OCR Text |
Show the combustion and heat transfer adversely affects either process. The combustion terminates at the intermediate stage due to contact of burning gases with the surface of the solid particles. The heat transfer rate becomes lower than that of the rated owing to decrease of temperature difference between the combustion products and charge. In most of the asphalt plants of the Tatarstan Republic (Russia) the combination scheme of combustion and heat transfer processes is used to warm up the load. This results in great amounting, on the average, 23... 25 m 3 of natural gas per year, specific fuel flow per ton of asphalt. The authors assume that the development of high thermal-power density combustion chambers, in which combustion process completes within their space, is a solution of the problems listed above. The combustion should be as complete as possible in the combustion chambers. Gaseous fuel can be burnt using a single-stage scheme pre-mixing it with the air (or oxidizer) establishing, therefore, a homogeneous combustion of fuel-air mixtures. In such combustion, a flame front separates a fresh fuel mixture from the combustion products. The shortcomings of the scheme are the following: 1) the steady-state combustion may be established only within narrow limits of fuel concentration that are estimated using the excess air factor a ranging within ct=0.5...1.5, where a=GJ(L0G{), Ga and Gf are air and fuel flow rates, respectively, L0=\7.2 kg of air/kg of fuel is the stoichiometric ratio of natural gas; 2) the flameout upstream the fuel mixture m a y take place resulting in the break-down of the combustion chamber. Gaseous fuel combustion can also be established without pre-mixing, i.e., using a diffusion combustion scheme. In this case, the flame front separates the fuel gas from the air (or from other oxidizer). The drawback of the scheme is an increase of diffusion flame length while enriching the fuel-air mixture, that is, the mixture composition approximates the stoichiometric ratio (a=l). Conditions, at which the industrial combustion chambers operate, (temperature of combustion products at the outlet is less than 1773 K, a>1.5, and wide range of power regulation) are more appropriate for diffusion combustion. Therefore, our studies were aimed at establishing the combustion process that utilizes a diffusion mechanism. Let us discuss some mechanisms of N0X formation. Three NOx formation mechanisms in combustion products have been documented [3]: thermal, prompt and fuel. The "fuel" oxides are formed through the oxidation of the nitrogen present in the fuel. This process m a y hardly be controlled. Besides, the portion of the "fuel" oxides is small because the portion of the nitrogen in the fuel is also small. Emission of the "prompt" N 0 X depends on the chemical reaction rate: the higher is the reaction rate, the higher the emission of the "prompt" N 0 X is. The "thermal" N O x form at high, higher than 1800 K [4], temperature of the combustion products. As the duration of stay of gas at such temperatures increases, their concentration is also subject to grow. On the basis of N0X formation mechanisms, the followings to establish the combustion process resulting in the lowest level of N O x emissions can be recommended: • the combustion process must be carried out at local temperatures not higher than 1800 K; • the residence time of combustion gas at the highest temperatures should be minimal. 2 |