On the Burning of Sawdust in a MILD Combustion Furnace

Abstract

The purpose of this work is to extend the applicability of moderate or intense low oxygen dilution (MILD) combustion to solid biomass fuels. A laboratory-scale furnace fitted with a parallel jet burner was operated in conventional nonpremixed flame mode, and in MILD combustion mode, using either natural gas or pine sawdust particles. Sawdust with particle sizes in the range of 212−355 μm were injected into the furnace using either air, CO2, or N2 as a carrier gas. Measurements of in-furnace wall temperatures and exhaust gas emissions of O2, CO, NO x , and ash are presented, together with visual observations at the burner exit region. It was found, through detailed comparisons, that MILD combustion was established without air preheat for both gaseous and solid fuels, suggesting that the parallel jet burner system is suitable for MILD combustion. A 3-fold reduction in NO x emissions and an increase in CO were recorded during the transition from conventional to MILD combustion using natural gas. The optimal equivalence ratio (ϕ) to reduce both CO and NO x emissions, when burning sawdust, was determined to be in the range of ϕ = 0.71−0.75, with CO2 as the carrier gas, and at ϕ ≈ 0.75, with N2 as the carrier gas. Ash content analysis showed that the extent of carbon burnout was low, which is thought to be due to the relatively short furnace residence times.

A review of MILD combustion and open furnace design consideration

“…Dally et al (2010) injected sawdust into a MILD closed burner and achieved MILD state. Although the concept of MILD combustion has been extensively studied experimentally and numerically, the challenge to accurately model the MILD combustion regime is due to the homogeneous mixing field and slower reaction rates.…”

“…All the nozzles and exhausts were at the bottom of the furnace. This MILD combustion setup has produced data on various experiments, including fuel tests, flame tests, NO x tests, and heat exchanger tests (Christo and Dally, 2005;Colorado et al, 2009;Dally et al, 2002Dally et al, , 2004Dally et al, , 2010de Joannon et al 2009de Joannon et al , 2010Flamme, 2004;Li et al, 2011b;Lou et al, 2007;Maruta et al, 2000;Medwell et al, 2007Medwell et al, , 2008Mi et al, 2009;Mörtberg et al, 2006;Oryani et al, 2011;Park et al, 2004;Stankovic, 2006). MILD combustion technology is still not fully commercialized and well adopted in the furnace industry, thus it is very important to conduct substantial fundamental and applied research (Cavaliere et al, 2008;Danon, 2011;Li et al, 2011b;Parente et al, 2011aParente et al, , 2011b.…”

Effect of Air-Fuel Ratio on Temperature Distribution and Pollutants for Biogas Mild Combustion

“…A new combustion process is able to contribute to the increase of combustion thermal efficiency and reduce of the emissions. MILD combustion produces higher thermal efficiency due to re-cycling of Exhaust Gas by using exhaust gas recirculation (EGR) [7,[9][10][11][12]. Others call it flameless oxidation (FLOX) [13,14], High-Temperature Air Combustion (HiTAC) [15,16] and Colourless Distributed Combustion (CDC) [17,18].…”

Low-nitrogen oxides combustion of dried sludge using a pilot-scale cyclone combustor with recirculation

“…The reduced peak temperature leads to the low emissions of NO x . The flameless oxidation is also called moderate or intense low-oxygen dilution (MILD) combustion or high-temperature air combustion (HiTAC) (Wünning and Wünning, 1997;Stadler et al, 2009;Dally et al, 2010;Mei et al, 2014). The term used in this paper is moderate or intense low-oxygen dilution (MILD) combustion.…”

“…The NO x concentration in the exhaust gas was lower in the 1073 K air than in the 623 K air, whereas the maximum temperature of each flame was similar. Dally et al achieved MILD combustion of natural gas and sawdust without air preheating (Dally et al, 2010). Their combustion system was designed to provide strong recirculation inside the furnace.…”