Flue gas compositions

Products of Combustion For lean mixtures, the products of combustion (POC) of a sulfur-free fuel consist of carbon dioxide, water vapor, nitrogen, oxygen, and possible small amounts of carbon monoxide and unburned hydrocarbon species. Figure 27-12 shows the effect of fuel-air ratio on the flue gas composition resulting from the combustion of natural gas. In the case of solid and liquid fuels, the... 

Alternate calculations can be made using equations to calculate flue gas compositions and partial pressures of H2O and SO3 to calculate the dew point. The alternate method is shown in the source document also. 

A 2-value smaller than 1 means that there is an excess of fuel in the mixture. In this case the air/fuel mixture is called rich. If more air is in the mixture than needed for a complete fuel combustion (2 > 1) the term lean mixture is used. Ideally the combustion is complete at 2 = 1. Real fuel cannot be combusted without an increase in CO and soot at 2-values smaller than 1.05. Due to changing operation conditions, for example a soiled burner, wear of the nozzle or leaky flaps, change of gas quality or changes of temperature and air pressure in the ambient atmosphere, the air/fuel ratio and thus flue gas composition can change over time. In order to minimize the risk of intoxication (see also chapter 5333), explosion and pollution real (uncontrolled) fuel burners are adjusted to operate far beyond this limit in the excess (lean mixture) region. However, unfortunately effi-... 

To prevent the flue gas from exploding, we need to proceed rather cautiously. If we just block in the leaking process tubes and fuel-gas supply, the fuel gas content of the flue gas will gradually decrease. The air fuel ratio in the flue gas will increase until the flue-gas composition enters the explosive region. If the firebox refractory walls are still hot enough to initiate combustion, the firebox will now explode. 

The analyzers available for the detection of C02, CO, and excess 02 are discussed in Chapter 3. Excess air can be correlated to 02, CO, C02, or combustibles present in the flue gas. The most sensitive indicator of flue gas composition is CO. As shown in Figure 2.2, optimum boiler efficiency can be obtained when the losses due to incomplete combustion equal the effects of heat loss through the stack. These conditions prevail at the "knee" of each curve. Whereas the excess 02 corresponding to these knee points varies with the fuel, the corresponding CO concentration is relatively constant. 

Multiple measurements of flue gas composition can further improve boiler efficiency. 

Table 1.2 Approximate flue gas composition from the combustion of various paraffin hydrocarbon fuels (water-free basis).

In the theoretical studies, a stoichiometric model was employed to describe the overall combustion process and to examine the effects of the biomass composition on a fuel s combustion behaviour in term of flue gas composition, heat output and combustion temperature under ideal conditions. Incorporation with the correlations derived from the experimental work allows prediction of heat efficiency likely to be realised in a real combustion situation for stoker combustors. Good agreement between predicted and empirical results gives confidence in applying the technique to evaluate combustion performance of various biomass fuels without have to undertake combustion trials. 

The quantities of flue gas produced, flue gas composition, ash distribution and unbumt carbon in ash were determined for the combustion of the five biomass fuels (Table 3). Key observations from the combustion tests were ... 

In Eqn 3 the gaseous products of combustion are limited to CO2, Oj, N2 and H2O. The CO and CH4 contents in the flue gas can be derived from the empirical correlations derived earlier and when combined with the stoichiometric equation, these can be used to calculate heat losses due to incomplete combustion. So doing allows the performance of different biomass fuel combustion to be described for the tested combustion rig in terms of flue gas composition, combustion temperature and useful heat output. 

Fig. 3 Comparison of flue gas composition between the predicted and empirical results for the five biomass fuels. Pred = predicted Emp - empirical.

Four experiments were carried out in the fluidised bed combustion pilot plant with a 16-18 hour test time and whose operation conditions were modified in order to obtain an efficient combustion process. These two conditions yield lower pollutant emissions. Two operation variables were changed mainly in this study feeding rate and air excess percentage. These two parameters, in turn, affect the others parameters. Table 2 shows the combustion operation conditions set in the experiments. In Table 3, the flue gas composition during the four tests is shown. The sampling flue gas temperature was 150 C, the probe, cyclone and filter was all held at the same temperature. The CO level emitted in Test 2 with a 40-60 air excess percentage is lower than in the other experiments. 

Some general observations were also made. When steady self-supporting combustion was possible, the flue gas composition was similar to that obtained when a low sulphur eoal was burned (the NO coneentration was not measured). The optimum temperature for the burning of the slag was over 900 °C, its exact value depending on the chemieal character of the waste and in particular, on the proportion of volatiles still present. When fresh coal is burned in a fluidized bed, the volatiles burn first, and their oxidation influences the subsequent combustion of the chair [2 ]. When practically no volatiles are present, this cannot take place. 

If this hydrocarbon compound contains three atoms of carbon, determine its chemical formula if the flue gas composition on a dry basis is ... 

Pyrolysis of Pulp and Paper Sludge. The filter cake containing about 80% moisture was supplied for the cracking reactor without predrying. Heavy oil was fed to the incinerator as the auxiliary fuel. This is different from the case of municipal refuse, but the combustible gas composition and calorific value, flue gas composition and ash were similar to that of municipal refuse. The chemical analysis of combustible gas and flue gas are shown in Tables IX and X. 

Drews et al. pointed out that the rate of SO2 conversion to SO3 over SCR catalysts for NOx reduction with NH3 depends on many factors, viz., temperature, V2O5 content of the catalyst, operating load, O2 and H2O concentrations of the flue gas, and the NOx conversion. The catalyst can be deactivated by the formation of small amounts of ammonium sulfates, which can coat the surface and block the pores. The authors stressed that knowledge of the flue gas composition is essential for determining the lowest temperature at which the catalyst can be used efficiently. 

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... 

Bussman and Baukal [36] have shown that the ambient environment can have a significant impact on the flue gas composition, specifically the pollutants NO and CO. The environment variables include ambient air temperature, relative humidity, barometric pressure, and wind velocity. Climatic changes in ambient air temperature and humidity can significantly impact the excess O2 in a heater or furnace. If the excess air is not properly controlled, it can ultimately lead to dramatic changes in NOx and CO emissions. 

In many cases, the performance of a combustion system can be quantified in terms of a few parameters, such as oxygen concentration and temperature of flue gases, pollutant emissions, steam production, and so on. However, these data may not be enough to describe the actual state of the combustion process. A clear example is the case of multiburner chambers, where global values do not represent necessarily the conditions at individual flames. Even for a single burner, bulk parameters (e.g., flue gas composition) can only afford a limited characterization of the flame. As it was pointed out in the introduction, a detailed description would require a vast amount of information in terms of the distributions in space (and maybe time) of many physicochemical variables. Imaging techniques are most valuable tools in this respect, since information on spatial patterns can be very helpful to describe, or even understand, important characteristics of a flame. 

The instrumentation and control system of the furnace controls and measures important parameters such as fuel flow rate, flue gas composition, and furnace temperature. Below is a list of instrumentation and control equipment functions that will be discussed ... 

Combustion Flue Gas Composition The combustion flue gas products are calculated as follows and expressed in Nm /kg cli ... 

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