Apr-2009

Ambient conditions impact CO and NOx emissions: part I

Changing ambient conditions can impact CO and NOx emissions without proper process adjustments. A wide range of operating conditions show how these emissions can be affected by ambient condition changes for fuel gas-fired natural draft process heaters

Wesley R Bussman and Charles E Baukal, Jr
John Zink

Article Summary

Since process heaters are typically located outside, their operation is subject to the weather. Heaters are usually tuned to a given set of conditions. However, the actual operating conditions can vary dramatically from season to season and sometimes even during a day. Wind, ambient air temperature, ambient air humidity and atmospheric pressure can all significantly impact the O2 level, which impacts both the thermal efficiency and the pollution emissions from a process heater. Unfortunately, most natural draft process burners are manually controlled on an infrequent basis. Some type of automated burner control, which is virtually non-existent today in this application, is recommended to adjust for the variations in ambient conditions.

There continues to be great interest in minimising pollutant emissions, particularly nitrogen oxides (NOx) and carbon monoxide (CO), from process heaters, because these pollutants pose a health risk to the public and harm the environment.1 In normal practice, most heaters are operated with about 3% excess oxygen (O2) to ensure low CO emissions, high combustion efficiency and to allow for variations in operating conditions that may occur during normal operation.2 These variations include daily and seasonal changes in weather and minor variations in the fuel gas composition or supply pressure.

Changes in ambient air temperature and humidity, atmospheric pressure and wind can all have a significant impact on heater operating conditions. These climate changes can result in variations in heater excess O2, which impacts the flame temperature. This can lead to substantial increases or decreases in both NOx and CO emissions.

Figure 1 shows several potential pathways in which changes in ambient conditions can lead to higher NOx and CO emissions. It shows that changes in ambient air temperature and humidity, atmospheric pressure and wind can all result in variations in the heater excess O2 (red lines). An increase in excess O2 typically produces a more compact, higher temperature flame that results in higher NOx emissions. A decrease in excess O2 tends to reduce the mixing rate of the air and fuel, causing poor combustion and leading to high CO emissions. Figure 1 also shows that changes in the ambient air temperature and humidity can lead to higher flame temperatures, resulting in higher NOx emissions.

Effects of excess O2 on NOx and CO
Changes in ambient conditions can cause significant variations in heater excess O2 that can impact NOx and CO. In this section, experimental data will show the effects of excess O2 on NOx and CO production. Mechanisms associated with these trends are discussed. Next, the impacts of ambient temperature, relative humidity, atmospheric pressure and wind on excess O2 are shown and demonstrate the potentially significant influence they can have on NOx and CO emissions.

Effects of excess O2 on NOx
The American Petroleum Institute (API) Publication 535 states that “NOx concentrations will increase as the excess oxygen increases in raw gas burners...this is true for typical refinery heater excess oxygen (1–5% O2, wet basis) rates.3” Figure 2 shows the API 535 curve (shaded area) demonstrating the effect of excess O2 on NOx emissions from raw gas burners. NOx covers a fairly broad range at a given excess O2. This variation in data may be due to burner type, fuel composition, burner design and so on.

The API curve is commonly used in industry as a guide to help estimate how NOx emissions vary with excess O2. For example, suppose a burner is operating at 3% O2 on a volume dry basis (vd) with a 20 ppmvd NOx level. According to the API curve, if the excess O2 increases to 6% while maintaining a constant furnace temperature, the increase in NOx can be estimated as follows:

This example demonstrates that excess O2 can have a significant impact on NOx emissions.
The API 531 curve does not characterise the effect of excess O2 on NOx emissions for all operating conditions. For example, Figure 2 also shows test results of a diffusion-type wall burner firing at 1.65 x 106 Btu/hr (0.483 MW) with two fuel compositions: 100% natural gas (NG), and 60% NG, 40% hydrogen (by volume). The data for each fuel are corrected for dilution to 3% O2 and a furnace temperature of 1600°F (870°C). When firing 100% NG, the effects of O2 on NOx follow the API curve fairly closely. However, when firing with the hydrogen mixture, the data deviate significantly from the API curve. These results show that increasing the hydrogen content increases the sensitivity of excess O2 on NOx production for this particular burner. For example, suppose this burner is operating at 3% O2 with NOx emissions of 20 ppmvd firing both the NG and high-hydrogen fuel cases. If the excess O2 were to increase to 6% at a constant furnace temperature, the NOx would increase to 27 and 32 ppmvd for the NG and high-hydrogen case, respectively.

Experimental data clearly show that increasing excess O2 results in a NOx increase for diffusion-style burners. The increase is attributed to the combination of high flame temperatures and excess O2 available to combine with N2 to form NOx.

Various methods are used in industry to lower flame temperatures. A common method, referred to as internal flue gas recirculation, uses the pressure energy of the fuel to entrain and mix flue gas with the fuel prior to ignition. The presence of flue gas within the combustion zone provides additional mass to absorb heat from the flame, which reduces the overall flame temperature, resulting in lower NOx production.

When the excess O2 increases, the air velocity through the burner throat also increases. This causes the air and fuel to mix more rapidly, intensifying the flame, making it shorter and hotter, as demonstrated in Figure 3. This is a diffusion-style wall burner firing 40% hydrogen (volume), with the balance natural gas (NG), at a heat release of 1.65 x 106 Btu/hr (0.483 MW). Notice the difference in the appearance of the wall and flame just at the burner outlet. The photographs show the wall is hotter and the flame more intense when operating at 6% excess O2 compared to 1%. For this case, the higher flame temperature and the increase in the amount of nitrogen and oxygen available within the flame zone resulted in over a two-fold increase in NOx.