Combustion air calculations

This technique is particularly useful in handling combustion calculations where the nitrogen in the combustion air passes through unreacted and is used as the tie component. This is illustrated in Example 2.8. 

Example 15.5 A gas, which can be considered to be pure methane, is to be used as fuel in a furnace. Both the fuel gas and combustion air are both at 25°C. Calculate the theoretical flame temperature if the methane is burnt in ... 

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. 

Solution First, calculate the mole fraction of water vapor in the combustion air. The partial pressure of water in the combustion air is given by ... 

Use GRI-Mech (GRIM30. mec) and a laminar premixed flame code to calculate the burning velocity of a methane-air mixture at 1.0 atm. Repeat the calculation, replacing the nitrogen in the combustion air with helium. Compare flame speeds and adiabatic flame temperatures. 

Here is a problem that came up on a sulfur recovery facility in Punto Fijo, Venezuela. The combustion air blower, shown in Fig. 28.8, was a fixed-speed, motor-driven centrifugal machine. The air intake filters were severely fouled. They had a pressure drop of about 8 in H/). The atmospheric vent valve, used to control the discharge pressure at a constant 12 psig, was 50 percent open. The unit engineer had been asked to calculate the incentive in electrical power savings that would result from cleaning the filters. 

One hundred mol/h of butane (C4Hio) and 5000 mol/h of air are fed into a combustion reactor. Calculate the percent excess air. 

Fig. 3 The efficiency decreases, when the combustion air ratio or the moisture content of the fuel increases. However, if the outlet temperature of the combustion gases is below the dew point of the water vapour, the efficiency calculated according to LHV lower heating value) begins to increase, when the moisture content of the fuel increases. Temperature of combustion air is 0 C, RH 60%, excess air ratio 1.4 and composition of dry fuel is CH i owOo.m).

The utilization of coal to produce gaseous fuels for heating and generating power as a supplement to the dwindling supplies of natural gas is a major research and development activity in the United States at the present time. Barriers to q nthetic fuel production are more economic than technological, but there are still many technical problems to be resolved. In a test of pilot plant coal gasifier with a production rate of 25.8 mVhr, the output gas with a composition of 4.5% CO2, 26% CO, 13% H2, 0.5% CH4, and 56% N2 is burned with 10% excess air. Calculate the Orsat analysis of the combustion product. ... 

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. 

Minimum fluidization velocities for particles in air calculated from Eq. (7.51) are shown in Fig. 7.10. Note that the dependence on Dp holds up to particles about 300 fim in size in many applications of fluidization, the particles are in the range 30 to 300 pm. However, fluidization is also used for particles larger than 1 mm, as in the fiuidized-bed combustion of coal. In the limit of very large sizes, the laminar-flow term becomes negligible, and VoM varies with the square root of the particle size. The equation for p > 10 is... 

It is also recommended that the experimenter thoroughly analyze what is being collected to ensure the necessary data will be available for the analysis that will be done. For example, if the thermal efficiency of a combustion process will be calculated, the experimenter needs to measure both the composition and the temperature of the exhaust products, among other variables such as the fuel flow rate and composition, the combustion air flow rate, the furnace pressure, and the furnace skin temperature. If the actual water content in the exhaust products is not measured, which is often the case, it can be calculated knowing the other components in the stream and the fuel composifion and flow rate. The furnace air leakage can be calculated based on the flue gas composition and combustion air flow rate. The point is that the experimenter should carefully check... 

The combustion air flow rate is also sometimes measured. Together with the fuel flow rate, the estimated exhaust gas flow rate can be calculated by using an O2 measurement in the exhaust stack to estimate air infiltration into the furnace (assuming that the measured O2 is higher than the calculated O2). 

While measuring the combustion air flow may not be necessary to calculate the exhaust gas flow, measuring only the O2 in the stack (either wet or dry) only tells how much air has gotten into the furnace. It does not tell where it came from so it is possible that the burners are running fuel rich with a large amount of air infiltration. Therefore, it is usually desirable to measure the combustion air flow to ensure the operating conditions of the burners. 

From geological studies to aerospace engineering, physical modeling has been widely used in the industry to study complex fluid dynamics where engineering calculations or computational fluid dynamics are deemed either unreliable (the former) or uneconomical (the latter). In the field of combustion, physical modeling is employed in studying flow distribution involving combustion air, over-fire air (OFA), and flue gas recirculation (FGR) as well as isothermal flows in combustion chambers of furnaces, boilers, heat recovery and steam... 

Natural and induction draft burners suck in combustion air directly from fheir surroundings and do nof have combustion air supply pipes where a flow rate meter could be placed. That is why the total flow rate of the combustion air can only be calculated retrospectively from the chemical composition of flue gas measured just behind the outlet from the combustion chamber. Therefore, when regulating the air supply, it is necessary to take into account the time lag between the change of the setting and the reaction of the flue gas analyzer, whereas the period corresponds to the residence time of flue gas inside the combustion chamber plus the reaction time of the flue gas analyzer. 

The fuel-saving rate, S, that represents the energy-saving effects, is obtained by preheating the combustion air. The fuel energy savings can be calculated as follows. 

The clinker cooler heat balance is exposed hereafter in Table 31.41. The shown secondary air temperature (955°C) has been calculated to balance the inlets with the outlets since it not possible to get reliable measures in the field. The cooler s efficiency, expressed as the ratio between the heat recovered as combustion air and the heat from the incoming material (clinker) is equal to 237/376 x 100 = 63%. Actual clinker cooler specific load is 42.98 tpd/m. ... 

Note Actually, there will be additional gas flow from the combustion air provided for burning auxiliary fuel. However, this does not enter into the basic fuel saving calculations since this air and this fuel would be used in a conventional boiler to generate steam if it were not being used in the CO gas recovery unit.)... 

Example 6.6 A pit fiirnace is being fired with natural gas and 10% excess air, and has a 2400 F (1589 C) flue gas exit temperature. The wall, hearth, and roof losses are calculated to be 1.55 kkBtu/hr. With cold air firing, there is a40°F (22°C) temperature difference from top to bottom of the ingots. Predict the corresponding temperature difference when using 1300 F (704 C) combustion air, and when using oxy-fuel firing. 

The ultimate analysis is used with the heating value of the coal to perform combustion calculations including the determination of coal feed rates, combustion air requirements, weight of products of combustion to determine fan sizes, boiler performance, and sulfur emissions. 

The combustion air and fuel gas flow to the gas burner were measured with flow meters. The flow rates were corrected for temperature and pressure pertaining to experimental conditions. The waste feed rate is calculated from the speed of the feeder motor. Two digital temperature indicators are used for continuously monitoring the temperatures in the furnace and exhaust duct, respectively. The furnace temperature is measured using a type R [Pt—Pt/13% Rhodium] thermocouple of wire size 0.01 inch while the exhaust gas temperature is measured using a type K (Cr—Al) thermocouple having 0.025 inch wires diameter. A microprocessor-based data acquisition and analysis system was used to measure the instantaneous and time-averaged temperature in the combustion chamber and exhaust duct. 

---