Air Emission Reduction from the Use of Alternative Fuels in Cement Production

Abstract

The brisk increase in recent global industrialisation and development has sourced similar demands for the provision of construction materials. Cement being a fundamental ingredient used, and the global rate of cement production is predicted to reach 6.1 billion tonnes in 2050. Cement manufacturing is a resource and energy intensive industry which utilises 9% of global industrial energy and is accountable for 5 per cent of carbon dioxide (CO2) emissions. Given the endothermic reactions required to produce 1 tonne of cement, it also consequently releases 1 tonne of CO2 into the atmosphere, propagating that future emissions from manufacturing are also set to double. With its effects towards climatic impacts receiving recent wide-spread attention, the agenda of avoiding CO2 emissions still requires immediate regulatory support and action to control a rising and irreversible baseline. Further to climatic effects, concerned health effects from pollutant exposure can include particulate matter (PM), oxides of nitrogen (NOx) and sulfur oxides (SOx). If manufacturing is not well engineered or emissions are not fully abated, various other air pollutants can also be released, including heavy metals, polycyclic aromatic hydrocarbons (PAHs), polychlorinated dibenzo –p dioxins and furans (PCDD/F) and polychlorinated biphenyls (PCB). Through research and experimental examination, the cement manufacturing process is identified with a great potential to reduce these pollutants and energy needs through the substitution of combustible waste fuels which contain sufficiently energy, namely alternative derived fuels (ADF). As the physical and chemical structure of ADFs may differ within and between material types, the formation, suspension, control and release of contaminants will largely dependent on a combination of fuel composition, process design and operation, and combustion conditions. The challenge, however, is that a change or increased influence on one or more of these attributes is shown to affect process stability (with upsets), heat loss or corrosion, increased emissions or a reduced production capacity (due to build-ups or blockages). Discovering process bottlenecks will pave the way for optimising waste co-incineration within cement manufacturing. This thesis investigated ADF co-incineration in several cement plants under normal conditions, and to identify factors which influence the formation of criteria air pollutants. The objective is to provide process data and changes to a pollutant’s unit mass emission factor (UMEF) can be minimal when compared to baseline emissions, and that the complete destruction and irreversible transformation of pollutants is achieved. The thesis also examines any impacts to the rate of clinker production and if any bottlenecks formed. In summary, the co-incineration trials of waste oil, carbon dust, waste solvents and shredded tyres within differing cement kilns were shown to have the minimal influence on baseline emission rates, or had significantly reduced pollutants’ UMEF. Specific to the waste oil trials, the reported levels were significantly lower than the baseline averages for total solid particulate (TSP), CO2, acid gases, volatile organic compounds (TVOCs), sulfur oxides (SOx), heavy metals, and polychlorinated biphenyls (PCBs). For the carbon dust trials, the results achieved less than the baseline averages for carbon monoxide (CO), PCDD/F, and PCBs. The co-incineration of waste chemical solvents resulted in lower emission levels than baseline averages for TSP, CO, TVOCs, heavy metals, polycyclic aromatic hydrocarbons (PAH), PCDD/F, and PCBs. For the burning of tyres, sample results showed to be less than baseline emissions TSP, CO2, SOx, PAH, PCDD/F, and PCBs. Furthermore, an increased rate of substituted fuel during the experimental also identified a consistent reduction to health-critical emissions of particulates, heavy metals, dioxins and dl-PCBs. The distribution of toxic isomers (TCDD/F and PeCDD/F) were shown to be predominate during waste oil, wood chips, and solvent trials. Whereas the use of TDFs consistently showed a lower toxicity contribution. The distribution of dl-PCBs toxic congeners showed PCB-126 (3,3',4,4',5-Pentachloro biphenyl) to be greatly present during the co-incineration of waste oil, wood chips, solvents and TDF trials. For each test burn, the rotary kiln presented the typical requirements for hazardous waste incineration, particularly high combustion temperatures, extended fuel-flame residence time, gas turbulence, stoichiometric mixing, thermal inertia, existing post-combustion pollutant techniques and no waste residual for further disposal. Of particular interest to this thesis is that even with the co-incineration of an increasing ADF%, monitoring has identified there to be no subsequent emission effects and that the key process parameters contributing to contaminant suppression included (1) precalciner and kiln fuel firing rate and residence time; (2) preheater and precalciner gas and material temperature; (3) rotary kiln flame temperature and residence time; (4) fuel-air ratio and percentage of excess oxygen; and (5) the mass flow of meal feed and clinker load. This thesis has shown the above various techniques and inputs are viable for suppressing the formation of air pollutants whilst providing the necessary combustion and calcination needs, and that regulatory agencies should encourage the technological innovations needed to meet air quality goals along with the co-incineration of alternative waste fuels.