NATURAL GAS SAVING BY REPLACEMENT THE LAST FOR PROCESS GASES WHILE HEATING MIDDLE AND HIGH TEMPERATURE FURNACES. PART 1. INFLUENCE OF LOW-CALORIE GASES CHARACTERISTICS ON FUEL FLOW RATE IN FURNACES
In the paper the gas fuel classification has been advanced basing upon Eurostat’s criteria. The category of «Gas fuels» includes the natural gas (NG) and derived or recovered gases: blast furnace (BFG), coke-oven (COG) and the gas works gas (GWG) but doesn’t include the biogases. We unite the most wide-spread of mentioned notions like BFG and COG in group called «process gases» PG. The thermodynamic analysis of application the low-calorific gas fuels and fuel mixtures in the industrial furnaces has been carried out by using the ideal gas equation. The main heat engineering characteristics of the process gases: theoretical combustion temperature TT and combustion heat Ql— have been calculated grounded upon general enthalpian attitudes for any fuels and in dependence on composition of the mixed fuels: BFG with natural gas or BFG with COG. Evaluation of opportunities the natural gas saving by transfer from NG as a fuel for mixed NG + PG fuel by heating the furnaces has been performed. The technique of definition the substituting fuel flow rate has been proposed in frame of author’s methodology of fuel interchangeability. The problem of saving or over expenditure the NG flow rate along with change of available and of combustion heat for mentioned gas fuel mixtures (NG with BFG and COG + BFG) has been studied and analyzed. The methodology of the fuels replacement has been advanced. Instead of traditional approach with the basic condition of conservation the heat flux introduced into the furnace by any fuel combustion, the novel condition of conservation the useful heat absorbed by the furnace charge is proposed by author (B. Soroka). It means an assumption the condition of invariable flux of the useful total enthalpy to be attained in case of fuel replacement and account of the furnace (fuel utilization) efficiency. An analysis of fuel saving by flue gases heat recovery has been fulfilled for the case of preheating the initial combustion components: an air or/and BFG. It has been stated that the role of fuel preheating for NG saving is increasing with respect to opportunities of the proper impact of an air flow as the share of BFG in the mixture of fuel gases will grow. The calculations have been carried out for the cases of furnace operation temperatures of 800 and 1000 °C by conditions of cold initial combustion components: fuel gases and an air-oxidant (25 °C) and under preheating one or both components (250 and 400 °C). It was stated that saving of NG is increasing by enhancement the portion of BFG in fuel mixture with NG while the working (furnace operation) temperature is of moderate level. For eligible cases the required rates of heat energy fluxes: for available heat (chemical and physical (sensible) energy of initial combustion components: fuel and air-oxidant) and for combustion heat — have been evaluated. The higher is the share of BFG in fuel mixture with NG the bigger are the required heat fluxes resulting in over expenditure for the last values in comparison with heat consumption for the cases of clean natural gas using by furnace operation while lowering the NG portion in mixed fuel gas. The calculations on replacement the natural gas for mixture of COG with BFG have been carried out by means of evaluation an available heat content and of combustion heat through the values of excess total enthalpy flows of initial combustion components: gas fuel and an air-oxidant. The similar procedure on comparison the combustion heat for mentioned fuels has been fulfilled as well. Bibl. 21, Fig. 3, Table 2.
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