• Thomas Meldgaard
4. semester, Energiteknik, Kandidat (Kandidatuddannelse)
The use of biomass power plants for energy generation is positive in terms of CO2 emissions, but is often connected with high levels of other pollutants because of for example incomplete combustion. For smaller grate fired boilers combusting high moisture fuel, high emissions of CO are often a challenge. Because of the environmental impact of CO and the additional species associated with the detection of CO, the emission limits are restrictive and are expected to be tightened further in the future. Concerning new plants, the emission limits are often in the range of 150 to 200 mg/Nm3 at 6% O2. Much research has been directed to improving the combustion, mostly focussed on mixing and air staging. Appropriate control of the furnace temperature or injection of species could possibly be useful in reducing the CO emissions. This is the concern of this report. Numerical models of the detailed chemical kinetics in a combustion process was used to study this in a basic manner, through simulations in Cantera. This was based on a representative high moisture fuel. The kinetics were described by a chemical mechanism, GRI-1.2, with 177 reactions. Studying the affect of different temperatures through simulations it was found that an appropriate temperature interval for CO burnout was 1300 K to 1800 K. Through investigation of the effect on the kinetics by adding different species, it was found that injection of steam could possibly increase the burnout rate of CO by increasing the amount of OH radicals in the combustion gas. Also, the effect of injecting steam with secondary air is of practical interest, as moist air, for example from a drying process, is used in some plants. Both subjects were investigated further through CFD simulations of a simplified biomass furnace. The detailed chemical kinetics were succesfully incorporated into the CFD simulations using the Eddy Dissipation Concept (EDC) model. Turbulence was modelled using the k-ǫ approach, and simulations were carried out in FLUENT. Four different cases were simulated, a reference case, a case with water injection and two cases with different parts of the furnace walls insulated.
Udgivende institutionN-study board
ID: 17642185