Factors governing particle number emissions in a waste-to-energy plant.

Particle number concentration and size distribution measurements were performed on the stack gas of a waste-to-energy plant which co-incinerates municipal solid waste, sewage sludge and clinical waste in two lines. Average total number of particles was found to be 4.0·10(5)cm(-3) and 1.9·10(5)cm(-3) for the line equipped with a wet flue gas cleaning process and a dry cleaning system, respectively. Ultrafine particles (dp<100nm) accounted for about 97% of total number concentration for both lines, whereas the nanoparticle (dp<50nm) contribution differed slightly between the lines (87% and 84%). The experimental data is explored statistically through some multivariate pattern identifying methods such as factor analysis and cluster analysis to help the interpretation of the results regarding the origin of the particles in the flue gas with the objective of determining the factors governing the particle number emissions. The higher moisture of the flue gas in the wet cleaning process was found to increase the particle number emissions on average by a factor of about 2 due to increased secondary formation of nanoparticles through nucleation of gaseous precursors such as sulfuric acid, ammonia and water. The influence of flue gas dilution and cooling monitored through the variation of the sampling conditions also confirms the potential effect of the secondary new particle formation in increasing the particle number emissions. This finding shows the importance of reporting the experimental conditions in detail to enable the comparison and interpretation of particle number emissions. Regarding the fuel characteristics no difference was observed in terms of particle number concentration and size distributions between the clinical waste feed and the municipal solid waste co-incineration with sludge.