FORMATION AND EMISSION OF FINE PARTICLES FROM TWO DANISH PULVERISED COAL FIRED POWER PLANTS

ABSTRACT
Measuring campaigns at two Danish pulverised coal fired power plants has been performed. The power plants are equipped with the same type og flue gas cleaning devices. The concentrations and chemical compositions of the formed particles were similar for the two plants. The measurements were performed simultaneously at three locations including both total dust measurements and fine particle characterisation with low pressure impactors and scanning mobility particle sizers. The size distribution of the particles formed during the combustion is clearly bimodal and the fine particles (< 1m) account for less than 1 % w/w of the total dust concentration. The fine particles are formed by condensation of vaporised species and show significant enrichment of several trace elements. Less than 0.5 % w/w of the total particle mass penetrate the electrostatic precipitators. The fine particles have penetrations of 4-6 % w/w clearly indicating the difficulties accompanied with the removal of fine particles from the flue gas, in good agreement with the theory for electrostatic precipitator performance. The flue gas desulfurisation plants have very little effect on the fine particles which easily penetrate the plants. Almost all particles emitted from the stacks are smaller than 10 m and the fine particles account for 19-38 % w/w of the total emitted mass.

1 INTRODUCTION
Several investigations have shown that elevated levels of fine particulate matter in the ambient air causes an increase in respiratory symptoms, cardiopulmonary disease and mortality [1,2,3,4]. In 1997, based on that knowledge the US EPA proposed a new standard for the levels of fine particulate matter in ambient air. This so called PM2.5 standard dictates limits for the concentration of particles in the ambient air with an aerodynamic diameter less than 2.5 microns at 15 g/m³ on an annual basis [5].

In 1998 a Danish investigation sponsored by the Department of Energy and in cooperation with FLS miljø a/s, Risø National Laboratory, The Technical University of Denmark and the power companies ELSAM a/s and ENERGI E2 was initiated to investigate the formation and emissions of fine particles from Danish power plants using different fuels (coal, biomass and Orimulsion).

In the following the results from field measurements at two pulverised coal fired power plants will be presented and compared. Both Avedøre Power Plant and Nordjylland Power Plant deliver power and district heating to the Danishconsumers. Key figures for the two power plants are shown in table 1.

Table 1 Features of the two power plants investigated with emphasis on units influencing the particles in the flue
gas. (DC = Direct Current)

The bituminous coals burned during the measuring campaigns at the two power plants were similar in chemicalcomposition.

The measurements were performed simultaneously at three locations on the plant (figure 1): at the inlet to the electrostatic precipitator, at the outlet of the electrostatic precipitator and in the stack. Data from these three positions give information about the formation of particles in the boiler, the electrostatic precipitator efficiency, the impact on particle characteristics of the flue gas desulfurisation plant and the emissions from the stack.