Achieving Efficient Combustion (part 2)
Common Challenges to Achieving Combustion Ratio Control
Part 1 in this series described the chemistry of natural gas combustion and how excess air introduced into the combustion process affected fuel efficiency, emissions and thermal transfer to the load. The benefits of managing the ratio of air and fuel are many, yet industry struggles to control these ratios.
The control challenges are many and often interrelated. Common impediments include:
- Equipment limitations
- Tuning the combustion system at high-fire
- Operating below high-fire
- Oversized systems
- Component wear
- Changes in atmospheric conditions
- Variations in fuel hydrocarbon content
What is meant by equipment limitations?
Burner fuel and air trains have imprecise components such as butterfly valves, linkage-driven valve actuators, and components that exhibit performance degradation over time (e.g., combustion air blowers).
Specifically, butterfly valves are non-linear with low turndown capabilities (predictable flow control over a very small flow rate range); linkages are difficult to set up and become loose and sloppy over time; and air blowers become dirty and their output decreases.
Many burners also have limited turndown capabilities and varying degrees of fuel- and air-mixing effectiveness at firing rates below high-fire. These burners typically require different fuel:air ratios at different firing rates. Imprecise components make precise and repeatable control nearly impossible.
Why are systems set/tuned at high-fire?
This is a result of the imprecise flow control described above. Since the control devices have both precision and response limitations, they cannot be adjusted as if they are linear. An operator has to start somewhere, and by setting the components to provide maximum efficiency at maximum heat output, the operator gains confidence that efficiency is maximized when the process is burning fuel at the highest rate.
Since the control devices do not respond in a linear manner, however, they are incapable of delivering fuel and air in correct ratios for efficient combustion as the system turns down to lower firing rates.
Because of this poor control, operators will err toward overly lean (high excess air) at rates lower than high-fire. They do this to avoid the risk of running rich, which produces carbon monoxide and soot. Soot creates a thermal insulator, reduces heat transfer to the load and shortens equipment life. As a result, any operating time at less than maximum output has high excess air and results in wasted fuel. This can be seen in Figure 3.
How often do boilers operate at less than high-fire?
The operational profile of an ICI boiler is dependent on many factors. In general, a boiler in intermittent use or with a high/low firing control will initially fire at maximum output for a given period of time. The output is then decreased by the firing controller to a very low value until the steam demand is satisfied. Fully modulating boilers will operate most of the time in the middle of the firing range. Even boilers that are firing constantly rarely fire at maximum output because of the reasons provided below. Figure 4 provides a visual representation of the different firing controls used on most boilers. Since efficiency is lower when output is in the low-mid range, significant fuel is wasted most of the time.
Why are burner systems oversized?
Equipment designers are historically conservative when sizing heating components and then add robust factors of safety to sizing calculations. In ICI boilers, systems may be sized to accommodate the largest load plus an additional capacity for future load growth. Regardless of specific applications, these systems tend to be significantly oversized, resulting in operation at less than high-fire efficiencies.
How does component wear affect efficient combustion?
Wear causes linkages to become out of adjustment, which negatively affects the repeatability of fuel:air ratio control and causes the drifting away from pre-established targets. This can affect both the air and gas controls since both the gas butterfly valve and the air damper are typically driven using linkages coupled to actuator motors.
How do changes in atmospheric conditions affect combustion efficiency?
Efficient combustion is a reaction of an optimum ratio of a mass of fuel and a mass of air. Any change in the mass of air, and hence the oxygen content, due to changes in atmospheric conditions results in less-than-efficient combustion ratios.
Combustion blowers deliver a given volume of air. As air heats up in summer, the mass of air delivered by blowers is reduced because hot air is less dense. This results in a drift toward rich combustion. As air cools down in winter, the mass of air delivered to burners increases, resulting in lean combustion. The same effects are generated by changes in barometric pressure – higher pressure is more dense, resulting in lean combustion.
High humidity means that there is more water vapor mixed in the air. This results in air of lower density, which in turn causes rich combustion. Low relative humidity, such as dry winter air, is more dense and leads to lean combustion. Throughout the industrial Midwest, burners that are tuned in summer are set to operate with hot and humid air, which has a lower density and therefore contains a smaller mass of oxygen. If these burners are not re-tuned for winter operation, the heavier cold, dry air will result in very lean combustion. Conversely, burners that are tuned in winter will burn very rich in the summer. Unfortunately, since environmental changes are continuous, semiannual tuning results in off-ratio operation most of the time.
Why are variances in fuel hydrocarbon content an issue?
Hydrocarbon content in natural gas can be different from day to day. Natural gas contains various and multiple hydrocarbons and is usually 85-95% methane. The other hydrocarbons typically present require more air than methane, so pockets of other hydrocarbons will cause the burner to operate rich. Natural gas also contains pockets of water vapor and inert gases, which will cause the burner to trend lean.
Watch for Part 3, “How to Make Improvements in Your Combustion Efficiency,” next month