Optimised combustion is achieved by reducing the oxygen setpoint (which reduces fuel demand at the burner) while also building in an adequate safety margin to operate above the combustibles breakthrough point. Figure 2 overlays the excess oxygen vs combustibles measurements and illustrates this optimal oxygen control setpoint. As an example, consider the scenario of a boiler operating at 10 MM Btu/ hr with natural gas, 75% firing rate and a flue gas
Combustibles losses
Excess air losses
EFFICIENCY COMBINED LOSSES
O & combustibles
O only operating set point
OPTIMUM CONTROL POINT
300 ppm 200 ppm 100 ppm
0%
1%
2%
3%
4%
Excess O
Figure 2 Measurement of excess oxygen and combustibles enables optimised combustion and lower fuel consumption and stack emissions
temperature of 600°F. Prior to optimising, the
energy efficiency: optimised combustion and waste heat recovery.
flue gas analyser measures 3% excess oxygen and 100 ppm combustibles. After optimising the combustion process, the boiler may now operate at 1% excess oxygen and slightly higher combustibles levels. In this case, the boiler now consumes 1.5% less fuel, directly resulting in a 1.5% reduction of CO 2 emissions. Waste heat recovery An alternative approach to energy efficiency is waste heat recovery, which focuses on the lever of ‘available heat’ from the combustion reaction. Waste heat refers to any unused heat generated from a combustion process. There is a great deal of waste heat in flue gas, and much of it can be recovered prior to venting to the atmosphere, reducing the demand for ‘new’ heat to be generated. Waste heat can be recovered in multiple ways: • Waste heat recovery boilers can generate steam and reduce the need for separate steam generation • Combustion air preheaters can preheat combustion air and reduce fuel at burner • Waste gas heat exchangers can preheat waste gas streams prior to incineration and reduce fuel As another example, consider a heater operating at 10 MM Btu/hr and venting the flue gas at 600°F to the atmosphere. By adding a combustion air preheater, the flue gas may now vent at 400°F,
Optimised combustion Optimised combustion refers to the optimal ratio of air and fuel to minimise fuel consumption without creating an unsafe condition. Optimised combustion primarily adjusts the ‘oxygen’ lever of the combustion reaction. In a standard fired heater, an oxygen measurement is made in the flue gas to ensure sufficient combustion air is available at the burner. Typically, the heater often operates within 2-3% excess oxygen, and this also provides a safety margin in case the fuel gas composition changes during operation. A combustibles measurement can also be made to monitor for incomplete forms of combustion, such as CO and H 2 . To achieve optimised combustion, a combustion flue gas analyser is required to measure both excess oxygen and combustibles concentrations. The oxygen measurement provides an initial setpoint for operation, and the combustibles measurement provides a mechanism to adjust oxygen to optimal levels. If excess oxygen levels are too high, the heater burns extra fuel to heat the greater quantity of air in the system. If the excess oxygen levels are too low, incomplete combustion can occur and can even cause an unsafe spike in high combustibles levels (often referred to as ‘combustibles breakthrough’).
www.decarbonisationtechnology.com
34
Powered by FlippingBook