Mathematical Modeling of Fixed Bed Biomass Combustion

An efficient and ecological friendly use of biomass for energy production is particularly important as biomass is CO2-neutral and safes the fossil fuel resources. In order to optimise fixed/moving bed combustion/gasification units with regard to efficiency and emissions various theoretical and experimental investigations have been performed in the past. Several mathematical models have been published, most of them focused on either the phenomena taking place inside a single fuel particle or inside the entire fuel bed. Both groups of models may be applicable for basic model fuels and valid/useful in certain ranges. In the case of biomass 85% or even more of the fuel converts during pyrolysis due to the high content of volatiles. It is well known that pyrolysis depends on many factors, e. g. particle size, temperature, heating rate, surrounding atmosphere, etc. In order to get realistic calculation results it is necessary that in a mathematical model both the history of the single fuel particle and the phenomena inside the fuel bed are considered adequately. In the project a combined reactor/particle model will be developed which allows the calculation of temperature and concentration profiles as a function of time and location inside both the single fuel particle and the fuel bed. The gases leaving the single fuel particle during pyrolysis determine the ignition and conversion behaviour of the fuel bed and furthermore the formation of pollutants. Most of the experimental investigations published in literature are focused on the mass loss of various fuels at different conditions and on the composition of the produced gases especially on the tar content. But from the energetic point of view the most important information would be the heating value and the oxygen demand of the gases leaving the single fuel particles as well as the heating value of the remaining char as a function of conversion. A calculation of these parameters based on the gas/char composition is nearly impossible as a detailed knowledge of the tar composition would be necessary. Within the project a calorimeter for online-measurements of the heating value of the gases leaving the single particle and their oxygen demand as a function of time will be built and a calorimeter for the measurement of the char as a function of conversion will be purchased. These parameters represent the main link between the single fuel particle and the entire fuel bed in the mathematical model and will be measured within the project. Finally experiments in a small- scale fixed bed reactor will be carried out for validation of the simulation results.

A considerable share of the energy demand is provided by decentred combustion units. Due to the Kyoto protocol the C02 emissions have to be reduced dramatically within the next few years. This is the reason for substituting fossil fuels by biomass in the combustion units mentioned above. The results of the project "Mathematical modelling of the biomass fixed bed combustion" provides an important contribution for an efficient use of biomass fuels. The computer model ReaMod for the simulation of fixed/moving bed combustion has been developed and simulation results have been compared with experimental data. Therefore a flow calorimeter which was developed by the same group was used to measure the heating value of the pyrolysis gases (as well as the tars) as a function of time. Furthermore the oxygen demand for the complete combustion as a function of time can be detected which plays an important role for the optimisation of a furnace. In addition to the original project proposal the two-dimensional computer model SEBOSS was developed. With this tool it is possible to calculate the transient temperature fields in stored solid fuels. The knowledge of the thermal behaviour of the stored fuels is important for the design of safe storage conditions in order to prevent spontaneous ignition. In the past few years refuse derived fuels based an plastics are used more and more for hegt and/or power production in addition to biomass. Especially the storage of those mixtures need to be analysed concerning safe storage conditions since an uncontrolled ignition/combustion would lead to gaseous and liquid (pyrolysis oils) emissions.