Combustion in industrial boilers and fluid heaters generates a significant amount of NOX. To fulfill the required ambient air quality standards, especially those for ozone, low-cost ultralow NOx burners are needed for these applications. It is a fact that emissions regulations have continued the downward trend that has been witnessed in the last decade. Present regulations in many countries are focused on single-digit NOX levels of 9 ppm. With our years of experience and knowledge, Rakhoh boilers have delivered innovative technologies designed to reduce nitrogen oxide (NOx) emissions for new or existing coal, oil, and gas-fired boilers.
Reduced NOx in Industrial Boilers:
In the case of industrial boilers, along with reducing NOx, it is equally important to achieve minimal greenhouse gas emissions while ensuring the highest thermal efficiency. Multiple advanced choices are available to assure both low emissions and high efficiency. It includes design and operational practices involving excess air, flue-gas temperature, low flue-gas recirculation, and selective catalytic reduction (SCR) technologies.
Usually, an industrial boiler system consumes the equivalent of its initial capital expenditure in fuel usage within its first year, depending on the continuous operation. Considering that factor, increases in a boiler’s efficiency with merely a few percentage points can lead to substantial cost savings. We have listed below six methods to increase a boiler’s efficiency while reducing NOX.
Tips to Reduce NOX in a Boiler System:
Reduction of stack gas temperature to increase efficiency:
One of the ways to immediately increase the boiler efficiency is by reducing the flue-gas temperature, or the temperature of the combustion gases that exits the stack of the boiler system. Boiler flue gas consists of useful amounts of energy that can be captured by using an economizer. Once stored in the economizer, the heat from the flue gas can be used through heat transfer to preheat the feedwater entering the boiler. In usual cases, a decrease of flue-gas temperature by 40°F (4°C) leads to an increase in efficiency by 1 percent.
Using Flue-Gas Recirculation for Lowering NOX Emissions:
Flue-gas recirculation (FGR) commonly is used to control thermal NOX. It is accomplished by lowering the burner flame temperature and staging the combustion of air and fuel. The step usually reintroduces 15% to 30% and in some cases as high as 45% of the flue gases into the mixing process leads to decreased production of thermal NOX.
However, operating with high FGR needs a significant increase in fan horsepower. Consequently, it results in reduced efficiency due to the rise in the volumetric flow and pressure drop of the combustion air and flue gas through the unit. During this process, the stability of the burner and response is compromised, causing high O2 concentrations. Undoubtedly, there are limitations on the amount of FGR that can be introduced based on the burner design.
Monitoring Excess-Air Levels for Optimal Performance:
Managing excess air levels or the amount of excess combustion air required to burn a given amount of fuel is important to improve efficiency. Increasing additional air provides process benefits. It includes improved air, flame stabilization, fuel distribution, and low CO levels. On the other hand, too much excess air can cause reduced efficiency. It is due to increased fan horsepower consumption and high heat loss up the stack. A burner operating with minimum excess air, at 3 percent O2, is ideal for optimum burner and efficiency enhancements.
Selective Catalytic Reduction for Low NOX Emission:
Ultra-low NOX emissions can be accomplished by utilizing selective catalytic reduction (SCR) technology. This method is post-combustion and involves using a single reactor unit with a catalyst and a reducing-agent delivery system. The unit passes the combustion gases through an injection system to add the reducing agent to the combustion gases, thoroughly mix, and then catalytically reduce them to eliminate the NOX. The process lets the reaction of NOX (NO or NO2) and NH3 (ammonia) chemically convert to resultant products of nitrogen and water vapor.
The usual performance of such units will see NOX levels reduced from 30 ppm to below 5 ppm or up to 95% reduction. SCR systems can also perform efficiently across a flue-gas temperature range of 325 to 1000°F (163 to 538°C) for industrial boilers, gas turbines, and fired refinery equipment. Particularly for boiler applications, SCR can reduce fan requirements by eliminating or significantly reducing the need for flue-gas recirculation. The savings in an electrical load along with a more stable burner during load swings over time could provide the payback when deciding on what equipment to purchase.
Combining Economizers and SCR Systems for Low Emissions:
The advantages of low emissions and high efficiency can be achieved when an SCR system is combined with an economizer and standard burner. The first phase of the SCR system utilizes a catalyst and a reagent to convert NOX to nitrogen and water. The next phase is gained with the finned-tube economizer, capturing waste heat and sending it back into the boiler feedwater or makeup water. The process can lead to significant reductions in operating costs. Its operational benefits are flame stability, higher turndown, and faster response to load swings.
Monitoring Emissions and Efficiency Performance:
Process plant owners nowadays keep tabs regarding emissions, efficiency, and carbon footprint. Additionally, such information is often a requirement for reporting purposes.
Most analyzers used in the package boiler market measure O2 and stack temperature, providing calculated CO2 value and corresponding efficiency. To measure NOX emissions, a secondary analyzer is usually required. Continuous emissions-monitoring systems (CEMS) are also used for reporting both NOX and CO, but they are generally large and complicated systems.
Rakhoh Boilers is one of the trusted boiler manufacturers in Pune since 1983. With 39+ years of experience and expertise in thermal solutions, we deliver highly efficient and reliable industrial steam boilers, waste heat recovery systems, thermic fluid heaters, and boiler accessories to over 20 process industries in 27 countries worldwide. We provide excellent boiler services to boost the efficiency and productivity of the boilers.
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