Atmospheric Emissions and Energy Metrics from Simulated Clamp Kiln Technology in the South African Clay Brick Industry

Abstract

The quantification of atmospheric emissions from clamp kilns in the clay brick industry has met with limited success globally. The complex configuration of clamp kilns using coal or other carbonaceous fuels, as well as the uncertainty regarding kiln combustion conditions, has proven to be a hurdle in measurement of emissions and standardization of clamp kiln conditions. To enable measurement and quantification of emission and energy metrics, a model kiln was designed to simulate operating conditions and configuration similar to a transverse slice of a typical full-scale clamp kiln, but with a lower capacity (20 000 – 35 000 bricks per firing cycle). The model kiln design ensures the adequate confinement and extraction of flue gases with the aid of a bifurcated fan forcing the draft through a horizontal extraction stack where monitoring occurs. The model kiln design, which comprise two adjacent sealed sides and two partially enclosing and sliding galvanized steel doors, provides adequate spacing for ‘packing’ and ‘un-packing’ of bricks and sufficient oxygen for combustion, while still ensuring minimum losses of emission via the semi-enclosed sides. Concurrent firing and hourly monitoring of flue gases in the flue duct was conducted for fourteen batches of bricks over 8 – 14 days using varying brick products and energy inputs from eleven South African brick factories that utilizes clamp kiln as firing technology. The model kiln was tested for its suitability in firing bricks that are similar to conventional South African clamp kilns, as well as its effectiveness in the capturing and channelling of flue gases through to the stack vent where monitoring of the flue gases took place. Hourly readings are recorded for process parameters, SO2, NOx, NO, NO2, CO and particulate matter (PM) concentrations in the extraction stack. PM size-segregated mass measurement was conducted to produce PM1, PM2.5, PM4, PM10, and PM15 fractions. SO2 monitoring results were also compared to mass balance calculations, using the analysis of sulfur in the coal to indicate that the model kiln design is effective in capturing emissions and standardizing emission factors, as well as providing an effective energy analysis tool for clamp kilns. A statistical mean efficiency for the model kiln emissions capturing and channelling capacity was calculated from sulfur mass balance results of the batches that lie within 95% confidence interval of the assumed true mean (100%) to give 84.2%. Therefore, 15.8% of emissions were considered to escape from underneath the semi-enclosed sides. Final emission factors (mean ± standard deviation) were quantified as 22.5 ± 18.8 g/brick for CO, 0.14 ± 0.1 g/brick for NO, 0.0 g/brick for NO2, 0.14 ± 0.1 g/brick for NOx, 1.07 ± 0.7 g/brick for SO2, 378 ± 223 g/brick for CO2, 0.96 ± 0.5 g/brick for PM10; as well as 1.53 g/brick for hydrocarbons (calibrated to propane emissions) and 0.96 g/brick for PM15, PM4, PM2.5 and PM1. Various kiln technologies were ranked from lowest to highest potential for atmospheric pollution based on available emission metrics as follows: Zig-zag < Vertical shaft < South African Clamps < US coal-fired < Fixed chimney Bull’s trench < Tunnel < Asia Clamps < Down draft < Bull’s trench. Energy analyses indicate that a significant reduction of 0.9 MJ/kg (36%) in energy use could be achieved by the South African clamp kiln industry, thereby reducing cost of input, and significantly reducing the quantity of atmospheric emissions. In addition, chemical reactions and thermodynamic processes occurring in the firing chamber of brick kilns were qualitatively linked to the amount and type of pollutant emissions released at different periods during a firing cycle. The sensitivity of brick kiln emission concentrations and process metrics to these reactions and processes was utilized to proffer emission control measures. These measures are aimed at reducing energy consumption; improving the clay material processing and drying technique; monitoring chemical constituents of input materials in order to eliminate less favourable options; monitoring firing temperature to modify firing process; as well as altering the combustion and firing process in order to favour chemical and thermodynamic processes that will result in the release of lower emissions. Screening dispersion modelling results was additionally employed in recommending the extent of impact zones from the clamp kiln area for small kilns (500 m), medium kilns (1000 m) and large kilns (2000 m). A general reduction in most pollutant emissions was observed when the external fuel (coal) was replaced with a locally available alternative, propane gas. CO, CO2, NOx/NO and PM10 indicated 87%, 7%, 41% and 10% reduction in emissions respectively, during propane gas firing. SO2 emission, however, indicated a 19% increase, which may be attributed to lower energy consumption that alters the complex thermodynamic reactions in the model kiln. Only CO and NOx/NO emissions provided significant reduction in emission rates to support the notion that substituting the external coal with propane gas will result in significant reduction in atmospheric emissions. PM10 and CO2 emission rate do not provide significant reduction to validate this notion, while SO2 emission rate analysis is inconclusive and may require further research.