Aug-2021

Strategies for decarbonising combustion processes

Operators can leverage the combustion reaction to decarbonise fired equipment, optimise energy efficiency, enhance process reliability, and reduce emissions.

Tim Tallon
AMETEK Process Instruments

Article Summary

Amidst the global movement towards renewable energy sources, combustion remains an important heating source across many industries, including power and steam generation, oil and gas. While many of these combustion processes continue to operate on fossil fuels, we can derive ‘greener’ forms of combustion by considering the available ‘levers’ inherent to the combustion reaction. With an intuitive working knowledge of these levers, we can more clearly reveal opportunities to meet short-term sustainability targets and long-term strategic roadmaps.

First, consider the primary elements of combustion: fuel, oxygen and heat. At sufficient air levels, the fuel combusts with the oxygen largely to produce carbon dioxide (CO2) and water (H2O) with trace part-per-million (ppm) amounts of combustibles (which includes carbon monoxide [CO] and hydrogen [H2]) and nitrogen oxides (NOx) both as a result of imperfect mixing and localised flame hot spots. The standard combustion reaction (using natural gas) can be summarised in Equation 1 or generalised (using a generic CxHy hydrocarbon) in Equation 2, as shown below:

CH4 + 2*O2 + N2 → CO2 + 2*H2O + N2 + ppm CO + ppm H2 + ppm NOx                   eq. (1)

CxHy + (x+y/2)*O2 + N2 → x*CO2 + y*H2O + N2 + ppm CO + ppm H2 + ppm NOx           eq. (2)

While it may seem simple for strategic discussions, the combustion chemical reaction provides an insightful framework for identifying the various levers available to decarbonise combustion.

Notably, each reactant component plays a direct role in the heat generated and consumed during the combustion process. For example, increasing fuel consumption directly increases CO2 emissions. In the same way, decreasing the fuel directly reduces the amount of CO2 generated.
From this lens, there are four critical levers that each offer a pathway to decarbonise combustion (see Figure 1):  

➊ Fuel
âž‹ Oxygen
➌ Available heat
➍ Carbon dioxide

The following sections highlight the importance of each lever and how specific adjustments to these levers work together to reduce CO2 emissions.

Energy efficiency
The first approach to decarbonising fired equipment is to make these assets more efficient. Through increased energy efficiency, the system produces equal or better performance while also consuming less fuel. There are two common approaches to decarbonise fired equipment via energy efficiency: optimised combustion and waste heat recovery.

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 H2.

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’). 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 temperature of 600°F. Prior to optimising, the 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 CO2 emissions.