Emissions performance of a 40 MW pulverised wood fired boiler

Authored by: Olof Stålnacke and Björn Zethræus

Corresponding Author:

Name: Olof Stålnacke

Affiliation:  TPS Termiska processer AB,
Box 624,
SE-611 10 Nyköping,
Sweden.

Tel.: +46 8 535 248 10
Fax: + 46 155 26 30 52

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The authors of this paper have developed a unique method of optimizing the performance of full-scale wood boilers using a small scale physical model and the data from the process control system for the boiler. The physical model study was able to identify flow anomalies in the boiler which hamper the mixing of the fuel and combustion air and affect the residence time in the combustor. By studying the relationship between quantities measured by the process control system the authors are able to make recommendations for the improvements to the boiler performance. Possibly this approach can be applied to other types of boilers. As with their previous paper (Experimental method to verify the real residence-time distribution and temperature in MSW-plants), this is a good example of the research community developing practical methods to improve combustor performance.

Key Words: Pulverised wood combustion, Carbon monoxide, Nitrogen oxides, Unburned carbon, Ash

Summary:

In this paper we study the characteristics of combustion in a 40 MW pulverised wood fired boiler in order to find measures that would enhance its performance in regards to the emissions of carbon monoxide (CO) and nitrogen oxides (NOx), as well as the amount of unburned carbon in the ash. The latter has historically been observed to be high, which led to this study. The main flows in the furnace were studied with a Plexiglas model. The residence time, temperature, oxygen level, mixing rate and the fuel’s particle size distribution were measured and correlated to the responses of the boiler performance. NOx was found to be formed mainly by conversion of fuel nitrogen as is common for biomass combustion. We conclude that CO burnout was limited by insufficient mixing and CO production by entrained particles in the combustion chamber exhaust. The reason for the high amount of unburned carbon in the ash was that the fuel particles were too large. Our results are in agreement with the flow studied in the Plexiglas model. To improve mixing and the staging of the combustion, we recommend changing the design of the secondary air inlets. Our study predicts that both NOx  and CO emissions can be lowered by taking this measure.

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