Technical Aspects of Gasification

Overview

• Gasification technology is mostly used in medium sized systems with about 150 - 500 kW. But it is principally well suited for small power plants in the range of 10 kW to over 100 kW also. In contrast to the information in company brochures of gasifier producers, it has to be stated that there is not yet any reliable, affordable standard gasifier technology appropriate for rural small-scale applications readily available off the shelf. There are still several unsolved technical problems. In general, the small-scale power-gasifier technology proved to be unreliable and expensive.
• Producer gas can be used as fuel for both Otto (gasoline) engines and diesel engines. In general, these engines have to be adapted slightly to this fuel. Otto engines can run exclusively on producer gas while diesel engines need admixing with conventional diesel fuel. Systems with diesel admixing seem to be more tolerant against fluctuation of the load and of the syngas quality and quantity.
• Wood fuel gasification systems in combination with Otto engines show overall system efficiencies (energy in the fuel/electrical energy produced) from 16 to 19 per cent. Gasification systems fuelled by rice husk show overall efficiencies of 7 to 14 per cent. By integrating gasifiers in combined heat and power systems (CHP) their efficiencies can approach far more than 50%.
• Clean operation of downdraft reactors can only be achieved in a small power range. Hence, steady full load operation of the plants is crucial for efficient operation and achieving tar-free gas production. (Maximum passable turn down ratios are in the range of about 50% of full load).
• The internal combustion engines have strict purity requirements regarding the generator gas. Too much particular matter, tar or other residues decrease the lifetime of the combustion engine and make frequent maintenance necessary. The main strategy to address this challenge is to equip gasifier systems with a gas filter. This raises the costs, requires frequent cleaning of the filter system, and often produces much carcinogenic waste, especially in the case of wet stripping of the gas. Nearly all reports document severe environmental threats by the disposal of these waste products. Nearly none of the plants that were monitored in the different studies took adequate measures to deal with the condensates. Instead, the pollutants were freely discharged into the environment. In addition, none of the operators dealing with these contaminated condensates used protective clothing or gloves.
• The remaining ashes are generally unproblematic and can be used as fertiliser, e.g. in fuel wood plantations.
  The gaseous emissions of a well-established and well-operated gasification plant are low. The gas is used as fuel for the combustion motor and its exhaust gases are similar to those of engines running on fossil fuels. If originating from renewable sources they can contribute significantly to reducing the GHG burden. However, one
  component of the generator gas is CO, which can constitute a serious threat in the event of leakages or improper management. Cases of CO intoxication are not unheard of.

Further Information

For more information on gasification please see Christa Roth's Gasification Manual Version 1 (2011) and Version 2 (2014)

as well as the following Energypedia pages:

• Micro Gasifiers
• Small-scale Biomass Gasification
• Feedstock Gasification