Numerisk undersøgelse med detaljeret kemi, af CO reduktion ved forbrænding af biomasse

Meldgaard, Thomas

4. term

Energy Engineering, Master

2009

2009-06-03

0 pages

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.

In the case where water was injected together with the secondary air, the outlet mass fraction of CO was increased by 50 %. This originated from local spots of low temperatures caused by the presence of water. The two cases where areas of the furnace wall were insulated, reduced CO emissions were obtained. The reductions was most significant in the case where 60 m2 (19%) of the wall was insulated, where CO emissions were reduced by 33 %. In practice, the CO reductions obtained by insulating furnace walls are more significant. This was not fully resolved by the model used. Most likely, the phenomenon causing high CO emissions in practice, are cold regions near the walls, why a better resolution of the boundary layer had been needed. In general, the effects of different temperatures, the creation of radicals etc. was excellent modelled using a detailed kinetic mechanism with the EDC, features that cannot be resolved with simple chemistry models.

biomass ; grate furnace ; CO ; carbon monoxide ; EDC

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