Finding the right balance between efficient power output from boilers and other energy producing equipment while also reducing environmental emissions is an important objective for power plant operators. Governments and environmental agencies around the world establish emission standards as part of air pollution regulations, but finding the right way to meet the standards can vary greatly depending on location, equipment and other operating factors.
Finding the optimal balance between reduced emissions and better fuel efficiency or better overall performance of the boiler and longer boiler lifespan is the holy grail of efficient power plant operations.
Power plant operators – including those in the utilities sector as well as private power operations for manufacturing facilities or universities – must adhere to the regulations created by local and national governments, while balancing the most efficient use of energy resources to operate effectively.
In the USA, the Environmental Protection Agency (EPA) has set National Ambient Air Quality Standards for six principal pollutants, which are called "criteria" air pollutants. The European Union (EU) has developed a similar set of air quality standards.
These emissions standards are concerned with measuring output of pollutants and carbon emissions and vary depending on whether you’re operating a gas, an oil, a coal burning or biomass boiler. Emissions standards cover:■ Carbon Monoxide
■ Nitrogen Dioxide (NO2)
■ Sulfur Dioxide
■ Lead (biomass/wood only)
■ Chlorides (biomass only)
■ Particulate Matter (PM10) and (PM2.5) (biomass only)
Finding the right balance between efficient power operations – and keeping equipment operating effectively and for a longer life-span without problems – takes a careful balance of many variables such as operating time, air temperature, tank pressure or amount of steam, all of which can impact the NOx and CO2 emissions (nitrous oxides and carbon dioxide).
Air Quality Standards
Emissions criteria are often based on limiting emissions of specific elements, as well as taking into account regional differences based on attainment levels (how much pollution or ozone exists in the area already).
In the United States, for example, the Clean Air Act, which was last amended in 1990, requires the EPA to set National Ambient Air Quality Standards (NAAQS) for pollutants considered harmful to public health and the environment.
The NAAQS are designed to protect humans and the environment from the adverse effects of air pollutants. Unlike emission limitations, which specify allowable pollutant releases from air pollution sources, ambient standards set forth maximum allowable concentrations of pollutants in the outdoor, or ambient, air.
The most common pollutant for which the NAAQS are exceeded is ozone. Ozone is not emitted directly from smokestacks, tailpipes, or other pollution sources. Instead, it is formed by the reaction of volatile organic compounds (VOCs) and nitrogen oxides (NOx) in the presence of sunlight. NOx and VOCs are released into the air by automobiles, factories, and several other sources, including industrial boilers.
Setting Standards for Emissions
Emissions for boilers and industrial power generators and factories are also regulated under the EPA’s new source of performance standards (NSPS). NSPS for industrial boilers regulate levels for NOx, SO2, and particulate matter. The regulated pollutants and requirements vary for different fuels and boiler sizes.
Finding the right combination of operating efficiency, along with fuel, air, flame and other operating variables, has to take into account the overall design and controls available for the boiler operations and overall plant design. Each of the variables could affect emissions (and performance) in ways that are compounded and impact each other.
Strategies for Reducing Emissions
Reducing emissions can involve both the design of the power operation system as well as how it’s run. Let’s take a look at some common factors that affect emissions.
Boiler design. To minimize emissions and obtain optimal combustion in the boiler, key factors that must be addressed include: availability of oxygen, time, temperature and turbulence. There is an optimum ratio of temperature, air and turbulence in boiler operations that minimize organic PM, NOx, and VOCs emissions.
Boiler size. Sizing is considered a critical design component when employing clean burning equipment and the general approach is to maintain boiler operations at above 50 percent of capacity.
Fuel parameters. Adjusting various aspects of the fuel such as feed stock, fuel composition, and air or moisture content, can affect both emissions and efficiency, and understanding the impact in combination with other factors can be important.
Nitrogen compounds. The principal nitrogen pollutants generated by boilers are nitric oxide (NO) and nitrogen dioxide (NO2), collectively referred to as NOx. The main reason NOx is considered an environmental problem is because it creates reactions that result in ozone and acid rain. In industrial boilers, NOx is primarily formed in two ways: thermal NOx and fuel NOx.
Controlling NOx Emissions
Many different factors can influence NOx emissions from boilers. The most significant ones are flame temperature and the amount of nitrogen in the fuel. Other factors affecting NOx formation are excess air level and combustion air temperature.
The two types of NOx emissions are formed in different ways:
Thermal NOx is formed when nitrogen and oxygen in the combustion air combine with one another at the high temperatures in a flame. Thermal NOx makes up the majority of NOx formed during the combustion of gases and light oils.
Fuel NOx is formed by the reaction of nitrogen in the fuel with oxygen in the combustion air. It is rarely a problem with gaseous fuels. But in oils containing significant amounts of fuel-bound nitrogen, fuel NOxcan account for up to 50% of the total NOx emissions.
NOx can be controlled, generally, in one of two ways: post combustion or with combustion control. Post combustion methods focus on reducing the NOx emissions after they occur (such as by using scrubbers) and are generally more expensive, whereas combustion control methods focus on reducing NOx from occurring in the first place. This is usually accomplished by lowering the flame temperature, but various combinations of air and temperature could be used to find the optimal mixture.
Some combustion control methods for NOx emissions include:
Low excess air (LEA) firing - this involves limiting the amount of excess air that is entering the combustion process in order to reduce the amount of extra nitrogen and oxygen that enters the flame.
Low nitrogen fuel oil - low nitrogen oils can contain up to 15-20 times less fuel-bound nitrogen than standard oils.
Other NOx control methods involve:
Burner modifications – changing the design of the burner to create a larger flame (enlarging the flame can result in lower flame temperatures which reduces thermal NOx formation).
Water/steam injection – introducing water or steam into the flame can lower flame temperatures, suppressing thermal NOx formation and reducing overall NOx levels (but this can also reduce efficiency of the boiler).
Flue gas recirculation – recirculating a portion of relatively cool exhaust gases back into the combustion zone in order to lower the flame temperature and reduce NOx. This may be the most effective method of reducing NOx emissions from industrial boilers with inputs below 100 MMBtu/hr.
Choosing the Best NOx Operating Methods or Technologies
Understanding the effect that NOx control has on operating performance, and finding the right balance between various factors such as flame temperature or air volume, can have a huge impact on the overall performance of the power plant operations. In some cases, making the right adjustments in operating procedures can reduce the need for additional capital expenditures or new technology.
Certain NOx controls can decrease boiler performance while other controls can significantly improve performance. Some areas of the boiler performance that could be affected include capacity, turndown, efficiency, excess air and CO emissions.
Turndown – a high turndown burner, which keeps the boiler at low firing rates, can reduce the energy loss that happens when a burner recycles many times a day and loses heat out of the stack.
Capacity – if the boiler is oversized, its ability to handle minimum loads without cycling is limited.
Efficiency – Some low NOx controls reduce emissions by lowering flame temperature, but this decreases the radiated heat transfer from the flame and could lower boiler efficiency. Flue gas recirculation is one way to offset this; another could be to use an economizer.
Excess air – NOx controls that rely on high access air levels can result in an oxygen deficient flame and increased levels of carbon monoxide or unburned hydrocarbons.
CO emissions – Reducing NOx levels by lowering flame temperatures or modifying air/fuel mixing patterns can result in higher CO levels.
Taking the Next Step in Boiler Efficiency
Optimizing variables such as air temperature, tank pressure or amount of steam could be the answer to reducing NOx and CO2 emissions. Many power plants operators have data available within their operations that could help improve the efficiency of boilers and reduce emissions if the data were to be used in the right way. Using multivariate data analytics (MVDA) is a very promising way to do that. In many cases, the existing data from facility operations can be used without the need for new or expensive equipment (like scrubbers) in order to make emissions reductions in power plant operations ranging from large municipalities to manufacturing plants. Data analytics will provide information on which measures are most efficient, which parameters have the biggest influence, also considering different operating conditions like high or low load.
See a Boiler Operations Example
Michigan State University, which operates a power production facility running four steam boilers and one recovery steam boiler, along with five steam turbines and one combustion turbine running on natural gas, used MVDA to uncover key optimization factors that led to major cost savings and emission reductions.