Biomass cofiring impacts on flame structure and emissions

Abstract

The impacts of cofiring biomass and coal on flame structure and NO emissions are investigated in the context of a swirl-stabilized, pilot-scale burner with straw and coal fired independently. The comparatively low energy density of biomass generally leads to higher transport air requirements per unit energy, increasing the momentum of biomass streams relative to an energy equivalent coal stream in burner feeds. Increasing the primary momentum in this manner alters the flow field and stoichiometry patterns of the burner. Detailed species concentration measurements as well as particle sampling were employed to investigate the flame structures of both high and low straw primary air flowrates. Large straw particles penetrate the internal recirculation zone at the high primary air flowrate, elongating the flame structure by forming fuel-rich eddies. The knees (relatively dense sections of straw) of the straw penetrated much further into the reactor, forming a secondary combustion zone. The NO emission was seen to decrease as the straw primary air flowrate increased because of increased numbers of fuel-rich eddies providing more reducing zone, where the fuel nitrogen from the large particles was released. It is also shown that the fuel-rich eddies served as reburning and/or advanced reburning centers, reducing the effluent NO emission further.

Introduction

Cofiring biomass with coal represents among the most cost effective and efficient means of producing renewable energy and reducing CO₂ [1]. Most commercial-scale investigations focus on fuel transportation, preparation, and feeding [2], [3]. Commercial work specifically regarding flame interactions between coal and biomass during cofiring is incomplete [3], [4]. When considering potential cofiring changes on flame structure, the method of biomass feeding stands out as important. Generally, twice the mass of biomass is required to substitute for a given mass of coal to maintain similar thermal input or firing rates. The amount of required primary air also increases because of a second fuel stream, though biomass requires less total air than coal, so the secondary requirements commonly decrease slightly. This results in an increase in primary momentum, potentially disturbing the optimized flowfield and stoichiometry of a given burner design [5], [6]. This work reports experimental flame structure results associated with varying primary particle and air momentum in an independently fed, swirled, cofired burner.

Past detailed gas-phase species, temperature, and velocity measurements in laboratory flames document swirl flame structures. Highly resolved data sets for pulverized coal flames under different firing conditions abound in the literature [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. Detailed data sets including biomass are less numerous. Recently, data have been obtained where coal and biomass were blended prior to the burner [18] and another data set including a pulverized wood flame [19]. Cofired data under conditions similar to the current project were obtained for independent injection of sawdust and coal [20].

This work adopts the same approach as is common for pulverized coal flames. A swirl-stabilized burner cofired with separate streams of coal and straw provides axisymmetric gas and particle data for several biomass primary air flowrates. The two cofiring conditions, one of a flame produced with a relatively high straw primary air flowrate, the other with a low flowrate, provide insight into the influence of a biomass feedstream on a traditional pulverized coal burner.