Air requirement

We assume that the oxygen from the air (assumed to be 20mol% oxygen and 80% nitrogen for simplicity) and the methane react as follows  

The exergy of the ambient air is nearly zero, will be small for the compressed air flowing into the combustor compared to the exergy of the fuel, and can be neglected (when chemical transformations take place, the bulk of the exergy is usually due to the chemical component of the exergy). 

Standard Exergy Value of Methane at Different Conditions 

Source Smoot, L.D., Fossil Fuel Combustion A Science Source Book, Bartok, W. and Sarofim, A.F. (eds.), John Wiley Sons, Inc., New York, 1991. 

Species Number of Moles per Second Gas Mole Fraction, y  

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When using a compressed air or diffused aeration system, the SORT has to be converted to a standard volume of air required per minute. This conversion can be accompHshed through the equation... 

The insensitivity of adiabatic flame temperature to heat of combustion does not necessarily apply to the operational flame temperature, T, which is the flame temperature found in an actual furnace (remembering that this refers to some average temperature). The higher excess air requirements at higher C/H ratios coupled with greater thermal loads on longer flames generally results in markedly lower operational temperatures as the C/H ratio increases. 

Soda. Ash Roasting. Some of the first processes to recover selenium on a commercial basis were based on roasting of copper slimes with soda ash to convert both selenium and tellurium to the +6 oxidation state. Eigure 1 shows flow sheets for two such processes. Slimes are intensively mixed with sodium carbonate, a binder such as bentonite, and water to form a stiff paste. The paste is extmded or peUetized and allowed to dry. Care in the preparation of the extmdates or pellets is required to ensure that they have sufficient porosity to allow adequate access to the air required for oxidation. 

The secondaiy ejector systems used for removing air require steam pressures of 2,5 bar or greater. When the available steam pressure is lower than this, an electrically driven vacuum pump is used for either the final secondaiy ejector or for the entire secondaiy group. The secondary ejectors normally require 0,2-0,3 kg/h of steam per kW of refrigeration capacity,... 

This results in a significant drop in power consumption by greatly reducing the compressed air requirements. 

Theoretical Oxygen and Air for Combustion The amount of oxidant (oxygen or air) just sufficient to burn the carbon, hydrogen, and sulfur in a fuel to carbon dioxide, water vapor, and sulfur dioxide is the theoretical or stoichiometric oxygen or air requirement. The chemical equation for complete combustion of a fuel is... 

Figure 27-11 gives the theoretical air requirements for a variety of combustible materials on the basis of fuel higher heating value (HHV). If only the fuel lower heating value is known, the HHVean be calculated from Eq. (27-6). If the ultimate analysis is known, Eq. (27-7) can be used to determine HHV. 

FIG. 27-11 Gomtilstion air requirements for various fuels at zero excess air. To convert from kg air/GJ fired to Ih air/lO Btii fired, multiply hy 2.090. 

Higher temperatures also require more cooling and, as temperatures increase, the single can or annular combustor design becomes more attractive. The tubo-annular combustor has a more even combustion because each can has its own nozzle and a smaller combustion zone, resulting in a much more even flow. Development of a can-annular combustor is usually less expensive, since only one needs to be tested instead of an entire unit as in an annular or single-can combustor. Therefore, the fuel and air requirements can Be as low as 8-10% of the total requirements. 

The ehanges in the various reaetions are never eomplete therefore, the amount of air required and, henee, the quantities of gas eireulated, are greater than those eorresponding to the theoretieal reaetions. 

Process requirements. Are there speeial startup and shutdown eonsiderations sueh as eatalyst loading and unloading and minimum air requirements ... 

These ai e, of course, the limits for the air-free mixture. But remember, the original mixture, before we removed the air, had 2% O2. So the lower limit for the original mixture (with the air added back) is 25.4% + 2/0.209 = 35.0 (the increased lower limit means greater percentage of the combustible mixture in air or lower percentage of air required with the combustible mixture to cause combustion). It is logical that less air is needed since the original mixture (with the air added back) has a head start on combustion (with the contained 2% O2). 

Air Requirement 15 - 20 cfnt per 1,000 cubic feet of digester capacity is adequate. The air supplied must keep the solids in suspension this requirement may exceed the sludge oxidation requirement A dissolved oxygen concentration of 1 to 2 ppm should be maintained in the aerobic digestion tanks. 

Any oxygen contained in a mixture may be considered as though it were a part of the air required for the combustion, and the analysis of the flammable mixture should be converted to an air-free basis before the flammable limits are... 

Certainly, some workplace operations involving highly toxic and/or valuable materials can be controlled more effectively by LVHV ventilation than by conventional local exhaust ventilation. These situations represent opportunities to improve worker protection, recover valuable materials, and to reduce replacement air requirements. Designers of local exhaust ventilation systems should be mindful of such opportunities and take advantage of LVHV control methods. 

Theoretical air quantity The stoichiometric quantity of air required for complete combustion of a given quantity of a specific fuel. 

In this chapter, cycle calculations are made with assumed but realistic estimates of the probable turbine cooling air requirements which include some changes from the uncooled thermal efficiencies. Indeed it is suggested that for modern gas turbines there may be a limit on the combustion temperature for maximum thermal efficiency [2,3]. 

The results of calculations for the cooling air flow fractions in the first (nozzle guide vane) row of the turbine, ba.sed on the assumptions outlined in Section 5.2 for film cooled blading, are illustrated in Fig. 5.1. The entry gas temperature Tgi was taken as the combustion temperature Tcoi = Ty and the cooling air temperature as the compressor delivery temperature T2. The cooling air required is. shown here as a fraction of the exhaust gas flow, i.e. as ip/( 1 + ip), plotted against compressor pressure ratio and combustion temperature for an allowable blade metal temperature, Tpi = 800°C. Also shown are... 

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