Precise Ratio Control Overcomes Environmental Impacts on Natural Gas Combustion
Many variables affect natural gas combustion, including environmental conditions. Corrective measures and enhanced controls can reduce the impacts of these variables and can provide optimum control continuously, maximizing the efficiency of industrial burners.
With the development of new extraction methods for natural gas, this abundant resource is poised to remain a significant source of energy throughout the world. In 2012, natural gas consumption in the U.S. exceeded 25 Bcf (billion cubic feet). Of this total, the commercial and industrial sectors accounted for nearly 40% of that usage. This fuel is used predominantly by steam and hot-water boilers in the commercial sector. Boilers, furnaces, ovens and kilns account for the majority of usage in the industrial sector.
Even though the combustion of natural gas is a well-established process, most control systems remain incapable of continuously providing the ratio of fuel to air that optimizes the release of thermal energy to the process. This lack of precise control leads operators to err on the lean side, resulting in large amounts of excess air. Excess air robs heat from the process and exhausts it to the atmosphere. An analysis of the many factors affecting combustion ratios provides an understanding of why operators rely on lean combustion.
Variables Affecting Combustion
Ideally, natural gas combustion can be represented by the stoichiometric reaction of methane and air (by mass):
CH4 + 7.5N2 + 2O2 → CO2 + 2H2O + 7.5N2 + heat
This equation indicates that one part methane requires 9.5 parts of air for complete combustion. However, natural gas is not pure methane. Depending on the geographical location, natural gas may contain significant percentages of higher order hydrocarbons (ethane, propane, etc.), inert gases, water vapor and dirt.
Realistically, the combustion of natural gas involves many variables that have direct impact on the required fuel/air ratio for complete combustion. The burner design itself controls how the fuel and air mix. Valves and dampers affect the volumetric ratio of fuel and air throughout the firing range. These components require regular maintenance to ensure correct operation over time. In addition to the aforementioned fuel composition variability, atmospheric conditions can have a significant impact. Variations in air temperature, pressure and water-vapor content impact the mass of oxygen available in a given volume of air.
Since combustion management involves the control of the volume of air and gas, changes in the density of air affect the mass of oxygen in the air and have a direct effect on the true ratio of fuel/air supplied to the burner. Temperature, relative humidity and barometric pressure changes result in direct changes in the mass of the oxygen in a given volume of air. To better understand atmospheric variations, consider the range of air properties of several major cities (Fig. 1).
An understanding of the impact of the temperature variations in a given region begins with visualization of the data. Figure 2 shows the relationship between temperature and air density. As air temperature increases, the density decreases. This means that there is a lower mass of oxygen in a given volume of hot air as opposed to cold air.
Examination of the impact of humidity is more complicated. By definition, relative humidity is the measure of water vapor in air at a specific temperature. Because the variation in temperatures would result in a very complex set of data, the density variation due to humidity in the air will be examined in general. For the U.S., the highest humidity levels occur in the summer months and the lowest levels occur in the late winter months. Figure 3 shows the relationship between the amounts of water vapor in dry air and air density. There is a slight decrease in the density as the amount of water vapor increases.
Barometric pressure changes occur relative to storm fronts. Low pressure and high pressure occur across a frontal boundary. In the Midwest, for example, a storm front moving from west to east has low pressure east of the front and high pressure west of the front. As the front moves through a region, a combustion process will tend to go lean after the front passes.
Combustion control systems for boilers and industrial operations use standard components and fall into a few well-recognized categories. Most burner manufacturers document the design of these systems in their literature.
On boilers, the typical system uses a single actuator controlling an air damper and a fuel butterfly valve through a single mechanical linkage. Industrial systems typically use a motorized air butterfly valve and an impulse fuel-pressure regulator. These systems require manual commissioning and adjustment. All of the control components vary only the volume of air and do not account for the variability in the density of the air.
Seasonal Change Causes Variation
Consider an operator in St. Louis. Seasonal changes can cause a variation of more than 12% in air density. Summer thunderstorms can lead to pressure changes of 5%. Since the control systems are only modifying the volume of air, changes in air density occur unseen. If this operator tunes the equipment in the summer when the air is relatively less dense, the burners will trend lean as the air density increases with the transition to winter. This wastes money because hot excess air is exhausted.
Conversely, equipment tuned in the dense winter air will trend rich with the approaching spring and summer, increasing the risk for soot formation and subsequent equipment damage. Weather fronts can cause temporary disruptions for many hours.
This variation due to seasonal change is experienced throughout the country, with some regions more severe than others. Experienced operators will often tune their equipment on the lean side, willing to reduce the overall combustion efficiency in order to protect the equipment from the negative effects of rich combustion.
Corrective Measures Can Solve Problems
Corrective measures are available for most gas-fired equipment. Boiler operators may install a parallel-positioning system to control both the air and gas independently through a complex setup procedure. This is a very expensive upgrade that does not compensate for environmental changes. In addition, they may add oxygen trim to provide closed-loop control of the combustion reaction. This additional enhancement continues to rely on imprecise valves and is generally cost-prohibitive on boilers less than 3,000 Bhp (boiler horsepower).
Industrial operators may choose to check their burners daily with a combustion analyzer and make manual adjustments, but this is very time consuming and costly. They may also operate in a high-fire/off firing scheme, with the assumption that the burners will operate at maximum efficiency at high-fire. This can lead to problems such as premature component failure and large temperature fluctuations in the process. The impact of environmental changes remains unaddressed. Lumec’s Automated Oxygen Control System (AOCS) is the only known system that can address these issues with an acceptable payback.
Achieving Optimum Air/Fuel Ratio Control
Environmental changes and geographical locations are not the only variables that have an effect on natural gas combustion. Efficient combustion requires a specific mixture of fuel and oxygen. Achieving an optimum air/fuel ratio is defined as the minimum amount of air necessary to complete combustion without producing harmful emissions.
Achieving this optimum ratio has traditionally been difficult, not only due to these environmental changes but also due to changes in the gas composition itself. Additionally, slow-reacting, imprecise components such as air dampers, gas regulators, old valve designs, dirty air filters and sloppy linkages can all reduce the chance to achieve the best air/fuel ratio possible. For industrial heating applications such as furnaces, ovens and kilns, the optimization of air/fuel ratios is critical. Increased thermal transfer in the process, with more heat to the load means fewer quality shortfalls. Additional benefits include longer burner-tube life, reduced maintenance costs (including downtime and reduced production) and reduced fuel consumption.
The AOCS from Lumec Control Products is an automatic, closed-loop, real-time solution for monitoring and managing the combustion ratio in gas-fired applications. With low-cost installation and a simple, rugged design, this system can be economically installed on boilers as small as 100 Bhp and is also suitable for most industrial direct-fired furnaces. IH
For more information: Contact Paul Luebbers, chief technology officer, Lumec Control Products, Inc., 8400 North Allen Road, Suite B, Peoria, Ill. 61615; tel: 309-691-IRIS (4747); e-mail: email@example.com; web: www.irisvalve.com. Paul is an industrial combustion expert with an MS in Material Science and Engineering.
1. Energy Information Administration Natural Gas Monthly, October 2013
2. Weatherspark.com, multiple pages
Another Unique Solution: Lumec AOCS for Boilers, Industrial Furnaces, Ovens, Kilns
Lumec Control Products, Inc., has approached this problem in a different way. Realizing that the root cause of poor control is the absence of a precise control valve, Lumec has developed a precise, fast, linear control valve specifically designed for combustion applications.
The IRISvalve™ uses an iris diaphragm control element that can make adjustments on the order of one-tenth of an inch of water column. Coupled with this superior flow-control technology is a rugged and long-life oxygen sensor capable of continuous in-situ oxygen measurement.
The Automated Oxygen Control System (AOCS) provides continuous monitoring of exhaust-stream excess O2 and adjusts the supply fuel or air to achieve the user-defined exhaust excess O2 (%) setpoints.
This technology, which can be applied to either the gas or air supply of the burner, provides real-time, continuous combustion management that compensates for all of the variables discussed in this article, including environmental conditions.
With these enhanced controls from Lumec Control Products, optimum ratio control can be achieved.