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.
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 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).
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.
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.
Less than optimal combustion efficiency will also increase VOC emissions. VOC emissions are minimized with proper combustion that promotes good mixing at the burners. More vigorous and turbulent mixing that promotes higher flame temperatures and longer residence time at high temperature will reduce VOCs but must be carefully balanced with NOx emissions. Trace amounts of VOC species contained in the raw fuel such as benzene and formaldehyde does contribute to VOC emissions with poor or incomplete combustion in the furnace.
Because natural gas is a gaseous fuel, particulate matter (PM) emissions are low. PM from natural gas is very fine, typically less than (1) one micrometer. Particulate matter while firing natural gas are usually higher molecular weight hydrocarbons that are not fully combusted. Considering this, increases in PM emissions are the result of poor combustion caused by poor mixing, excessive boiler setting air ingress, burner mechanical problems and/or imbalanced combustion air to the burners.
Natural gas contains very little sulfur and as a result, produces very low SO2 emissions. The most common additive to add odor to natural gas is Mercaptan (CH3SH : Methanethiol).
Natural gas firing flame emissivity is lower than that for coal. This means the furnace water wall absorption is typically lower with natural gas firing. The amount of heat absorbed by the water walls regulates FEGT. When less heat from combustion is absorbed by the furnace water walls, higher FEGT entering the convection pass (AKA back pass) is the result.
Depending on a number of factors including coal properties, boiler efficiency typically ranges between 87% and 89% while firing coal. The boiler efficiency is usually reduced to 84-85%, when a coal fired boiler is converted to natural gas. The biggest change is due to the combustion of Hydrogen. This is normally a 4% heat loss while firing coal and about a 11% heat loss while firing natural gas. When firing natural gas, other losses such as dry gas loss, moisture in fuel and unburned carbon are lower when compared to coal firing, about 1% for each of these losses.
Natural gas boilers operate at lower excess air (O2) and a different airflow requirement for stoichiometric combustion. Coal fired boilers normally operate with 3-3.5% O2 (about 20% excess air). Whereas natural gas boilers normally operate at 1-1.5% O2 (about 7% excess air).
Upon switching from coal to natural gas, heat transfer characteristics of a boiler will be impacted. The characteristics of these impacts are highly dependent on unit design and configuration and are unit specific. Coal contains ash whereas natural gas does not. Ash as well as fouling or slagging deposits on the heating surfaces occurs while firing coal. These deposits do not exist while firing natural gas and the heat transfer in the convection pass can increase by 10% to 20% depending on a number of factors.