Role of Primary Freeboard on Staged Combustion of Hardwood Pellets in a Fixed Bed Combustor

Abstract

In staged fixed bed biomass combustion, primary air is supplied beneath the fuel bed with secondary air then provided above in the freeboard region. For fixed bed configurations, the freeboard is further divided into a primary freeboard length (LI), which is upstream of the secondary air and a secondary freeboard length (LII), measured from the secondary air all the way to the exhaust port. Despite extensive research into fixed bed configurations, no work has been successfully completed that resolves the effects of changing LI on fuel conversion, both in the fuel bed and within the freeboard of batch-type biomass combustors. In this study, experiments on a 202 mm diameter and 1500 mm long batch-type combustor have been conducted to determine the effects of changing primary freeboard length over three secondary to total air ratios (Qs/Qt) and two total air flow rates (Qt). The impact of these conditions has been studied on (i) intra-bed fuel conversion, measured through burning rate (kg/m−2 s−1), fuel bed temperature (°C) and ignition front velocity (mm-s−1), as well as (ii) post-bed fuel conversion in the freeboard, expressed through freeboard temperatures and emissions (NOx ppm, CO2%, CO ppm, O2%). The fuel used throughout the above experiments was Australian hardwood pelletised biomass. Results show that changes to primary freeboard length over LI = 200 mm, 300 mm and 550 mm, or LI/D = 1.00, 1.48 and 2.72, respectively, affect both intra-bed and freeboard (post-bed) performance indicators. The highest values of burning rate, ignition front velocity and fuel bed temperature were observed for interim values of LI/D = 1.48 at Qs/Qt = 0.25 and Qt = 0.358 kg/m−2 s−1. Primary freeboard lengths of LI/D = 1.00 and 1.48 were found to have higher freeboard temperatures, NOx and CO2 as well as lower CO and O2 values as compared to LI/D = 2.72 at Qs/Qt = 0.50 and 0.75. Increasing Qs/Qt from 0.25 to 0.50 for LI/D = 1.00 and 1.48 initially increased freeboard temperatures, with an accompanying increase in NOx and CO2 as well as decrease in CO values. However, further increase in Qs/Qt to 0.75 lead to lower freeboard temperatures for all primary freeboard lengths.

Secondary air induced flow structures and mixing in a fixed bed combustor

“…The presence of stronger recirculation zones for Q s /Q t = 0.50 case can result in improved combustion characteristics. This observation is also supported by the previous work 9 conducted on wood pellets in fixed bed combustor in which higher freeboard temperatures and lower CO emissions were observed for Q s /Q t = 0.50.

Figure 3.Axial velocity contours for secondary air injection at h/D = 0.64: (a) left Q s /Q t = 0.18, center 0.25 and right 0.50, (b) left Q s /Q t = 0.06 and right 0.12.

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“…The presence of stronger recirculation zones for Q s /Q t = 0.50 case can result in improved combustion characteristics. This observation is also supported by the previous work 9 conducted on wood pellets in fixed bed combustor in which higher freeboard temperatures and lower CO emissions were observed for Q s /Q t = 0.50.…”

“…8 In the case of fixed bed biomass combustors, the main flow primary air, passing through the packed fuel bed is further supplemented in the downstream with secondary air, thus resulting in a configuration involving jets in a cross-flow. 9 The role of secondary air in biomass combustion becomes very important due to high volatile matter content of biomass, which requires a further supply of air at the second stage above the fuel bed. 10 Whilst a number of work have already been published on biomass combustion in a fixed bed on the current configuration recently, 9,[11][12][13] there remains ambiguity on the exact flow dynamics induced through the use of secondary air.…”

“…9 The role of secondary air in biomass combustion becomes very important due to high volatile matter content of biomass, which requires a further supply of air at the second stage above the fuel bed. 10 Whilst a number of work have already been published on biomass combustion in a fixed bed on the current configuration recently, 9,[11][12][13] there remains ambiguity on the exact flow dynamics induced through the use of secondary air.…”