Preheating of combustion air

Avoid cold blowing effects through preheating of combustion air... 

The flue gas generated in the tubular reformer is used for preheat of reformer and prereformer feed, natural gas preheat, steam superheat and preheat of combustion air. The synthesis gas generated in the tubular reformer is cooled by high-pressure steam generation (4), preheat of boiler feed water and reboiling in the distillation section (5). 

Natural draft burners are supplied with combustion air from their surroundings and they do not allow for fhe preheating of combustion air. They utilize furnace nafu-ral draft (stack effect) and often simultaneous ejection effect of fuel streaming from burner jefs under a high speed. This is a rather frequent method of combustion air supply and is used for bofh gas fuel and liquid fuel burners (and possibly for combined burners). The burners are relatively difficult to operate under a low excess of air since the amount of supplied combustion air can only be controlled by a chimney damper or in some cases by flap valves on individual burners, which is rather demanding on the operators. Due to complicated control they are predestined to be used in operations with constant heat consumption without any sudden changes of heat supply, as it would cause product quality deterioration that is, process furnaces in chemical and pefrochemical industries [2]. 

In drying processes using flue gas as heating medium preheating of combustion air and, if necessary, product by exhaust gas. 

It is possible to reduce the steam production from a hydrogen plant based on tubular steam reforming [208], Introduction of a prereformer with reheat increases the thermal efficiency for reforming from 50% to about 60%. Another part of the flue gas heat content can be used for preheating of combustion air. 

Due to the high outlet temperatures of the reformer, only about 50% of the fired heat is absorbed by the reformer tubes. The so-called convection zone follows the radiation zone. Here, the energy of the flue gas, which is still up to 1000 °C hot, is used for the preheating of feedstock, for the generation of steam and optionally for the preheating of combustion air. Nowadays, steam reformers with up to 1000 tubes and a hydrogen capacity of up to 300 000 m h are used in technical plants [5.21]. 

Another type of combustion unit operates at about 1600°C to produce a molten slag which forms a granular frit on quenching rather than the usual ash. The higher operating temperature is obtained by preheating the combustion air or by burning auxiUary fuel. 

Smelting. The fuel suppHed to the reverberatory furnace is in the range of 5—6 GJ/t (4.7-5.7 x 10 Btu/t) concentrate. Steam produced in the waste heat boiler is equal to ca 60% of the energy suppHed by the fuel. The additional heat recovered from the exit gases in the recuperator to preheat the combustion air is equal to ca 10% of the energy from the fuel. Hence, the heat recovered from the furnace is equal to ca 70% of the heat from the fuels. 

Cracking reactions are endothermic, 1.6—2.8 MJ/kg (700—1200 BTU/lb) of hydrocarbon converted, with heat supplied by firing fuel gas and/or fuel oil in side-wall or floor burners. Side-wall burners usually give uniform heat distribution, but the capacity of each burner is limited (0.1—1 MW) and hence 40 to 200 burners are required in a single furnace. With modem floor burners, also called hearth burners, uniform heat flux distribution can be obtained for coils as high as 10 m, and these are extensively used in newer designs. The capacity of these burners vary considerably (1—10 MW), and hence only a few burners are required. The selection of burners depends on the type of fuel (gas and/or liquid), source of combustion air (ambient, preheated, or gas turbine exhaust), and required NO levels. 

Incineration with Heat Recovery Heat contained in the gases produced from the incineration of solid wastes can be recovered as steam. The low-level heat remaining in the gases after heat recoveiy can also be used to preheat the combustion air, boiler makeup water, or solid-waste fuel. 

In these calculations, the fuel and combustion air were both at the standard temperature of 25°C. If the temperature of either had been below 25°C, then AllK would have acted to decrease the theoretical flame temperature. If either had been above 25°C, the effect would have been to increase the theoretical flame temperature. One energy conservation technique sometimes used in furnace design is to use waste heat to preheat the combustion air. This has the effect of increasing the theoretical flame temperature, and as will be seen later, increases the fuel efficiency. 

The furnace is fired by gag from the producer to 500—550 °C. The arch of the muffle is heated first and afterwards the flue gas enters the flues below the bottom. Before entering the stack the flue gases preheat the combustion air in a recuperator. 

Other measures, involving more additional hardware and engineering work, are introduction of combustion air preheating and reducing the primary reformer load. 

Air Preheating. The majority of the gas burners installed in cracking furnaces are natural draft burners. As combustion air is introduced at ambient conditions, a part of the heat released from the fuel has to be used to heat up the combustion air to the fire box temperature. Therefore, preheated air reduces the fired duty. Figure 6 shows the fuel consumption as a function of combustion air temperature. For example, preheating combustion air from 0° to 100 °C reduces the fired duty from 100 to 95.5%, which means savings of 4.5% fuel. 

Table 1 contains characteristic data and dimensions of the pilot plant. The reactor is manufactured with stainless steel and is insulated. The wann up is carried out with electrical preheating of all air streams and by combustion in both zones. The whole warm up lasts about 4 hours. An oil feeder was installed into the riser, which gives the possibility to change the temperature level of the system without varying other operation parameters. With this installation parameter studies can be carried out very easily. 

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