NATURAL GAS FIRED BOILERS
For economic reasons as well as regulations associated with natural gas fired boilers MACT/MATS, natural gas is rapidly replacing coal as the fuel of choice. A majority of the new gas-fired capacity is combined-cycle units that incorporate a combustion turbine and HRSG.
There are however, a large number of wall-fired or tangentially water tube utility boilers that were previously burning coal. Wall units have multiple burners on a single wall or opposing walls of the furnace while tangentially fired boilers have vertical spaced rows of burners in each of the four corners of a furnace.
What is Natural Gas?
Natural gas consists mostly methane (typical >85%) with the balance being varying amounts of ethane, propane, butane and some inert components (nitrogen, carbon dioxide and helium). Average gross heating vale is 1,020 BTU/scf.
Emissions while firing Natural Gas
Emissions while firing natural gas can include nitrous oxides (NOx), carbon monoxide (CO), carbon dioxide (C02), methane (CH4), nitrous oxide (N2O), volatile organic compounds (VOCs) and trace amounts of sulfur dioxide (SO2) and particulate matter (PM).
NOx
The principal mechanism of NOx formation while firing natural gas is thermal NOx, which is very different than firing coal with a modern boiler where NOx is mostly fuel NOx. Thermal NOx occurs through the thermal disassociation and subsequent reaction of nitrogen (N2) and Oxygen (O2) molecules in the combustion air which is approximately 78% N2 and 20.9% O2.
Most NOx is formed in the high temperature burner zone that is affected by 3 factors:
- Excess air (Excess Oxygen).
- Peak flame and furnace temperatures.
- Time of peak temperature exposure.
Carbon Monoxide (CO)
Less than optimal combustion efficiency causes high CO levels. Non-optimal boiler tuning, insufficient combustion air, poor airflow distribution, poor mixing in the burner zone or burner mechanical problems are the few things that cause high CO emissions.