Volumetric combustion of biomass for CO2 and NOx reduction in coal-fired boilers

Abstract

To meet the urgent environmental targets, substituting coal with biomass has been considered to be an effective and promising method over the last decades. In this paper, a new concept of volumetric combustion is proposed and further developed to achieve 100% fuel switching to biomass in large scale coal-fired boilers. Volumetric combustion not only changes the in-furnace flow but also affects the combustion reactions by the intensive mixing and internal recirculation of the flue gases. Firstly, the volumetric combustion properties of the wood pellets were investigated experimentally. An Aspen model was then used to thermodynamically describe and study the volumetric combustion with three different types of fuel, and the emission properties of CO 2 and NOx were compared. Finally, two applications of volumetric combustion were discussed. It is concluded that the wood pellets ignited and combusted much faster than the coal pellets and had a larger combustion volume when combusted under lower oxygen concentration conditions, and the ignition time was almost independent of the oxygen concentration when the oxidizer was preheated to 1000°C. In addition, the NOx emissions decreased as the recirculation ratio of the flue gas increased, and as the percentage of biomass used in co-firing increased, the amount of flue gas that needs to be recycled for reduction of NOx decreased. Thus, the volumetric combustion is beneficial as it reduces the operation cost of NOx reduction. The volumetric combustion would be an attractive technology for co-firing a large proportion of biomass in coal-fired boilers with high boiler efficiency and effective emissions reduction.

Flame characteristics of pulverized torrefied-biomass combusted with high-temperature air

“…Moreover, volumetric combustion has been proposed and developed to realize a high replacement ratio of biomass in co-firing boilers, which is expectedly accompanied by a reduction in pollutants [21]. The volumetric combustion of both gas and liquid fuels has been successfully realized for conditions of High Temperature Air Combustion (HTAC) [22][23][24][25].…”

Curbing Air Pollution and Greenhouse Gas Emissions from Industrial Boilers in China

“…A number of studies focused on switching from coal to biomass in boilers and covered many related aspects including composition of biomass, comparison between biomass and other fuels, combustion of biomass, co-firing of biomass and coal, transportation of biomass, densification of biomass, economic and social impacts of biomass, and future trend of using biomass energy (Sims et al, 2003;Demirbas, 2005;Zhang et al, 2010;Saidur et al, 2011;Li et al, 2012b). Most researchers agree on the climate benefits of using biomass but most researchers also agree that "care should be taken" when co-firing biomass in coal-fired boilers.…”

“…Most researchers agree on the climate benefits of using biomass but most researchers also agree that "care should be taken" when co-firing biomass in coal-fired boilers. Environmental impacts of using biomass in industrial boilers, such as land and water resources, soil erosion, loss of biodiversity, and deforestation were discussed in the literature along with the assessment of technical issues such as fouling, marketing, low heating value, storage and collections, and handling (Laursen and Grace, 2002;Sims et al, 2003;Li et al, 2010a;Li et al, 2012b). Existing literature also discussed some of the issues that are largely impacted by regulations and environmental requirements and discussed the way to improve existing methods of using biomass in order to improve efficiency and address the associated problems.…”

“…The authors found that the volumetric combustion would be an attractive technology for co-firing a large proportion of biomass in coal-fired boilers with high boiler efficiency and effective CO 2 and NOx emission reduction. Li et al (2012b) summarized the current technical challenges with co-firing biomass, including cost-effective methods for getting more and wider varieties of fuels into the boiler, occurrence of insufficient gas mixing and stratified flows in the boiler, fouling and corrosion of the boiler, continuation of fly ash utilization, and impacts on performance of the flue gas cleaning.…”

“…High uncertainties in both arenas may make investment in biomass co-firing technologies difficult to justify. Li et al (2012b) also discussed issues such as lack of financial incentives, uncertainty of fuel price/availability, lack of legislation support, public perception of co-firing biomass, and difficulty of getting permits.…”