Fine PM_2.5 and ultrafine PM_0.1 particle emissions from shared and industrial boilers - Report + Synthesis, August 10, 2014

This project, entitled 'EMIPAR', aim to extent the knowledge concerning the fine particulate matter (PM_2.5) and ultrafine (PM_0.1) emitted by biomass fired boilers with a power range up to 20 MW (district heating/industrial boilers), in order to formulate recommendations/solutions for the abatement of these particles. This requires a detailed characterization of the main parameters controlling their origin, i.e. the combustion. Ultimately, this would allow the reduction of PM emissions from biomass combustion using primary techniques. In order to achieve this, the PM samples have to be collected at high temperature levels (600 deg. C and above). Thus, the first step of the EMIPAR project is the development of a high temperature sampling probe. This step is followed by the investigation of the combustion of different types of wood chips at different scales, ranging from laboratory (10 kW), industrial pilot (470 kW) up to full industrial scale (4.8 MW reciprocating grate boiler and a 20 MW spreader-stoker). During each running test, biomass and the resulting ash samples were collected. The PM were sampled at high temperature within the furnace (for all scales) and at the outlet of the first heat exchanger (from industrial pilot to full scale). All the collected samples (biomass, ash and PM) have been submitted to an extensive physical and chemical analysis. The results concerning the PM emissions (concentration, size distribution and composition) have been analysed with respect to the biomass and ash composition, as well as the main operating parameters of the investigated combustion facilities, in order to identify and enhance the comprehension of fine and ultrafine particles formation mechanisms. Thus, the evolution of these particles is influenced mainly by the moisture yield of the biomass and the O₂ concentration level in the furnace. The composition analysis results show that collected PM contain mostly water-soluble ions, found as particulate salts (KCl and K₂SO₄), for all the wood chip types. It seems like these particles are intrinsic combustion products of the investigated biomass fuels and combustion technologies (grate firing or similar). The important quantity of water-soluble ions found in the fine and ultrafine PM open the way of the fuel pre-processing using leaching techniques. This would eliminate these ions that are present in the biomass and would thus lead to an important PM abatement. Another solution would be the wet scrubbing of the flue gas using normal water. However, the last is not the most common flue gas treatment technique, due especially to the difficulties raised by the resulting liquid effluents. Finally, it seems very difficult to define a primary technique for PM abatement. Nevertheless, a more precise control of the oxygen set value imposed at the boiler outlet would contribute to the reduction of these emissions. An investigation of the boiler's automatic control loop would seem necessary. (authors)