Polytechnik

When biomass is exposed to temperatures above 200 degrees Celsius in the absence of oxygen, synthesis gas is produced. Synthesis gas is an important by-product of a process for the thermal treatment of biomass in reactors in the absence of oxygen (= pyrolysis), such as torrefaction or carbonization, for several reasons. During further processing, considerable energy is released, which can be used for various purposes. Not utilizing this energy would be a direct waste of energy resources and, consequently, money. Furthermore, due to its harmful nature, synthesis gas must not be released into the atmosphere in its original form. Its treatment requires higher investments, which also increases losses in this indirect way.

In this case, the question arises as to what would be a sensible use for synthesis gas in a torrefaction or carbonization plant. The answer is simple: burn synthesis gas in specially designed chambers and extract energy from this process. The crucial question, however, is how to extract the maximum amount of energy from a low-grade energy source, justify the investment, and transform synthesis gas into a sustainable energy source.
In addition to water, hydrogen, and nitrogen, the typical composition of synthesis gas also includes carbon monoxide, carbon dioxide, various hydrocarbons and methane. Depending on the input raw material for pyrolysis, the gas may also contain chlorine, sulphur and other pollutants. Depending on the process, the gas may also be heavily laden with tar and dust. The typical calorific value of synthesis gas is between 7-15 MJ/kg.

Polytechnik has developed a special combustion chamber that can extract the maximum amount of energy from synthesis gas produced during torrefaction or carbonization. To illustrate this innovative technical solution to a complex problem, three basic areas are distinguished.

• Adiabatic combustion chamber
  The design and properties of the combustion chamber are tailored to the synthesis gas to be used, the composition of which varies depending on the raw material and production process. Synthesis gas contains various elements with different combustion speeds, which requires flame stabilization. Certain elements of the synthesis gas also require a higher temperature for ignition, while others reduce the calorific value and influence the flame temperature. The moisture content, i.e. water vapour, directly affects the stability of the flame and emissions. Combustion optimization is achieved through a special design that adapts to the properties of the synthesis gas.
  Polytechnik’s synthesis gas combustion chambers are lined with extremely temperature-resistant fireclay. The fireclay protects the chamber from rapid wear, reduces heat loss, thus increases combustion efficiency, distributes heat evenly, prevents hot spots and stabilizes the temperature in the chamber.

• Optimal gas flow
  CFD computer simulations are used in the development of Polytechnik’s solutions. These use numerical methods and algorithms to analyse the synthesis gas flow as a process of heat and mass transfer within the chamber. These virtual experiments provide detailed insights into the processes in the chamber and enable the development of optimized solutions. Based on the results, specially designed burners for synthesis gas are developed, enabling optimal combustion and thus optimal energy release.

• Optimisation of the combustion process through controlled air and recirculation gas admixture
  The special nozzle geometry optimises the synthesis gas-air mixture. The resulting swirl increases turbulence and mixing, accelerates combustion and reduces the formation of hot spots that can lead to NOx emissions. The distribution of the recirculation gas, which can be injected into the primary or secondary air or directly into the flame zone, influences the quality of combustion. The recirculated flue gases are chemically controlled by optimizing and stabilizing the flame temperature and reducing the oxygen concentration. Finally, the positioning and geometry of the nozzles for secondary and tertiary air achieve a staged combustion process, which, among other things, leads to a significant reduction in pollutant emissions.

Practical example:

The recently completed Taaleri project in Finland, currently the largest torrefaction plant in Europe, is an example of such a chamber.