Pyrolysis and thermal Gasification are related technologies. Gasification is the thermal decomposition of organic material at elevated temperatures in an oxygen restricted environment. The process, which requires an initial heat supply to get underway, produces a mixture of combustible gases (primarily methane, complex hydrocarbons, hydrogen and carbon monoxide). This producer gas can then either be used in boilers or cleaned up and used in combustion turbines or in generators. The gasification process is either self sustaining once the operating temperature is reached or it can be maintained by recycling a small proportion of the energy produced from the combustion of the fuel gases.

Figure 3: Process flow diagram for Gasification

The process flow diagram for Gasification is shown in Figure 3 (the diagram for pyrolysis being remarkably similar). Thermal pyrolysis of MSW is only different from the gasification process in that the thermal decomposition takes place in the absence of oxygen. The only necessary modifications to the gasification process diagram are that there will be no air input into the pyrolyser which will result in the creation of an energy rich oil and combustible solid residue (known as char) together with the fuel gas.

When comparing to mass burn, there are several clear advantages. It can be a more efficient technology - one studied biomass gasifier has a 36% efficiency (statistics from www.thermogenics.com leading to the quoted efficiency) compared 21% for mass burn. Using this and the load factor for this type of process (65% from www.bwea.com), and assuming once again 90million tonnes of waste per annum, there is the potential to develop over 6.7 GW (over 9% of the UK's installed capacity).

It should also be easier to develop these technologies on a smaller scale than that which is demanded by a mass burn incinerator as capitol costs are smaller and individual gasification/pyrolysis units generally deal with only between 25,000 and 40,000 tonnes per annum. More than one unit would likely operate together to make best use of the necessary pre-screening and to boost output. It might only be economic for a plant to incinerate by mass burn a minimum of 150,000 tonnes per year. The difference is it could prove practical to gasify just 50,000 tonnes per year. There is, therefore, greater scope for application of this technology in smaller communities than the large cities which typically support incinerators today. Likewise, there is less need to keep the gasifier running 100% of the time as start-up periods are less than for mass burn incinerators which allows for possible plant closing at nights or weekends. These systems are also able to operate at less than 100% of capacity so there is flexibility when there is a decline waste availability. (www.thermogenics.com)

The major environmental benefit of these processes are that they retain pollutants (the sulphur, heavy metals etc.) in the ash instead of them being moved to the gas phase and discharged to the atmosphere. Therefore the emissions form this technology are much lower than produced by conventional incineration and will require less flue gas treatment. In fact, there is often no need for a smoke stack as the emissions only come from the burning combustible gases in a turbine or boiler. (www.thermogenics.com)

One disadvantage with the process is the necessary fuel preparation. The fuel material is required to be shredded before being inputted to the gasifier, which entails cost. However, with the savings made by not requiring the level of emission controls, the process can prove economically profitable.

The major drawback to these systems is with getting past the planning stage. Advanced thermal technologies like these are still categorised as incineration when it comes to planning. The public generally have misconceptions as to the word "Incineration" as most expect huge visual intrusion and dangerous smoky emissions. This is the reason why the granting of planning permission is often resisted by the local community. The absence of a smokestack, however should help with planning permission applications.