Cement Energy and Environment

Combustion conditions Optimum combustion conditions in cement kiln systems occur when kil n exit gas oxygen and carbon monoxide emissions m·c as low as possible. Stated another way, optimum combustion conditions occur when excess air is as low as possible and complete combustion still occurs. A kiln operating with low excess aiJ· may cause prutial combustion of fueL A kiln system operating with high excess air increases the heat loss in the kiln system ex it gases. In e ither case, the net effects are higher specific fuel consumption and lower clinker production . The effects of reducing kiln exit gas oxygen percentage on kiln production capacity, specific fuel consumption and secondary air temperature for a wet process kil n with a planetary cooler are presented in Fig.5. In this example, kiln exit gas flow was constant and the kiln fuel firing rate, kiln production and secondary air were allowed to vary. Effect ive co ntrol of excess air is accomplished when an optima l leve l of oxygen and carbon monoxide exist in the kiln system ex it gas . Optimal combu s ti o n conditions can be maintained by various means; for example, measuring oxygen or carbon monoxide in the kiln system ex it gases and adjusting kiln system gas flow rate accordingly. Typical install ation costs range from US$ 75 000 to US$ 300 000. The potential savings achievable by reduci ng 6 3 ~ 0 J) :; /) ~ .J 'J ..... -6 :.J '-' -o -o -9 ;a ~-12 <;l <1.) t; - 15 .s - 18 the kiln exit gas oxygen percentage for a wet process kiln system with a planetary cooler are shown in Table 5. Table 5. Potential savings of reducing kiln exit gas oxygenpercentage for wet process kiln system with planetary cooler. -: Before After Effect change change Kiln exit gas 3 l - 2 oxygen, % 0 2 ' Fuel consumption, net 1575 "1500 -75 kcallkg Production capacity, tpd 390 500 + 28% Annual production 330 129000 165 ()()() + 36 000, days/yr, tpa ...... "t.. ., Fuel cost, US¥[ 12.5.8 12:00 ~US$ 0.58 ....... •, ~ ~· - ~- '" ' Potential savings and benefits are as follows: • US$ 75 000/yr is saved in fuel costs based on a production of 129 000 tpa . • US$ 720 000/yr in additional savings or income can be realised from the addi tional production ac hieved. • The secondary a ir flow rate decreases thereby resulting in increasing secondru·y air temperature. • Lower oxygen levels reduce the oxidation rate of chains in the chain system of wet or long dry process kilns, thereby increasing the life of chains. • Social benefi ts include reduced pollution emission levels fo r nitrogen oxides per ton of clinker produced. _ Clinker production - Secondary air temperature - Specific fuel consumption 3.0 2.5 2.0 1.5 Oxygen in the kiln exit gases (%j 180 150 120 90 60 - 30 0 1.0 Fig.S Ef ects ofdecreasing oxygen in the kiln exit gases Alternative fuels G '0 c.. E ~ .... ·o; ~ -o c:: 0 ll <1.) "' .5 "' <1.) "' ro <1.) ..... ll ..:; In all the examples illustrated above, the obj ective was to reduce specific fuel consumption and increase clinker production. This example presents alternative fuels that may be fully or partially substituted for existing fuels to reduce fue l costs. Fuels used in some cement plants as either the ma in fuel or as a supplement include natural gas, fue l oils, coal, petroleum coke, tyres, domes tic refuse, rice husks, wood c hips and a wide range of waste solvents and other organic liquids. One of the first steps in evaluating an alternative fuel is to determine whether or not it is economically feasible. If it is, the next step is to evaluate the effects of using the alternative fuel. Items to review before using an alternative fuel are as follows : 13