Achieving Efficient Combustion (part 1)
The Importance of Oxygen
The chemistry of combustion is well researched and generally accepted. Efficient combustion of natural gas requires combining fuel gas and air in precise ratios. Stoichiometric combustion (a chemistry-based equation) occurs when fuel and air are perfectly balanced. Maximum thermal efficiency occurs with a small amount of excess air.
The natural gas combustion profile (Fig. 1) shows the interaction of certain constituents of interest occurring in a typical burner exhaust stream when firing at a unique, fixed rate under a given set of environmental conditions. The profile illustrates the relationships between exhaust stream oxygen (O2), unburned fuel (hydrocarbons), carbon monoxide (CO), carbon dioxide (CO2) and burner efficiency.
For any particular burner and firing rate, various inputs of excess air produce predictable and repeatable levels of CO2, CO, unburned fuel (if any) and combustion efficiency. Optimum excess air input results in the most desirable combustion, where CO2 is at its peak, CO is minimized to a safe level, fuel is completely burned and efficiency is at its peak.
The amount of CO, unburned fuel and other harmful emissions is strongly dependent on how well the burner mixes the fuel and air. For a high-performance burner, the maximum combustion efficiency typically occurs between 1-2% excess oxygen.
Optimum excess oxygen levels are achieved in the field by measuring exhaust components with a multi-gas analyzer and making adjustments to the fuel and air control devices.
Managing excess air input at optimum levels by measuring and controlling the resulting oxygen output is scientifically “closed-loop.” The input and output are linked directly by oxygen. When air and fuel are provided to a burner at rates that create the most efficient combustion, the burner is said to be operating “on-ratio.”
Another way to visualize natural gas combustion efficiency is the Available Heat Chart (Fig. 2), which shows the relationship between exhaust gas temperature, excess air/excess oxygen and percent available heat (percent of the total energy content of the fuel) to the process. The greater percent available, the more efficient the combustion process.
Percent available heat for a particular operating condition is found on Figure 2 by reading up from the field-measured exhaust gas temperature to the curve representing the excess air or O2 (the field-measured exhaust O2 content) and reading left to percentage available heat (AH).
This AH chart supports the analysis from the combustion profile and confirms the increasing heat loss/wasted fuel resulting from higher levels of excess air. The heat is lost because it is transferred to the exhaust gases that contain air not used for combustion. The opposite of this is also true. In other words, reducing excess oxygen in the exhaust stream reduces the temperature of the exhaust as well.
The AH chart can be used to estimate combustion efficiency gains by evaluating alternative levels of excess air introduced into the combustion process. Fuel savings are calculated by the formula:
% Fuel Savings = 100 x %AH Optimum - %AH Actual / %AH Optimum
For example, assume a burner is operating with an exhaust temperature of 400°F and 80% excess air (field-measured 10% exhaust oxygen). Available heat is 78.2%. If excess air can be reduced to 9.3% (2% exhaust oxygen), which will result in a temperature drop of about 80°F in a typical application, AH increases to about 84.6%. Fuel savings of 7.6% (100 x (84.6% – 78.2%)/84.6%) is realized.
Increasing available heat makes more heat available for transfer to the load instead of being exhausted. Less fuel is required to achieve target process temperatures, and exhaust temperature drops. In other words, when you capture more of the available heat, steam production is achieved with less fuel. This increases the fuel savings because the operating load is further reduced.
This example demonstrates the impact of controlling burner ratios to reduce excess air introduced into the combustion process. Lowering excess air yields significant improvements in energy use and fuel reductions.
Controlling combustion ratios to the optimal minimum excess air reduces fuel consumption, maximizes heat transferred to the load and reduces toxic CO emissions to safe levels. Measuring burner exhaust oxygen content and temperature allows for the assessment of efficiency-improvement potential and fuel savings. Evaluations should be made at all firing rates as optimum ratios typically vary across the firing rate profile.
Watch for part 2, “Common Challenges to Achieving Combustion Ratio Control,” next month.