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Description of the biomass CHP technology based on biomass gasification

BIOS BIOENERGIESYSTEME GmbH, Graz

The objective of the thermo-chemical biomass gasification process is the best possible conversion of solid biomass fuels into a high calorific product gas. Thereby, biomass reacts with a fumigator (air, oxygen, steam or CO2), which provide oxygen for the process. Due to the thermal cracking and the partial oxidation a product gas is formed. The composition of the product gas depends on the biomass fuel, the reaction conditions and the fumigator and consists of different concentrations of hydrogen (H2), carbon monoxide (CO), steam (H2O) and methane (CH4). In case of air as fumigator the product gas includes nitrogen (N2) as well. Char coal, ash with varying carbon contents and condensable low molecular hydrocarbons are produced besides the product gas. The char coal and the hydrocarbons (summarised as tar) are the products of an incomplete gasification.

The gasification process comprises four stages:

  • Drying
  • Pyrolysis
  • Oxidation
  • Reduction

Typical product gas composition

Gasification technologies

Figure 1 shows an overview of the different gasification technologies.

Figure 1: Overview of the different gasification technologies

The fuel particles in fixed bed gasifiers are not moved by the gas flow and thus the fuel in the gasifier is arranged as fixed bed. The fuel feeding of most reactors is positioned above the fuel bed while the char coal and the ash are extracted from the bottom of the fuel bed. The four stages of the gasification process take place in a distinguishable drying, pyrolysis, oxidation and reduction zone. The biomass fuel moves from the top to the bottom of the fuel bed resulting in relatively long residence times of the fuel in the gasifier. A special design of fixed bed gasifiers comprises a fuel feeding from below the fuel bed. Depending on the direction of the product gas flow relative to the direction of the fuel transport the fixed bed gasifiers are classified into co-current, counter-current or cross flow gasifiers. Figure 2 shows the three basic designs of fixed bed gasifiers and the characteristic reaction zones of each gasifier.

Figure 2: Different designs of fixed bed gasifiers including the characteristic reaction zones of each gasifier design

Explanations: left: co-current gasifier, middle: counter-current gasifieer; right: cross flow gasifier

Source: OLOFSSON I., NORDIN A., SÖNDERLIMD U., 2005: Initial Review and Evaluation of Process Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels, ISSN 1653-0551 ETPC Report 05-02, Energy Technology & Thermal Process Chemistry, University of Umeå, Sweden

Fluidised bed gasifiers are operated with significantly higher gas flow velocities than fixed bed gasifiers. The fuel bed and a carrier material (e.g. sand) are fluidised by the gas flow (fumigator and recirculated product gas). Thus, the gasification reaction takes place in a fluidised bed but only 5-10 wt% of the bed is fuel. Since the fluidised bed allows an intensive mixing and a good heat transfer, there are no distinguished reaction zones. Hence, drying, pyrolysis, oxidation and reduction reactions take place simultaneously. The temperature distribution in the fluidised bed is relatively constant and typically ranges between 700°C and 900°C. Since the fluidised bed causes a relatively high reaction surface, the residence time of the fuel in fluidised bed gasifieres ranges between a few seconds and a few minutes and is clearly lower than the one of fixed bed reactors. Thus, higher fuel throughput rates are achievable.

Figure 3: Overview gas cleaning technologies

Gas cleaning

  • Cyclone
  • Tar cracker
  • Gas cooler
  • Ceramic filter
  • Bag filter
  • Gas scrubber
  • ESP
  • Compressor
  • Shift-reactor

The product gas usually contains different impurities which need to be separated before further utilisation in order to avoid erosion, corrosion and deposits in plant components upstream the gasifier. Such impurities are condensable hydrocarbons (tar), particles (dust, ash, sand from fluidised beds), alkali metal compounds (mainly potassium and sodium compounds), nitrogen compounds (e.g. NH3, HCN), sulphur compounds (e.g. H2S, COS), halogen compounds (e.g. HCl) and heavy metal compounds (e.g. Cd, Zn and Hg; especially when waste wood is applied).
The concentration of these impurities in the product gas strongly depends on the gasification technology, the operation parameters and the composition of the fuel applied. The necessary product gas quality depends on the product gas utilisation. The residues from the gas cleaning process need to be disposed of adequately.

Figure 3: Overview gas utilisation possibilities

Gas utilisation

  • Burner
    • Boiler
    • Furnace
    • Co-Firing
  • Gas engine
  • Gas turbine
  • Fuel cell
  • Combined processes
  • Synthesis-reactor
  • Feed-in into gas grid

For the utilisation of the product gas different utilisation technologies are applicable in order to produce electric energy, thermal energy for space and process heating and other sources of energy (fuel, synthesis gas).
The simplest way of utilisation is burning the gas for the production of heat. In order to produce electricity and heat technologies such as gas engines, gas turbines, steam turbine processes or stirling engines are applied. Furthermore, it is possible to use the product gas for Co-Firing in fossil fired power plants.
Further utilisation possibilities include the production of standardised liquid or gaseous fuels such as Fischer-Tropsch-(FT)-Diesel or synthetic natural gas (SNG) in catalytic reactors. Moreover, the compounds CO and H2 could be used as base material to synthesise other chemicals.

Integration of the ORC-unit in a biomass gasification plant

The implementation of an ORC-unit for the production of electric energy based on waste heat of a biomass gasification plant is an innovative possibility to increase the electric efficiency of gasification plants.
Figure 3 shows the implementation of an ORC-unit into a fluidised-bed-steam-gasification process which seems to be energetic and economical feasible for plant sizes greater than 2.5 MWel. By coupling these different CHP technologies, an improvement of the electric plant efficiency of about 20% can be expected.

Figure 3: Integration of an ORC unit into an fluidised bed steam gasification process

Source: REPOTEC Umwelttechnik GmbH, Vienna and BIOS BIOENERGIESYSTEME GmbH, Graz

Realised projects and proposals under design

  • Biomass CHP plant based on the integration of an ORC-process into a CFB steam gasification process – Oberwart (Burgenland, Austria); Preliminary design and preparation of permit application regarding thermal oil cycle and ORC unit
  • Biomass methanisation plant (production of Bio-SNG) based on a CFB steam gasification process Güssing (Burgenland, Austria); Engineering thermal oil cycle
  • Technical, ecological und economic evaluation of new biomass fixed bed gasification technologies
  • Gasification and pyrolysis of solid biofuels for the production of heat and power – State of development and technical-economic evaluation (internal project)

Papers:
DOWNLOADS – Biomass gasification