Axial concentration profiles and N 2O flue gas in a pilot scale bubbling fluidised bed coal combustor

Abstract

Atmospheric Bubbling Fluidised Bed Coal Combustion (ABFBCC) of a bituminous coal and anthracite with particle diameters in the range 500–4000 μm was investigated in a pilot-plant facility (circular section with 0.25 m internal diameter and 3 m height). The experiments were conducted at steady-state conditions using three excess air levels (10%, 25% and 50%) and bed temperatures in the 750–900 °C range. Combustion air was staged, with primary air accounting for 100%, 80% and 60% of total combustion air. For both types of coal, virtually no N 2O was found in significant amounts inside the bed. However, just above the bed-freeboard interface, the N 2O concentration increased monotonically along the freeboard and towards the exit flue. The N 2O concentrations in the reactor ranged between 0–90 ppm during bituminous coal combustion and 0–30 ppm for anthracite. For both coals, the lowest values occurred at the higher bed temperature (900 °C) with low excess air (10%) and high air staging (60% primary air), whereas the highest occurred at the lower bed temperature (750 °C for bituminous, 825 °C for anthracite) with high excess air (50%) and single stage combustion. Most of the observed results could be qualitatively interpreted in terms of a set of homogeneous and heterogeneous reactions, where catalytic surfaces (such as char, sand and coal ash) can play an important role in the formation and destruction of N 2O and its precursors (such as HCN, NH 3 and HCNO) by free radicals (O, H, OH) and reducing species (H 2, CO, HCs).