Study of the reduction of particle emissions and borate black liquor gasification in bioenergy systems

Abstract: The use of biofuel for district heating and for electricity generation is expected to increase consistently in the coming decades due to the neutral impacts on the greenhouse gases. Domestic wood boilers are first studied in this thesis. The particle emissions from these boilers are high and encounter health problems. This requires better understanding of the combustion process in the boiler. CFD software has been used as a tool for simulating the conditions inside the boiler for design and optimisation of the wood fired boiler. With the CFD model studied in this thesis, it is possible to get a detailed knowledge of the conditions inside the boiler, without excessive computational time and with acceptable accuracy. To decrease the emission of particles from a domestic wood-log boiler, particle traps based on inertial separation could be used. To find out whether the particle tracking option in the commercial CFD-code is reliable, a low-pressure impactor has been studied, and each stage of the impactor has been simulated. By comparing with experiments, it is possible to determine the accuracy of the particle tracking option. It has been shown that the particle tracking procedure in the CFD software Fluent V6.0 offers good accuracy at velocities below 12 m/s with Reynolds numbers 790-2150. Acceptable accuracy was shown for velocities 20-170 m/s at Reynolds numbers 960-2980. The particle tracking with Fluent is then appropriate for the domestic wood-log boiler where the velocities usually do not exceed 30 m/s and the Reynolds numbers are low. 3D simulations with either the RSM turbulence model, for accuracy, or the RNG k-ƒÕ model, to save calculation time, should be used together with the stochastic particle tracking. Knowing that tracking particles with an acceptable accuracy is possible for a domestic wood-log boiler, flow conditions in such a boiler were studied. The three-dimensional velocity and temperature fields were simulated in the secondary combustion zone of the boiler. The results from the simulations were validated against measurements. The temperature field was well described and the velocity field was predicted with acceptable accuracy. Chemical reactions were not included in these simulations. Turbulence was modelled using the RNG k-ƒÕ model, which is relatively easy to use. This thesis also performs a study on a new model of black liquor gasification with borate autocausticizing, which has the potential to improve the production performance of pulp mills. The earlier model from Jansson did not have the efficiency expected, and was found unacceptable as a means of replacing the lime cycle. The new process has a real advantage compared to earlier borate autocausticizing processes; it can theoretically be led either on a full scale or as a part of the process to an ordinary causticizing and with moderate borate setting. The booster gasifier is studied. The material and energy balance is formulated for a sulphate- recycling block with this process and is compared with the conventional techniques. The results of the usual black liquor gasification were very close to reality and very promising. Regarding the black liquor gasification with borate, lack of data for the orthoborate, like the Gibbs free energy, did not enable good accuracy of the results. The model is so far a good start in black liquor gasification, but it needs to be improved as soon as some new data are available.