M. Purmalis’s research while affiliated with University of Latvia and other places

Electric field effects (EFE) on combustion characteristics, heat energy production and composition of polluting emissions have been investigated experimentally for different types of fuels (natural gas, biomass) providing experimental study of the EFE in a district heating boiler (DKVR) and complex modelling experiments in a small-scale pilot device. The DC field-induced variations of the produced heat energy, efficiency of heat energy production, flame characteristics and the composition of polluting emissions have been studied for a positively biased axially inserted electrode and negatively biased (grounded) heat surfaces by varying the applied DC voltage, net current and consumed electric field power. Experiments in the district heating boiler have shown that electrodynamic control of the heat production and combustion characteristics depends on the applied field voltage, power and on the flame region, where the top of the axially inserted electrode is located. The most pronounced EFE was observed when the top of the electrode was placed in the primary mixing zone intensifying the mixing of the flame compounds and thereby completing combustion. The mechanism of the electric field effects on the combustion characteristics is discussed with reference to the analysis of electric field effects on the flame characteristics observed in modelling experiments.

The paper presents the results of an experimental study of the effect of high-frequency electromagnetic (EM) field oscillations on combustion and emission characteristics of biomass pellets with the aim to provide active control of the processes of biomass gasification and combustion. The effects of high frequency oscillations on the combustion characteristics were studied experimentally providing complex measurements of the combustion characteristics, produced heat energy and product composition by varying the electric power and duration of biomass preprocessing by EM oscillations. It is found that high-frequency oscillations promote enhanced biomass gasification and combustion of volatiles completing the combustion of the volatiles with a correlating increase of the combustion efficiency and produced heat energy. The mechanism of high-frequency oscillations on the combustion characteristics is analyzed considering the field-enhanced variations of biomass thermal decomposition.

Combustion characteristics of microwave (2.45 GHz) pre-treated biomass (wood) pellets are investigated experimentally with the aim to find out the effect of pellets pre-treatment on the biomass composition and the feasibility of their thermo-chemical conversion. A complex study of the main combustion characteristics of the pre-treated biomass samples was carried out using a thermo-gravimetric method and a pilot-scale combustion test facility. The study includes time dependent measurements of mass loss at the primary stage of biomass gasification, variations of the flame temperature and heat production rate, combustion efficiency and composition of pollutants. The results have shown that the low-temperature microwave pre-treatment of wood pellets enriches the biomass carbon content from 50 % to similar to 60 % (d.b.) for biomass pre-treated for 180 s accompanied by the increasing higher heating values (HHV) from 19.9 up to 23.4 MJ/kg. The microwave pre-treatment promotes a faster thermal decomposition of biomass pellets with an enhanced heat energy production (up to 30 %) and a correlating increase of the volume fraction of CO2 in the products up to 25-28 %, while the mass fraction of the main volatiles (CO and H-2) in the products decreases, indicating a more complete combustion of the volatiles. The shear of the char combustion stage increases with the increasing pre-treatment time. The data of thermo-gravimetric analysis coincide with the results of the pilot-scale tests.

Active electric control of combustion dynamics at different stages of wood fuel burnout have been investigated experimentally, using a small scale pilot device, the main elements of which are a wood fuel gasifier and a combustor with swirl-enhanced mixing of flame compounds and combustion of volatiles. The electric field (DC, AC) effect on wood fuel gasification and combustion of volatiles downstream the combustor is studied experimentally for the regimes of field-enhanced mass transfer (ionic wind effect) and corona torch discharge using a flame-attached electric field. Variations of strongly swirled flame response to the applied electric field, combustion characteristics and combustion efficiency under conditions of DC and high-frequency AC electric field are detected and analyzed with approbation of the field effects on the flame characteristics for a domestic heating furnace.