score of 0.65 or above for installations granted a permit after 31st December 2008.
The proposals to be submitted to Swindon Borough Council will not use municipal waste exclusively, but will utilise RDF which originates from a number of sources, all of which have already been subject to intensive recycling systems.
It is also important to note that R1 classification is not a requirement to obtain planning consent for a gasification facility. Even so, Rolton Kilbride is confident that the proposals can obtain R1 status during the operational phase, as the facility is designed and intended to be capable of exporting heat to local consumers.
Further information can be found at the following Environment Agency website:
Q What about the ash left over after gasification – is that harmful?
No. The bottom ash from the gasification process is an inert or inactive material that remains at the end of the cycle and represents around 17% of the intake tonnage. This ash can be recycled in a variety of ways, to comply with the Environment Agency operating permit requirements. Examples of reuse include as a secondary aggregate replacement material, as a sub-base for roads and as material for temporary road construction on landfill sites. As with the rest of the process, the recycling of ash is strictly regulated and the system is audited on a regular basis.
Q What else is left as a residue from the process?
There is a small amount of APCR (air pollution control residue), which is sometimes called fly ash. APCR is typically a mixture of ash, carbon and lime (or bicarbonate). It is classed as a hazardous waste because of its high alkaline content from the spent lime, which is used as part of the filtering and cleaning process to remove acid gases. In the past it was disposed of at a hazardous waste landfill but nowadays it may undergo further processing such as washing or stabilisation before being sent to a non-hazardous landfill. It is a small volume of material from the process.
However, landfilling of APCR will be discontinued as new regulations come into effect. Many chemical treatment companies have anticipated this situation, with new facilities being opened which are now capable of achieving ‘end of waste’ status for the washed and cleaned aggregates contained in the APCR.
There is an amount of ferrous and non-ferrous metal which can be extracted from the ash or by the mechanical treatment facility (MTF) which will be on site, and sent for reprocessing by a scrap merchant.