Excess air for combustion

Excess Air for Combustion More than the theoretical amount of air is necessary in practice to achieve complete combustion. This excess air is expressed as a percentage of the theoretical air amount. The equivalence ratio is defined as the ratio of the actual fuel-air ratio to the stoichiometric fuel-air ratio. Equivalence ratio values less than... 

The vapor pressure of hexane (CeHw) at —20 C is 14.1 mm Hg absolute. Dry air at this temperature is saturated with the vapor under a total pressure of 760 mm Hg. What is the percent excess air for combustion ... 

Plugged burners require more excess air for combustion, but too much excess air could lift off flame. Sulfur deposit is the common cause of burner plugging, and a solution is to prevent oxygen from entering the fuel gas system as it could combine with hydrogen sulflde in the fuel gas to form NH3CI. 

The primary air flow rate per jet necessary for smokeless combustion depends on the molecular weight and degree of unsaturation of the flare gas. Experience indicates that it varies linearly with percent unsaturates, from a minimum of 20 % excess air for a flare gas containing 0 % unsaturates to 35 % excess air for a gas containing 67 mol % unsaturates. Based on this relationship and a gas flow rate of 72.2 mVh per jet, the required primary air flow rate can be computed directly from the gas composition, or approximated conservatively from the following equation ... 

Biomass has some advantageous chemical properties for use in current energy conversion systems. Compared to other carbon-based fuels, it has low ash content and high reactivity. Biomass combustion is a series of chemical reactions by which carbon is oxidized to carbon dioxide, and hydrogen is oxidized to water. Oxygen deficiency leads to incomplete combustion and the formation of many products of incomplete combustion. Excess air cools the system. The air requirements depend on the chemical and physical characteristics of the fuel. The combustion of the biomass relates to the fuel bum rate, the combustion products, the required excess air for complete combustion, and the fire temperatures. 

The feeder and injector produced a thin pencil-like p.c. stream which passed down through the hot zone. The total combustion air supplied was approximately 3 liters/min for the bituminous coals, giving between 10 and 25 percent excess air for p.c. feed rates of 0.24 to 0.28 g/min. The flow and heat transfer conditions were modeled using the methods described by Pigford (16) for conditions of superimposed natural and forced convection at very low mass flow rates. Particle residence times were calculated by summing the centerline gas velocity and terminal velocity using Stokes s law (17). The error introduced using this method should never have exceeded 10 percent, even when pyrite was tested and particle Reynold s numbers approached one. The residence times thus calculated were found to be between one and two seconds. 

Another difference between thermal oxidizer burners and other types of burners is that they are frequently required to fire much higher quantities of excess air. The high amounts of excess air are required because wastes downstream of the burner may require significant quantities of air for combustion. Also, high excess air may be required to control the combustion chamber temperature since heat is not withdrawn through the chamber walls. In addition, thermal oxidizers sometimes employ burners that are designed to operate sub-stoichiometrically. 

Since liquid fuels and natural gas have better atomisation and stable flame characteristics, they need only a small amount of excess air for proper combustion. The volume of flue gases generated is therefore less as compared to the volume of flue gases produced when solid fuels are used. 

The residence time was computed by dividing the gas volumetric flow by the furnace volume. The calculated residence time for all types of waste was somewhat lower (<1 s) than that often used in many commercial incinerators (1-2 s). The actual fuel-air ratio was evaluated from the waste and air flow rates. The theoretical fuel-air ratio was obtained from SOLGASMIX calculation for combustion and used in the calculation of equivalence ratio and excess air for the test operating conditions. 

Results of equilibrium thermochemical calculations for the thermal destruction of nonplastic and plastic materials show the effect of material composition on the flame temperature, particulate emission, metals, dioxins, and product gas composition. The effect of waste composition has greater influence on adiabatic flame temperature, combustion air requirement, and the evolution of products and intermediate species. The combustion of waste in air produces higher flame temperature for 100% plastic than for nonplastic and mixtures. The 100% plastic requires lower number of moles of oxidant than 100% nonplastic and mixtures. Plastic produces HCl and H2S with concentration levels ranging from 1000 to 10,000 ppm. Emission of NO and NO2 from 100% nonplastic showed an increase with increase in moles of air while that from 100% plastic a slight decrease with increase in moles of air. The higher theoretical flame temperatures predicted with plastic waste corresponds to lower waste feed rate requirement of plastic at constant furnace temperature. This resulted in higher excess air operation with plastic waste and hence lower equivalence ratio. The gas residence time calculated for all the samples was found to be about 1 s. Variation of residence time more or less follows the same trend as excess air for all the samples. 

Heater efficiency can be affected by the excess O2 content or extra air for combustion and a high stack temperature. Inappropriate O2 content could be caused by lack of control, air leaks, and poor burner performance, while a high stack temperature corresponds to high heat loss in flue gas. A heater approach temperature, defined as the temperature difference between flue gas to the stack and heater feed inlet, could be caused by heater fouling in operation and by heater design. 

I feel the need to provide additional comments on excess air as many plants have an O2 reduction program. O2 reduction (or minimum excess air) must be built upon the basis of proper draft control. Minimum excess air for the fired heater can be obtained when it is reduced to the point where combustibles begin to appear in the stack. For modern fired heaters, this occurs at 8% excess air equivalent to 1.8% of oxygen level in the flue gas. However, practical constraints prevent achieving this minimum excess air in operation, and these constraints include variations in fuel quality, feed rates, and other process variables. Thus, operation without flame impingement sets the limit for practical minimum excess air. The optimal flue... 

Excess Air Pulverized coal requires more excess air for satisfactory combustion than either oil or natural gas. An acceptable quantity of unburned combustible coal is usually obtained with 15 percent excess air at high loads. This allows for the normal maldistribution of primary-air, coal, and secondary air. 

Ib/h of air is theoretically required for combustion in this boiler. To this theoretical requirement must be added allowances for excess air at the burner and leakage out of the air heater and furnace. Allow 25 percent excess air for this hoUer. The exact allowance for a given installation depends on the type of fuel burned. However, a 25 percent excess-air allowance is an average used by power-plant designers for coal, oU, and gas firing. Using this allowance, the required excess air is 200,000(0.25) = 50,000 Ib/h. 

Given the mechanisms and temperatures, waste combustion systems typically employ higher percentages of excess air, and typically also have lower cross-sectional and volumetric heat release rates than those associated with fossil fuels. Representative combustion conditions are shown in Table 11 for wet wood waste with 50—60% moisture total basis, municipal soHd waste, and RDF. 

Figure 4 illustrates the trend in adiabatic flame temperatures with heat of combustion as described. Also indicated is the consequence of another statistical result, ie, flames extinguish at a roughly common low limit (1200°C). This corresponds to heat-release density of ca 1.9 MJ/m (50 Btu/ft ) of fuel—air mixtures, or half that for the stoichiometric ratio. It also corresponds to flame temperature, as indicated, of ca 1220°C. Because these are statistical quantities, the same numerical values of flame temperature, low limit excess air, and so forth, can be expected to apply to coal—air mixtures and to fuels derived from coal (see Fuels, synthetic). 

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