Rice Husk as a Substitute Fuel in Cement Kiln Plant

Abstract

A simple mathematical model of heat balance was applied to a cement kiln plant with a precalciner to estimate the effect of using rice husk as a substitute fuel for natural gas on specific heat consumption. Effects of the husk ash on the characteristics of the raw mix and clinker of ordinary Portland cement were also evaluated. Referring to Egyptian kiln plants data, it was found that the weight of rice husk required to supply fuel heat in the precalciner represented about 11-13% of the raw mix weight and about 90% of the total fuel weight (natural gas + husk). Specific heat consumption increased by about 3.7%, and the amount of combustion flue gases increased by about 20% when natural gas was completely substituted by rice husk. The use of rice husk as a substitute fuel in a cement kiln plant was accompanied by a marked decrease of lime saturation factor of the raw mix, a drastic decrease of tricalcium silicate and an increase of dicalcium silicate in the clinker. This data can provide the basis for the formation of a new type of cement such as high belite cement. The raw mix design was adjusted using pyrite as a correcting factor to keep the characteristics of the raw mix and the clinker similar to the factory data. KeywordsAlternative Fuel; Rice Husk; Natural Gas; Raw Mix Design; Clinker Mineral Composition; Raw Mix Parameters; Specific Heat Consumption Nomenclature ms1 = mass of the inlet raw mix to the preheater (kg) hs1 = specific enthalpy of the inlet raw mix to the preheater (kJ/kg) mf = mass of the fuel (kg) hf = low calorific value of the fuel (kJ/kg) hu = specific enthalpy of the fuel (kJ/kg) mA3 = mass of tertiary air from the cooler to the calciner (kg) hA3 = specific enthalpy of tertiary air from the cooler to the calciner (kJ/kg) mA5 = mass of secondary air from the precooling zone in the kiln (kg) hA5 = specific enthalpy of secondary air from the precooling zone (kJ/kg) mA6 = mass of secondary air from the cooler to the kiln (kg) hA6 = specific enthalpy of secondary air from the cooler to the kiln (kJ/kg) mG1 = mass of outlet flue gas from the preheater (kg) hG1 = specific enthalpy of outlet flue gas from the preheater (kJ/kg) ms6 = mass of clinker leaving the firing zone to the cooler (kg) hs6 = specific enthalpy of clinker leaving the firing zone (kJ/kg)  HR.c, HR.K = heat of reactions in the calciner and the kiln, respectively (kJ) Qw.p.c, Qw.k, Qw.c. = wall heat losses from the preheatercalciner, kiln, and cooler, respectively (kJ) Vaf = theoretical amount of fuel combustion air (Nm /kg fuel) Vgf = theoretical amount of fuel combustion gases (Nm /kg fuel)  = excess air factor for fuel combustion MCO2 = mass of evolved CO2 from calcination Cpg, Cpa = specific heat of flue gases and air, respectively (kJ/Nm 3 C) Tg, Ta = temperature of flue gas and combustion air, respectively ( C) h.f.o. = heavy fuel oil cli. = clinker LSF = lime saturation factor SIM = silica modulus AM = alumina modulus C3S = tricalcium silicate C2S = dicalcium silicate International Journal of Energy Engineering Apr. 2015, Vol. 5 Iss. 2, PP. 16-27 17 DOI: 10.5963/IJEE0502001 C3A = tricalcium aluminate C4AF = tetracalcium aluminate ferrite Borders of the heat balance zones, as shown in Fig. 2: [1] [2] = preheating zone [2] [3] = precalciner zone [3] [4] = bottom stage cyclone [4] [5] = heating zone of the rotary kiln [5] [6] = precooling zone [6] [7] = cooler