The composition and heating values of fuel gas can vary from one moment in time to another. Using a Coriolis flow meter to obtain a mass flow measurement, rather than relying on a standard volume flow measurement, allows for more precise control of the air-to-fuel ratio, which improves efficiency, reduces emissions, minimizes downtime, and increases safety.
Every power generating facility strives for smooth, consistent power output by avoiding shutdowns or upsets as much as possible. However, variations in fuel gas are often unavoidable because of different suppliers or different sources from which the fuel gas is received. These variations will impact safety, as well as efficiency and emissions of turbine generators, fired heaters, and engines alike.
The air-to-fuel ratio (AFR) mix control that feeds combustion requires continuous monitoring and control to maintain safe levels. If the level of oxygen present in the combustors is too low, this will result in unburnt hydrocarbons that represent both a loss in fuel efficiency and the potential for explosion. Too much air in the mix will introduce a higher volume of nitrogen, which also reduces heating capacity and can increase NOx emissions (Figure 1). Any excessive deviation from the AFR setpoint can cause a trip in the system and interrupted service. It should be noted that, although a very conservative setpoint can help prevent risks to safety, such an approach is often inefficient and can contribute to an elevated emission of pollutants.
1. Example showing 20% reduction in NOx by reducing excess oxygen from 6% to 4%. Courtesy: Emerson
To optimize combustion control, which is needed to ensure safe, smooth, and consistent operations, requires live measurements. The best process variables to use for control systems input turns out to be mass flow, and where possible, energy flow. These measurements can be complicated in situations where composition and heating values of the fuel gas vary.
Maintaining consistency and safety is the goal but is hindered at times by some unique challenges, which will be explained further below before different technology solutions are presented that can optimize efficiency and improve safety.
Challenges of Gas Energy Measurement
The two main challenges that can be overcome by moving a setpoint closer to optimum and safe operations are better fuel efficiency and lower emissions. Measurement is the primary hurdle that must be overcome, regardless of gas composition.
The first challenge to measurement is that the gas energy content might be unknown due to a composition measurement error. This is not uncommon, because fuel composition changes over time and regular sampling is needed to know the correct composition. When this is a manual process, it can be prone to errors due to contamination of the sample or poor equipment. In addition, there is the possibility of errors due to the composition measurement method and calibration of equipment used in sampling.
The second challenge often encountered in power generation is that the gas energy content is unknown once it reaches the combustion chamber because of the distance it has traveled from the composition measurement point. When the gas content or other composition measurement is made far from the combustion point, a change in the fuel source, which might occur, will not immediately arrive at the combustion point. There will be a lag time in these cases before the measured composition change reaches the AFR control and the combustion point. Similarly, if the composition of the fuel gas varies continuously, the energy content at the point of combustion will never match the measured value precisely if it is far downstream.
In each of these cases, the efficiency and safety of the process can and often does suffer if not addressed effectively.
Measurement Solutions with Mass
There are several solutions available today that can not only mitigate these challenges, but can also open the door to greater flexibility in accepting fuel gas from different sources.
Using a mass flow measurement, obtainable with a Coriolis flow meter, instead of standard volume flow measurement, allows for more precise control of the AFR. Because energy varies more by standard volume than by mass, energy content measurement errors are generally smaller if converting a mass measurement to energy than when converting a standard volume measurement to energy. This difference between mass and standard volume alone can dramatically reduce errors in heating value and energy flow caused by a slow update rate of composition measurement data, errors in composition measurement, and differences in composition caused by transit time between the sampling point and the combustion point.
Another option is to install a local specific gravity meter (SGM) and/or an energy content meter (such as one comprised of the combination of a mass flow meter together with an SGM). The SGM can be used in lieu of or in combination with a composition measurement device, such as a gas chromatograph (GC), because an SGM by itself can render a heating value without the need for a separate composition measurement.
The SGM does this by applying the following relationship between the heating value in Btu per standard cubic foot (scf) and the specific gravity, which can be derived from the American Gas Association (AGA) Report No. 5 Natural Gas Energy Measurement (Figure 2). This value can also be converted into units of Btu per pound-mass by dividing by the base density in units of pound-mass per scf, which is also known from the SGM.
2. Energy content of individual pure components from the American Gas Association (AGA) Report No. 5 Natural Gas Energy Measurement. Courtesy: Emerson
When an SGM is combined with a mass flow meter, the result is a measurement system that can output flow rate to the AFR control in units of the energy that is being carried by the incoming fuel gas (such as Btu per minute). With energy flow rate as a control input, the best AFR control for combustion can be achieved.
To further improve accuracy of energy flow measurement, consider the flexibility and benefits of installing a local energy measurement system, such as the one just described, closer to the combustion point. The ability to locate the energy content input measurement close to the AFR and combustion point helps resolve the variations in combustion caused by:
- ■ Transit time between the sampling point and the combustion point.
- ■ Slow composition measurement update rates from the last GC or at the source.
- ■ Errors in composition measurement at the source or last GC.
Alternatively, to solve the problem of errors caused by fuel gas transit time, it is possible to use a calculation and adjustment for transit time of the fuel gas by applying the closest composition measurement point results only after accounting for the transit lag. One way to do that is by characterizing the total volume of the pipeline between the source of the closest composition measurement and then using the flow rate measurement to determine when that volume of gas has passed from the composition measurement point to the AFR control point.
When implementing any or all of the above solutions—and especially just by switching to a mass-based measurement over a volume-based one—improvements will be seen in increased output efficiency, considerably less downtime, reduced emissions, and greater safety.
The Right Technology to Aid Optimization
To ensure a safe and optimized system of power generation using fuel gas, applying the right technology makes a difference. Coriolis flow meters allow for an accurate mass flow measurement that helps reduce the measurement errors caused by either variations in composition of the fuel gas, or errors caused by distance and calibration. The Coriolis flow meter selected is important in providing the best possible mass-based measurement when switching AFR control to mass versus volume measurement. The increased accuracy gained by that switch ensures greater optimization of the process and a reduction in emissions. Combined with a local SGM measurement, the energy measurement system approach offers the best possible solution for the application.
In general, installing the right technology improves a facility. It provides greater automation of processes, which in turn improves efficiency, which helps meet regulatory compliance standards that govern power generating plants. ■
—Marc Buttler is the Oil and Gas Application Innovation Director at Emerson for Micro Motion Coriolis flow meters.