Excess air ratio

Corrosion of metals by fuel ashes only occurs where the fuel ash contains a liquid phase. Temperatures at which the first liquid will form are inversely proportional to the oxygen partial pressure. Thus, when firing fuels at high excess air ratios, fuel ash corrosion occurs at lower temperatures than when firing fuels with low excess air ratios. 

Apart from the operational problems associated with closely matching the fuel and excess air ratio requirements and the ignition and combustion temperatures for any given steam demand and local atmospheric condition, the composition of the fuel, with all its variables, adds considerably to the puzzle of providing continuously nearperfect combustion. 

At present, the volume fraction of 02 in the flue gas is already being used in some cases in the field as a value for adjusting the excess air ratio in combustion processes. A control system based on monitoring the 02 fraction in the flue gas can help control compliance with a desired value. 

Several methods have been investigated to find correlations between physical properties of fuel gas mixtures and the excess air ratio to optimize the combustion procedure. In spite of the varying composition of natural gas it is said to be possible to control a heater system by measurements of the dynamic viscosity of the gas [7]. One explanation could be the correlation between Wobbe number and viscosity With increasing Wobbe numbers the viscosity decreases, and if the Wobbe number of a gas is known, the excess air ratio can be adjusted, resulting in an open loop control. 

The sensor needs an ambient temperature of 700 to 900 °C over a longer period of time to provide stable and reliable signals (see Fig. 3.25). Therefore, it is necessary to find a position in the combustion chamber, the temperature range can be maintained at all times (i.e. with variations in burner loads and excess air ratios). 

Where p is the excess air ratio used for the combustion. For gasoline and diesel engines, p> 1. 

Fig. 14.4 Comparison between experimented data (points) and modeling predictions (curves) for methane (CH4), ethane (C2H6), ethylene (C2H4), and acetylene (C2H2) oxidation in a flow reactor under very dilute, slightly fuel-rich conditions [148]. The excess air ratio X is about 0.9, and the residence time is of the order of 100 ms.

The SO3/SO2 ratio is important for the corrosive potential of a flue gas. Find, using appropriate software, the equilibrium composition of a flue gas at 1300 K, assuming the fuel consisting of 99% CH4 and 1% H2S is combusted with an excess air ratio of 120%. 

Assume that the combustion process occurs under well-mixed conditions. Use perfectly stirred reactor software together with the GRI-Mech mechanism (GRIM30. mec) to estimate the formation of NO in adiabatic combustion of CH4 with an excess-air ratio of 1.1... 

Use GRI-Mech (GRIM30. mec) and a laminar premixed flame code to calculate the flame speed and the flame temperature of a methane-air mixture at 1.0 atm, varying the excess air ratio (the air/fuel ratio normalized by the air/fuel ratio at stoichiometric conditions) from 0.7 to 1.4. 

Because a complete conversion of the fuel cannot be achieved in practice within the fuel cell, the SOFC stack can be treated like a power generating burner so as to integrate it easily into a system model. The cooling of the stack depends on the excess air ratio. 

The residual carbon contents at different axial locations of the combustor were measured in the pilot plant tests (Li et al., 1991), as shown in Fig. 18. These data show that axial variations in carbon content with temperature (from 810 °C-923 °C) are as a whole rather slight, but mean carbon content increases with decreasing excess air ratio. Besides, for excess air ratios greater than 1.2, the carbon content at the top of the combustor is somewhat less than that at the bottom, while for excess air ratio less than 1.2, the opposite tendency is evident. In conclusion, for this improved combustor, an excess air ratio of 1.2 is considered enough for carbon burn-out, leading to reduced flue gas and increased heat efficiency as compared to bubbling fluidized bed combustion. That is probably attributable to bubbleless gas-solid contacting for increased mass transfer between gas and solids in the fast fluidized bed, as explained by combustion kinetics. 

High excess air ratio will result in increasing sensible heat loss in the flue gas, while insufficient air supply means low combustion efficiency. To gain high heat efficiency, a rational excess air ratio needs to be specified. Figure... 

Fig. 32. Relation of combustion efficiency to excess air ratio (after Lin, 1991).

C) at a low excess air ratio Extrusion at 300°C mixing with liquid product recycled from the pyrolysis reactor thermal decomposition in the reactor catalytic cracking in a fixed bed reactor using a zeolite-based ZSM-5 catalyst at 400°C... 

The diesel engine is more attractive for conversion to producer gas as the reduction in power and efficiency is less compared to an Otto engine. This is due to the higher compression ratio of the diesel cycle and also the operation conditions with high excess air ratios which reduce the difference in the volumetric energy content of diesel/air mixtures and producer gas/air mixtures. 

To investigate flow conditions in the furnace chamber of wood him aces CFD codes have been applied in the past [1,2]. It could be shown, that mixing conditions between raw gas and secondary air still need to be improved to reduce the excess air ratio during the combustion of wood. 

It should be noted that the combustion tests were conducted with only primary air supply. Attempts were made to keep the feed rate and excess air as similar as possible between trial burns, however, the operation conditions were very hard to keep fixed, as the excess air was controlled manually and these biomass fuels do not have good flow characteristics due to their inconstant particle shape, uneven particle size distribution and high moisture content. Many studies related to incomplete combustion have focused on processes with two-stage air supply and have identified that excess air is a key factor influencing the formation of CO and CH (2, 4, 5. 13). It has also been shown that each wood furnace has a typical correlation between the CO-emissions and the excess air ratio regardless of fuel type (13). 

Based on a similar approach, a statistical analysis was conducted to determine whether correlations of CO and CH4 emissions could be established with a number of variables including fuel moisture content, fuel particle size distribution, fuel feed rate and excess air ratio. With the limited number of observations, the best model for CO was derived as a function of fuel moisture content (Fig, 1) for the combustor used for these studies. There was no evidence for correlation of the other variables. The measured CH4 contents also had an increasing trend with higher moisture contents of the fuels, but could not to be fitted in any correlation equation. 

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).

Excess air ratio was measured by the oxygen content of the flue gas. The excess portion of dry air volume is derived from the oxygen content of the flue gas and the stoichiometric oxygen demand. 

As far as the multi-fuel burner in the second conpartment is concerned, measurements were obtained when firing only sawdust and without any fuel supply on the grate. The purpose of this test was to estimate the burner s influence on the experimental results. The emissions recorded are given in Table 6 and with reference the results of the co-combustion tests (see Fig. 2), NO emission is increased, because of the significantly higher excess air ratio. 

The evaluation of the co-combustion behaviour was based on the emissions of CO, NO and SO, and the unbumt fuel content of the ash samples. The emissions of CO, NO and SO, measured at the stack, were converted at [mg/Nm, dry, 6% O,], and are shown in Figure 2, as a function of the excess air ratio. The corresponding results for the unbumt fuel content of the ash samples collected from the first conpartment of the combustion chamber and the cyclone are shown in Figure 3. 

Fig. 2 CO, SO2 and NO emissions measured during the co-combustion tests at the chimney, as a fiinction of the excess air ratio.

With higher excess air ratios there is a slight decrease in size but an increasing total number concentration. The increase of particle concentration was more distinct towards enhanced surplus air. 

The equipment for measuring and analysing emissions is shown in Fig. 1 and 2. The oxygen content "as measured" is 17.5 %, The excess air ratio is in the diesel fuel operation 3.3 and in the dual fuel operation about 6. 

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