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Show 3.2 Upgrading of Kiln Firing Systems Queensland Alumina in Australia is the largest alumina refinery in the world. They operate nine rotary kilns and one fluid bed calciner to process alumina and two rotary kilns to manufacture lime. Prior to 1990 the plant was oil fired and consumed over 350,000 tons of oil per year and sulphur dioxide emissions were a problem. In the mid 1980' s a proposal was developed to exploit the natural gas from a number of small gas fields in the centre of South Australia and on the Queensland border. To utilize this gas required that the gas fields be piped together and a pipeline laid to Gladstone on the coast, a distance of some 1,500 miles. The conversion from oil to natural gas would eliminate the sulphur dioxide problem and reduce C02 emissions, but introduces other problems for kiln operators, since conversion to natural generally involves some derating of the kilns owing to the lower net calorific value, higher exhaust gas flowrate per Btu, and poorer radiant heat transfer characteristics of gas flames. The plant had been constructed in stages from the 1960's and as a result of process optimization and de-bottlenecking the kilns were operating at, or close to, the limit of their capacity, hence no derating could be tolerated. FCT was therefore retained as consultants to investigate and advise on how the kilns could be converted to gas firing without reducing the output or significantly increasing the fuel consumption. 3.2.1 Minimizing the Effect of Changing from Oil to Natural Gas The rate of material calcination in a rotary kiln is proportional to the heat transfer rate in the burning zone (18). This takes place primarily as direct and indirect radiation from the flame and combustion gases (19). These radiative effects are mainly determined by the temperature and emissivity of the flame for a given system. To achieve the appropriate charge heating rates the flame must produce the correct heat flux. Too low a heat flux produces a high quantity of uncalcined material since insufficient heat is supplied to sustain the calcining reaction, whilst too high a heat flux produces overburnt, unreactive product. Thus, the heat flux pattern produced by the burner is critically important (20). Kiln flames are turbulent jet diffusion flames, with a jet of fuel and primary air issuing from the burner and causing the remaining combustion air (secondary air to mix with the fuel by jet entrainmen~, figure 9). The different fuel characteristics of natural gas and heavy fuel oil result in different flame lengths and heat flux patterns. The properties of the two fuels are given in table 3. The higher hydrogen content of natural gas means that it requires slightly more air for combustion than fuel oil. As can be seen from Table 3 the stoichiometric air requirement for a typical natural gas is 16.75 lb/lb compared to 13.68 lb/lb for the oil. Whilst the gas has a greater nett heating value per lb than the oil, implying a lower mass flowrate of fuel, it still requires additional combustion air of some 5% for every Btu (nett) of energy released. The increased air requirement and greater water formation which occurs with natural gas means that the volume of flue gases liberated will be higher when firing on gas compared to oil. |