Emission characters of fine particles from coal-fired power plants

PM10 is one of the principal atmospheric pollutants, and it is mostly generated from coal combustion in China. These years much attention has been paid to study its formation and emission control. In this investigation, particulate emission control devices (PECDs) classification collection efficiency, particulate matter emissions and size distribution were determined experimentally at the inlet and outlet of PECDs at four different coal-fired power plants. In these power plants, one uses anthracite coal and the others use bituminous coals. Electrical Low Pressure Impactor (ELPI) with a sampling system, which consisted of an isokinetic sampler probe, pre-cut cyclone, two-stage dilution system and sample line to the instruments, were used to measure in situ. Size distribution is measured on the range from 0.03µm to 10µm in aerodynamic diameter.

Before and after all the PECDs, the particle number size distributions display the bimodal distribution which contained the fine mode and the coarse mode with a peak around 0.1µm and 1µm, respectively. On one side, the mass concentration of PM10 is mainly dominated by the particles which are larger than 1 µm; on the other side, the number concentration is dominated by the particles which are smaller than 0.1 µm. Before the PECDs, the mass concentration of PM1 and PM2.5 are about 1~9% and 14~33% of PM10; furthermore, PM1, PM2.5 and PM10 are respectively about 0.1~0.8%, 1.3~4.5% and 8.7~22% of TSP. After the PECDs, these numbers were changed as about 2~17%, 12~52%, 0.8~14.5%, 8~45.8% and 62~87%, respectively. The generation factor and emission factor of TSP is 74.2~121.8g/kWh and 0.073~1.17 g/kWh, and which of PM10 is 6.4~17.4g/kWh and 0.045~0.78g/kWh.

Electrostatic precipitator (ESP) collection efficiency of TSP and PM10 is 98.6~99.9% and 96.0%~99.5%; bag-house collection efficiency is 99.94% to TSP and 99.57% to PM10. The collection efficiency minimum of ESP and bag-house both appear in the particle size range of 0.1~1µm. In this size range, ESP and bag-house collection efficiency is 72~99% and 99.4%.

Time resolution of ELPI is lower than 5 seconds, which makes it is possible to measure the emission instantaneous changes such as ESP collection plate rapping and bag-house cleaning pulse. Mass size distributions measured after ESP can be seen that the emission caused by ESP cleaning operation is primarily of particles larger than 1µm and no significant increase of submicron particles. The same phenomenon can be seen from emission caused by bag-house cleaning pulse operation.

The effect of the first-stage diluter temperature on particle number concentration is experimented. The results indicate that heating the first-stage dilution air to sample temperature can prevent condensation of species, such as water, sulfuric acids and some hydrocarbon compounds.