Pyrolysis & Gasification

Pyrolysis and gasification are processes for treating materials containing combustible and volatile components to derive gases that have a fuel value. Suitable materials include:

• biomass, such as wood chippings, bark and dried straw
• fossil fuels, such as coal, lignite and oil
• waste, such as RDF, tyres, car 'fluff', and MBM

Pyrolysis involves heating the materials indirectly with no air or oxygen being present. Gasification involves passing a limited amount of air either over or through the material so that partial combustion takes place. This partial combustion generates sufficient heat to volatalise the gaseous fuel from the material. Both processes generate a gas with some combustible content and, often, entrained solids such as ash.

Pyrolysis, gasification and incineration can be considered as a spectrum of combustion. Pyrolysis, with zero combustion, is at one end and incineration, with complete combustion is at the other. In practice, the combustion chambers of many incinerators are actually gasifiers and the combustion process is completed only by the secondary combustion stage. It will be noted that a gasifier which feeds a boiler in which the fuel gas is burned is very like an incinerator with a secondary chamber and energy recovery In both cases a fuel is completely burned and the hot gases used to generate high pressure steam. This steam is expanded through a steam turbine, turning a shaft to generate electrical power.

However, proper gasification and pyrolysis processes aim to generate a fuel that can be burned directly in an engine. This is inherently a more efficient process than using steam as an intermediate.

If the pyrolysis / gasification process produces a gas then it must be used immediately. Smaller units will use reciprocating engines, whereas larger systems will drive gas turbines. In both instances the power generation train will produce more power, at a lower capital cost, than the equivalent steam raising system.

Condensation of a proportion of the gas to give a liquid fuel is an important target for research. This will allow renewable fuels to be used for transport.

However, in many of the emerging pyrolysis and gasification processes, the gas produced is contaminated with ash and soot. These materials would have adverse effects downstream and are best eliminated, by filtration, at an early stage of the process.

The gas, when it is generated, is hot. Cooling the gas before filtering it is not normally an attractive option for several reasons:

• the entrained solids will quickly foul the heat transfer surfaces
• cooling will condense water from the gas stream, producing sticky solids
• the heat is potentially valuable, if it can be retained until the fuel is burned

So, it is better to filter the fuel gas when it is hot. Caldo have supplied several filters for the cleanup of fuel gas from gasification processes. There are a number of challenges, because the gas is combustible and hot, so it is better for potential operators to come and talk to Caldo rather than going it alone.

Caldo have produced a Powerpoint presentation about hot gas filtration in gasification and pyrolysis processes. This was originally delivered to a Filtration Society meeting but can now be downloaded as a 70 kB pdf file.