| The most important processes to heat treatment is the preparation of controlled atmosphere. When the propane gas or propane-butane mixture is replaced with natural gas in heat treatment industry, the raw materials cost will be decreased and the increasing serious energy problems will be alleviated.Based on materials conservation and chemical equilibrium, a mathematical model on the endothermic gas preparation with gaseous hydrocarbon is established. The changing effects of the producing gas components are analyzed by this model, when the raw is changed. The results showed that some of the changes have happened for the content of the atmosphere components. Carbon monoxide and hydrogen are changed from 23% and 31% to 20% and 41%. The content of carbon dioxide is changed from 0.5～0.9% to 0.1～0.3% comparing to that of moisture is change from 0.3～0.8% to 0.1～0.6%. At the same time, the contents of carbon monoxide and hydrogen are unchanged despite of different temperatures. But carbon dioxide and moisture contents have an obvious fluctuation with changing temperature. According to the requirement of carbon content, the optimum temperature conditions for natural gas preparing atmosphere are obtained: 1050-1100℃. The thermal efficiency of the generator has also been analyzed based on the energy equilibrium. The thermal efficiency has been improved from 45～68% to 54～89% obviously when the natural gas is used.The creation of this thesis is that the mathematical model can beused to analyzing the components of controllable atmosphere prepared with all gaseous hydrocarbon. The theoretical basis and predicting tool, on the traditional raw gas replaced with natural gas in the heat treatment industry, are provided.
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