About Gasification

The technology of gasification was first commercialized using various grades of coal, but biomass resources such as wood have a unique environmental advantage over traditional fossil fuels in that the gasification of biomass has a mitigating effect on global warming, when a renewable biomass fuel is used instead of a fossil fuel. The importance of gasification can also be derived from the history, the existence of fuel gas was a ‘catalyst’ for the development of the gas engine. All over the world, we use the term gas pedal, even in diesel cars.

Biomass gasification is an endothermic conversion technology where a solid (organic) fuel composed of the main elements Carbon, Hydrogen, and Oxygen, is converted into a combustible gas. A limited supply of oxygen, air, steam or a combination serves as the oxidizing agent. In practice, conversion of biomass and waste is closely related to carbonization, pyrolysis and combustion. The differences can be explained by a combined effect of the amount of oxygen supply to the biomass material and the three T’s (temperature, turbulence and time). To explain and predict the gas composition, the dimensionless parameter ER, the equivalence ratio is introduced. This is the amount of oxygen used relative to the amount required for complete combustion.

Each type of biomass has its own specific properties, which determine its performance as a fuel in gasification plants. A large difference of biomass with coal is the high volatile content of biomass and its low fixed carbon content. The most important properties for gasification are:

  • moisture content
  • ash content
  • ash composition
  • elemental composition
  • heating value
  • bulk density
  • fuel morphology
  • volatile matter content
  • other fuel related contaminants like N, S, Cl, alkalies, heavy metals, etc.


Due to the differences in these characteristics of various biomass resources, some pretreatment like drying, sizing, densification, etc. is needed in order to be able to use the original biomass resource as feedstock for a particular gasifier design. Different types of gasifiers can be distinguished:

  • fixed bed: downdraft, updraft, crossdraft, multi-stage
  • fluid bed: bubbling, circulating, twin-bed
  • Working principle of a downdraft fixed bed gasifier
  • entrained flow
  • stage gasification with physical separation of the pyrolysis, combustion and reduction stage
  • slagging gasification
  • plasma gasification

Advantages of Gasification

Gasification has the potential to produce a clean homogenous fuel from a contaminated and inhomogeneous solid fuel. The clean gas is derived from the intermediate raw gas after cleaning and conditioning of the raw gas. This offers significant advantages during the secondary conversion processes and/or usage of producer gas:

  1. contaminants like N, S, Cl, etc. are produced in a reducing atmosphere as NH3, H2S, HCl, etc. They can be removed during the gas cleaning process before being combusted to harmfull emissions of NOx, SOx and HCl
  2. The gas volumes to be conditioned after gasification (producer gas) are much smaller than those after combustion (flue gas)
  3. producer gas can be utilized in multiple ways
  4. the different stages in gasification offers the possibility to separate the different steps (drying, pyrolysis, combustion, reduction) involved in the gasification process
  5. it is possible to remove the CO2 from the producer gas before subsequent usagpotential for de-centralized medium scale Waste-to-Energy applications as an alternative to large scale waste incineration plants

Health, Safety and Environment

Pilot and prototype biomass gasifiers often operate under temporary (trial) environmental licenses for which emission limits are usually somewhat ‘relaxed’. For gasifiers intended for commercial operation permitting authorities have a tendency to impose unreasonably strict emission limits and safety measures due to their lack of familiarity with and understanding of the technology. For permitting authorities and other key market actors it appears difficult to properly appreciate Health, Safety and Environmental (HSE) risks as shown in the figure. This lack of knowledge and poor appreciation of HSE hazards was identified since the early 2000’s by international organisations working on promoting biomass gasification technology. Through an European sponsored project this issue was addresses which ultimately resulted in an accepted Guideline on HSE aspects regarding biomass gasification.

Non-technical barriers are amongst others:

  1. Lack of knowledge and awareness on potential HSE risks associated with the construction, operation and maintenance of gasification plants;
  2. Lack of separate emission legislation (in most cases the Waste Incineration Directive has to be fulfilled);
  3. The general negative public perception on bio-energy as a clean energy source;
  4. Lack of knowledge at permitting authorities;
  5. Lack of standards and certification.
The Guideline and other related issues can be found at http://www.gasification-guide.eu

Gas Cleaning

Raw syngas contains contaminants that must be mitigated to meet process requirements and pollution control regulations.  A multitude of technologies exist to purify the raw synthesis gas stream that is produced by gasification. Some methods are capable of removing several contaminants in a single process, such as wet scrubbing, while others focus on the removal of only one contaminant. Contaminants to be removed from syngas generally include particulate matter, condensable hydrocarbons (i.e. tars), sulphur compounds, nitrogen compounds, alkali metals (primarily potassium and sodium), and hydrogen chloride.

For removal of tar and particles, basic technologies are:

  1. Cyclone filters
  2. Barrier filters
  3. Electrostatic filters (dry and/or wet ESP)
  4. Wet scrubbers
  5. Thermal (catalytic) cracking

Tars are the most cumbersome contaminants in producer gas. Most of the R&D projects are related to this aspect. Several methods are examined as shown in the image below.

More information about Gasification

More information about biomass gasification can be found in the two Handbooks (ISBN90-810068-1-9 and ISBN 9 789081 938501).