While new power generating plants have the opportunity to be built based on modern technology and efficiency standards, the majority of our nation’s power-producing facilities are 40-50 years old, making upgrades to meet current emissions and efficiency regulations often costly. Fortunately, there are fundamental design and equipment options that can result in significant emissions reductions, energy efficiency improvements and, in turn, valuable cost savings. Assessments of – and adjustments to – the fuel delivery and draft system can create significant emissions reductions without the huge capital replacement costs of upgrading to a new boiler system.
For stoker coal-fired boilers, maintaining consistency in coal flow is the first step toward minimizing emissions. Uneven mounds on the boiler grate are a key indicator of downstream emissions issues. Simple monitoring of the boiler grate on stoker boilers can help identify feed system inefficiencies. Uneven fuel beds on the stoker grate also will enable the combustion air to blow excessive levels
Sources
|
In addition to increasing the particulate matter and greenhouse gases, UBC is a loss of potential energy that reduces the boiler system’s efficiency. While implementing the proper feed equipment will help to reduce UBC, a correctly sized overfire air system (OFA) recaptures the inevitable loss of much of the UBC before it reaches the fly ash. OFA systems are designed to provide the proper mix of fuel and oxygen to burn the UBC char particles, releasing additional BTUs into the system and minimizing particulate carryover.
For facilities considering OFA retrofits, employing Computational Fluid Dynamic (CFD) modeling might help pinpoint airflow and combustion problems, as well as aid in the design of the new OFA system. CFD modeling can help determine the exact amount and location of OFA needed to maximize the effect on combustion. An increase of just a few percentage points in combustion efficiency can result in big dollars and emissions compliance, without expensive backend equipment on the boiler. Also, minor modifications to the burner might minimize combustion issues.
The retrofit of an OFA for the Baltimore Gas and Electric Company (BGE) not only helped recapture the UBC energy, but created NOX reductions of 44 percent at full load, without affecting boiler performance. In this particular case, retrofitting the OFA system began with a CFD analysis of the boiler and air system. Through this modeling, the OFA manufacturer identified likely flow issues that could create flame instability. To minimize this effect, the manufacturer developed an enhanced burner mixer (EBM) based on the unit’s specific needs. The new EBM and OFA system, including new and relocated OFA ports, ductwork and dampers, all combined to provide better control of the mixing and recirculation patterns. With the OFA retrofit, BGE was able to reduce NOX emissions by 30-44 percent, depending on boiler production levels.
BGE’s OFA retrofit was implemented without the need to make FD fan adjustments. However, many facilities should consider retrofits to industrial fans and leaky draft systems. With so many plants designed with draft systems that are 30-50 years old, retrofitting components like ineffective impeller blade designs with more modern and efficient blades can result in major reductions in carbon emissions, as well as significant fan horsepower reductions.
Each impeller blade style has its distinct advantages depending on the specific application, but typically radial blades are the least efficient blade – with airfoil blades topping out as most efficient.
In addition to focusing on the blade design, assessing the cut-off point between the tip of the impeller and housing clearance can improve the performance and help minimize excessive noise problems. A 6-12 percent gap, based on the effective diameter, typically avoids these issues.
