Anybody who knows that there is a power plant in Boitzenburg, Germany in which telegraph poles, rail sleepers, fence posts and fibre boards impregnated with wood preservatives are disposed of without environmental pollution (up to 25 000t annually) would perhaps expect to see large power plant buildings, chimneys and flue gas cleaning systems. None of these can be found at this site.

A visitor to Boitzenburg is faced with an unconventional round building, in which the forest waste, the fuel, is stored. From there the wood chips are transported to a neighbouring building and thence to the underfeed reactor, the furnace where carbonization occurs.

The thermo-chemical process of generating biogas is initiated by process air with a high degree of water vapour. This is the only point in the process where energy has to be introduced, to get it started. The air plus vapour flows from below at a temperature of between 700 and 900°C through the wooden chips into the oxidization zone where it is partially combusted. The conversion of the fuel into process gases is primarily carried out by the vapour. Features of this process stage are:

  • Precise supply of fuel and exact control of vapour content;
  • High power density;
  • Mastery of the temperature profile without formation of slag;
  • Generation of a large proportion of pure charcoal, which is discharged with the flow of gas. The plant produces 170 g of super-clean activated carbon per kg of wood – about 2800 t annually, which is suitable as source material for industrial products.

    The resulting process gas is decomposed in the next process step, a reforming process developed by Herwig Michel-Kim, Managing Director of Easymod Bioenergiesysteme AG, of Gustrow, Germany, for conversion of hydrocarbons with vapour into hydrogen, carbon monoxide and carbon dioxide. The gas reformer process results in:

  • Reduction of the share of condensible hydrocarbons to 5 to 10 mg per cubic meter of gas (fluidized-bed gasifiers have a hydrocarbon concentration which is about a thousand times higher);
  • Complete conversion of chlorinated hydrocarbons without the danger of dioxin formation;
  • Stable water-gas and methane gas reactions which can be calculated and controlled.

    The final process step in the gas production phase occurs in the reduction generator in which vapour and carbon dioxide with carbon is converted to water gas (hydrogen and carbon dioxide). Heat is required for this purpose, which is obtained by burning the charcoal produced during carbonization.

    The process gas is combusted in a Jenbacher gas motor which is used to generate electricity.

    The end products of the plant are thus charcoal, heat and electricity, produced from wood wastes contaminated to varying degrees with heavy metals or other toxic materials.

    Rapid signal transfer

    The heart of the biogas production process is the advanced process control system, using the Interbus field-bus, which takes over control of the process two minutes after charging with fuel and initial ignition.

    More than 4000 signals are active in the plant – many of them associated with sound sensors which transmit their signals in real time via the Interbus to the control room. The specially developed process control system allows all the actuators to be controlled in real time. The “whole plant would be unthinkable without Interbus”, says Mr Michel-Kim who emphasizes the importance of rapid data transfer to the success of the project. The sensors are read by the Interbus remote I/O modules, rendering terminal blocks just as superfluous as thick cable harnesses running through the plant, thus reducing the commissioning period, potential fault locations and the quantity of plant documentation.

    The signals are all transferred through two cables as thick as a finger. This includes a reserve, since each of the Interbus lines can transfer up to 4096 I/O signals. And the control room itself is by no means packed with modern computer technology: two 200 MHz Pentium PCs are sufficient. “The decisive factors are the real-time operating system, the rapid signal transmission via Interbus and our software”, Mr Michel-Kim notes.

    The current power plant, a pilot unit, has an electrical capacity of 3.5 MW and a heat output of 4.5 MW. The biogas is converted into electric energy with a gross efficiency of 38.5 per cent and a net efficiency of 32-33 per cent. When heat output is taken into account the efficiency is 85 per cent.

    A second plant, with an output of 5 MW, is planned. This will use fuel wood.

    Expected emissions