Characterisation and Mitigation of Particulate Emissions from a Biomass Boiler using a Novel Non-Thermal Plasma Air Ionizer

Abstract

Biomass is commonly used for residential heating; however, it produces particulate emissions that are harmful to human health and dangerous to the environment. Though there are control techniques for large-scale applications, control of these emissions at the residential scale remains a problem. This study aims to investigate the usage of a novel non-thermal plasma (NTP) technology for the control of particulate emissions from small-scale (<25 kW) biomass boilers. A novel non-thermal plasma air ionizer (NTPAI) was developed and evaluated for particulate agglomeration. The NTPAI was assessed using both laboratory equipment in a “model” combustion particulate flow and on a commercial 15 kW biomass (wood pellet) boiler, located at Newcastle University’s Cockle Park Farm site. The laboratory-scale experiments showed that the NTPAI generates almost 4 million number of positive and negative ions even at low plasma power. The total ion generation increased with increase in NTPAI power. The ion concentration was demonstrated to decrease with increasing residence time. These laboratory-scale experiments demonstrated that the NTPAI significantly affected the size range of particulates. Nanosized particles were reduced by as much as 68% particle mass change (15W, 25L/min NTPAI ioniser flow rate; 1.2s residence time). This corresponded to a significant increase in the number of larger particles, indicating that agglomeration was taking place. The cumulative increase in the coarse and large particles were 62%. It was also noted that increasing the flow rate through the ionizer increased the number of ions generation, which is further useful design/operation information for the NTPAI. The emission of particulates from a 15 kW biomass boiler was investigated as a function of 5 operating modes: cold start, steady-state, shutdown, de-ashing, and warm start. This is the first-time that biomass boiler de-ashing mode has been identified as a separate mode and investigated as such. Generally, the NTPAI exhibited a substantial effect on particles agglomeration; the percentage particle mass concentration of submicron and fine particles decreased, resulting in an increase in the percentage particle mass concentration of coarse and large particle. The particle number concentration studied during steady state showed that coarse and large particles increased by 65% while ultrafine and tiny particles decreased by 75% and 30%, respectively. Overall, the NTPAI increased the average particle size of ultrafine and fine particles of the combustion particulates from the biomass boiler, making them less toxic to humans and probably easier to remove with current technologies. Ionizer design was also studied. Three designs were evaluated: the cylindrical, double test tubes electrode 100, and double test tubes electrode 50. It was shown that cylindrical ionizer generated 200 % ions more than the double test tube electrodes. Investigation conducted using cylindrical electrode showed that the total ion generation increased with increase in plasma power. Overall, at laboratory-scale and on a 15-kW biomass boiler flue, the NTPAI appears to be a reliable device for combustion particulates agglomeration. It significantly increases the average size of combustion biomass particulates which could then be easier to remove by the current downstream technologies.