Article pubs.acs.org/EF
The Impact of Biomass Cofiring on Volatile Flame Length Melissa L. Holtmeyer,† Gengda Li,‡ Benjamin M. Kumfer,† Shuiqing Li,‡ and Richard L. Axelbaum*,† †
Department of Energy, Environmental & Chemical Engineering and Consortium for Clean Coal Utilization, Washington University in St. Louis, St. Louis, Missouri 63130, United States ‡ Department of Thermal Engineering, Tsinghua University, Beijing 100086, P. R. China ABSTRACT: The physical characteristics and composition of biomass can vary significantly from coal and from one fuel to another, and these differences can impact the structure of the resulting pulverized fuel flame, particularly the volatile flame. The length and location of the volatile flame, which is the zone dominated by the combustion of volatiles, is important not only to flame stability and length but also to the formation of pollutants such as NOx. In this work, the effects of biomass volatile fraction and particle size on the length of the volatile flame were investigated for cofired flames of pulverized coal and wood waste. The volatile flame length was measured experimentally in a 35 kW combustion facility via axial measurements of CO and CO2 made along the centerline. Numerical simulations were employed to aid the interpretation of the impacts of wood waste cofiring on the volatile flame. The length of the volatile flame zone was found to be sensitive to the location of volatile matter release and the amount of volatile matter released within the volatile flame zone. Large biomass particles, with high axial momentum and long heating times, break through the fuel-rich volatile flame zone, releasing volatiles downstream into an oxygen-rich environment. The delayed release of volatiles reduces the amount of volatile fuel in the fuel-rich region, leading to shorter volatile flames, which in turn augments breakthrough of particles before complete release of volatiles. On the basis of CO and CO2 concentration measurements, cofired flames with 20 wt % wood waste and 80 wt % coal were found to have 21% shorter volatile flames compared with coal-only flames. Numerical simulations verified that volatile reactions take place past the end of the volatile flame. To reduce the amount of particles that pass through the volatile flame before complete devolatilization, the wood waste particle size was reduced. The resulting volatile flame was 27% longer for the 20 wt % cofired case compared with coal-only volatile flames. Increased volatile matter content, characteristic of biomass fuels, leads to an increased volatile flame length when all of the volatiles are released in the near-burner region.
1. INTRODUCTION Pulverized coal flames have been optimized for large-scale power generation to minimize criteria pollutant emissions and attain maximum fuel conversion and boiler efficiency. Coal-fired power plants are facing new challenges due to concerns over CO2 emissions, new regulations for criteria pollutant emissions, and renewable portfolio standards (RPS). Biomass cofiring is a technology that can potentially be incorporated into existing power plants to reduce emissions and meet RPS. However, utilizing biomass fuels to supplement large-scale coal-fired power generation faces both infrastructure and combustion challenges. This article will focus on the combustion-related impacts on the flame structure when cofiring biomass fuels with coal. The composition of biomass can vary widely from one source to another and can differ significantly from that of coal, and thus, it can impact the flame structure, temperature distribution, and pollutant formation within the furnace. Pulverized fuel flames typically have two distinct combustion zones: a volatile reaction zone and a heterogeneous char burnout zone. The volatile reaction zone, or the “volatile flame zone” as it will be called in this study, is dominated by homogeneous combustion of the gaseous volatiles with air and is typically identified by a region of intense radiative emission enhanced by soot formation.1,2 The length of the volatile flame zone is important not only for determining the location of the maximum temperature and the highest radiative emission but also for the formation of gaseous pollutants and particulate matter. For example, the reduction of NO formed from fuel-bound nitrogen, which relies on © 2013 American Chemical Society
homogeneous reactions with nitrogen-containing species in a reducing atmosphere,3 can benefit from a longer fuel-rich zone. Furthermore, the formation of ultrafine particulate matter is influenced by the combustion conditions, including the oxygen concentration and the time−temperature history of the particle.4 The structure of the volatile flame is influenced by the burner geometry and fuel characteristics. In swirl-stabilized burners, the flame length decreases as the ratio of swirl to axial momentum (swirl number) increases.5 The burner design can also impact the volatile flame length by altering the residence time of particles in the fuel-rich near-burner zone.6 Fuel properties that impact the volatile reaction zone include fuel composition, moisture content, and particle size.7,8 Of particular interest to this study are the effects of volatile matter content and particle size. The goal of this work was to develop a better understanding of how the cofiring of biomass can affect the length of the volatile flame using both experimental and numerical methods. The impacts of varying volatile fuel fraction and biomass particle size on volatile flame length were characterized in this study. This understanding can be used to develop fuel preparation and/or burner design strategies that prevent undesirable combustion conditions leading to, for example, increased NOx emissions and incomplete burnout. Received: July 16, 2013 Revised: November 7, 2013 Published: November 8, 2013 7762
dx.doi.org/10.1021/ef4013505 | Energy Fuels 2013, 27, 7762−7771
Energy & Fuels
Article
Figure 1. Experimental setup. All dimensions are in centimeters.
2. EXPERIMENTAL METHODS
Table 2. Particle Sizes
2.1. Experimental Setup. Experiments were conducted in a cylindrical, horizontally fired 35 kWth combustor as described in a previous work.9 The combustion test furnace featured a 16.7 cm ID combustion section that was 2.43 m in length followed by a 37 cm ID burnout section that was 120 cm in length (Figure 1). The secondary oxidizer (SO) flow was introduced with both axial and tangential components, creating a swirling flow to assist in flame stabilization. Pulverized coal and wood waste were fed to the primary oxidizer (PO) stream using separate volumetric screw feeders (K-Tron and Schenk AccuRate, respectively). The two feeder outlets were connected to the PO with an eductor, which entrained the coal and wood waste particles into the PO air stream and was contained within an enclosure maintained at atmospheric pressure. The air flowing into the enclosure of the eductor was controlled and measured. The wood waste was comingled with coal in the PO before entering the combustion chamber for all cases included in this study. No buoyancy-induced asymmetries in the flow field were observed. In all of the experiments, conditions were set such that the thermal input was 18 kW. Sub-bituminous Powder River Basin (PRB) coal and wood waste from a local sawmill were utilized as fuels. Sieve, proximate, and ultimate analyses are provided in Table 1. The higher heating value (HHV) is
sieve analysis (wt % retained)
PRB coal
wood waste
29.7
20.2
7.5 43.4 49.1
0.6 84.5 14.9
69.51 4.61 17.02 0.97 0.4
49.28 5.79 44.14 0.15 0.05
PRB coal
wood waste
18 mesh 30 mesh 40 mesh 50 mesh 70 mesh 100 mesh 200 mesh
0
