Firing and Drying / Resource Management

Measuring Fuel Energy

April 1, 2011
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The continuous monitoring of the calorific value of mixed gaseous fuels is on the rise.

Table 1. Calorific Values of Pure Fuel Gases

It is becoming increasingly important to measure fuel energy, which is formed by complex mixtures of combustible and non-combustible gases, and vapors. These gases and vapors can vary in concentration and composition over time.

Whether a non-traditional fuel source is an undesirable byproduct of chemical processes (and therefore should be destroyed before it is released into the atmosphere) or a potential alternative energy source from a landfill or biomass, it will likely present some measurement challenges. Possible challenges can stem from the rate of change or the presence of a wide range of water vapor at different process temperatures. Perhaps the combustibles vary in composition under different process conditions. Maybe the heating value itself even varies from very lean at some times to very rich at others. In any case, measuring the calorific value can often be difficult.

For fuel mixtures, the measurement should be fast and continuous, with a universal response to any gaseous fuels over a wide measurement range. A heated sample handling system is essential.

Table 2. Response factor for a number of fuel gases.

Background

Calorific value, also known as heating value, is the energy density of a fuel or the amount of heat energy released when a given amount of fuel burns (see Table 1). For gaseous fuels, common units of measurement include BTU per cubic foot or megajoules per cubic meter (26 BTU/ft3 = 1 MJ/m3).

Note that for gaseous fuels, two calorific values are defined: the lower heating value excludes the heat energy present in the water formed by combustion, and the higher heating value includes that energy. This is necessary because, in addition to heat energy, fuels that contain hydrogen atoms create hot water vapor as a byproduct of combustion. (Calorific values that involve the effects of moisture and ash-producing substances on the amount of energy released by burning-"as received," "dry," "ash-free," "dry and ash-free," etc.-can be important for solid and liquid fuels, particularly wood and coal, but they are not pertinent to most gaseous fuels.)

For the higher heating value, the heat energy present in hot water vapor (known as heat of vaporization or heat of condensation) that is produced by combustion can be 5-15% of the total energy released by the fuel. If this can be recaptured and put to use in the process (e.g., by condensation back into liquid), it can be an important contribution to the total energy available from the fuel. Processes that do not recapture this heat only realize the lower heating value of the fuel.

It is important to note that all hydrocarbon fuels produce some water vapor, which contains about 5-10% of the energy yield, but there are exceptions. Carbon monoxide (CO) has no hydrogen and burns without producing water vapor, so its higher and lower heating values are equal. Hydrogen, which produces proportionally more water vapor than other fuels, has a larger difference between its higher and lower heating values than other fuels; about 15% of the energy from burning hydrogen is contained in hot water vapor.

Figure 1. Temperature effects on process readings.

Measurement Processes

In non-conventional fuels, significant and varying amounts of water vapor might be present in a gaseous fuel mixture before it is burned. This may mean that a wet-basis measurement should be made; a wet-basis measurement involves analyzing a sample of the fuel without removing any water vapor through condensation from temperature or pressure changes. Otherwise, the loss of water vapor erroneously increases the concentrations of the remaining gases in the mixture before reaching the analyzer, thereby producing a large error. (It is possible, but not preferable or practical, to recover and measure the removed water vapor to correct for dry basis measurement error.)

At atmospheric pressure, saturated water vapor may constitute up to 20% of the mixture's volume at 60°C (140°F), but only 2% at 20°C (68°F). As shown in Figure 1, if a sample saturated with water vapor is taken at 60°C and is allowed to cool to 20°C prior to measurement, 18% of the sample volume turns to liquid. When the remaining gases are measured at 20°C, the concentrations are 22% higher in the partially dried sample than in the true 60°C mixture. Measurements taken on partially dried samples have large errors.

Under most conditions, keeping all parts of the sampling system and analyzer at 120°C (250°F) prevents this error. In addition, a heated analyzer prevents condensation of heavier, less volatile hydrocarbons. Keeping all sample-wetted parts of the sampling system and analyzer at a high temperature ensures that these combustible vapors are properly measured.

A related effect occurs in a sample pump system (see Figure 2). Even without a drop in temperature, the pump's pressure rise lowers the dew point and condenses out water, thus unintentionally increasing the concentration of the remaining gases. For a 60°C mixture saturated with water vapor at atmospheric pressure, a 15+ psig pressure rise condenses half of its water volume. The concentrations of the remaining gases increase, which causes an error of about 10%.

Since many fuel mixtures contain a variety of combustibles gases and vapors, along with nitrogen, carbon dioxide, and water vapor, it is important to consider the response of the analyzer to each component. An analyzer's response to a particular gas relative to a standard reference gas is known as the response factor. The response often varies. A calibration procedure can be used to correct the reading for a single gas or specific mixture, but it is not practical for mixtures that change composition or are not well known.

In an ideal analyzer, all response factors would be identical. In practice, no analyzer is ideal, but it is possible to have response factors that are close enough to minimize errors. Because it actually burns a sample to determine the calorific value, a micro-combustion calorimeter is a direct measurement and provides good response factors (see Table 2).

In addition, an analyzer should feature a fast response time so that it can quickly respond and activate controls for gas-blending optimization. Applications include flare stacks, biofuels, turbine engines and feed-forward control.

Figure 2. Pressure effects on process readings.

Accurate Performance

The performance and measurement of a combustion calorimeter can be greatly improved by adding a known, controlled amount of hydrogen to the sample stream prior to combustion. This stabilizes the flame and keeps the measurement accurate over a wide range of calorific values. The flame would then be lit and measured accurately even when the calorific value is zero.

By necessity, the flame in a hydrogen-fueled combustion calorimeter is small, and the hydrogen fuel requirement low. Careful control and compensation for fuel and sample flow allows a precise measurement from a small flame size in an apparatus that is small enough to be thermostatically heated to a high temperature.

The small flame responds quickly to changes in concentration by using temperature detectors with low thermal mass. This analyzer has the characteristics needed to quickly and continuously measure the heating value of a variety of combustible gases over a wide range of measurement.

Summary

Several key points should be considered when designing a system for the continuous monitoring of the calorific value of mixed gaseous fuels:
  • Heating prevents errors that result from the condensation of water vapor. The sample stays intact during measurement.
  • Heating allows combustible gases with low vapor pressure (high boiling points) to be measured without losses in the sampling system. False low readings are avoided.
  • The aspirated system does not increase pressure above the process conditions, which prevents condensation of water vapor or combustible gas that would otherwise condense. The sample is not affected.
  • Combustion calorimeters provide good response factors, which is especially useful for unknown mixtures or "anything that burns."
  • The use of hydrogen fuel stabilizes the flame and widens the measurement range (including zero). The flame is always ready to measure.
  • The instrument's speed provides a continuous measurement that is useful when process conditions can change quickly. Speed can sometimes be as important as accuracy.


For more information, contact Control Instruments Corp. at 25 Law Dr., Fairfield, NJ 07004; call (973) 575-9114; fax (973) 575-0013; e-mail sales@controlinstruments.com; or visit www.controlinstruments.com.

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