Most plant staff members periodically take a CPR course as part of their ongoing qualification program. It's a short and simple class that folks may take for granted after repeating it so many times. You may never use the skill, but when you do, your response must be nearly automatic. So here's a pop quiz: What's the first step in performing CPR? Know your ABCs: Check the
airway and then
breathing and
circulation. The mnemonic is easy to remember and can save a life.
A similar simple but effective approach can be used to assess the performance of a coal-fired steam generator. Manage the airflow first and then the fuel flow to obtain the best combustion results possible given the constraints of the boiler design. Hopefully, the results will be similar to those achieved with CPR: a long and productive life.
Later in this article we present a case study for a typical 500-MW pulverized coal (PC) boiler and apply a set of best practices to measure, balance, and control furnace inputs to achieve higher combustion efficiencies and lower NOx emissions. When you see the results, you may decide that CPR stands for "coal plant revived."
Get the airflow right
Ideal pulverized coal combustion occurs when a coal particle is burned completely and all of the carbon is converted to CO2, all H2 is converted to H2O, and all sulfur is converted to SO2. Deviations from ideal combustion are indicated by higher-than-desired carbon in ash, secondary combustion at the superheater, and objectionable CO levels in the flue gas. Most large utility boilers were originally designed to operate with 15% to 20% excess air (Figure 1) to make up for air and fuel imbalances in the burner belt. Critical tolerances for the combustion airflow paths to the boiler are noted in Figure 2.
1. Different paths. Air commonly takes three different paths, but it all ends up in the furnace—whether you want it there or not. The key to achieving excellent combustion efficiency is properly managing the amount of air supply and plugging the leaks. Source: Storm Technologies
2. Short course. These are the key design and operating measures for combustion air, from the pulverizer to the furnace, for a typical 500-MW coal-fired plant. Source: Storm Technologies
Within most utility furnaces, the residence time for coal particles to completely burn out is only between 1 and 2 seconds (Figure 3). Boilers designed in the 1970s had pre-low-NOx burners, and the short furnace residence time was compensated for by intense and turbulent mixing, with a very high flame temperature. The intensity of burner belt combustion, with the resultant high NOx production, is unacceptable today. Today's typical low-NOx burners have evolved into systems that inject combustion air into two, three, or four air zones at the burners (secondary air). For further reduction, most low-NOx firing systems utilize multiple levels of combustion airflow for staged combustion. A best practice is to ensure that combustion is completed in the furnace with a stoichiometry of 1.15 to 1.20 (excess air of 15% to 20%) before the products of combustion reach the superheater.
3. Burning desire. Best practices dictate staged combustion with multiple air zones in the furnace to manage NOx production. Many boilers have relatively short furnace boxes with residence times below the desired 1 to 2 seconds. Source: Storm Technologies
You may remember how automotive emissions were significantly reduced by adopting electronic fuel injection systems to precisely regulate the amount of air and fuel reaching each cylinder. Later, combustion chamber designs were altered to stage and slow down combustion. That reduced peak combustion temperatures and thereby lowered the rate of NOx formation. Consumers observed reduced emissions but also a significant improvement in automobile performance. Today, a computer can control individual piezoelectric injectors to create five or more injections per cycle in modern diesel engines to optimize performance and emissions under any conceivable operating condition. And it all started with applying combustion fundamentals to the internal combustion engine and leveraging IT advancements for more precise control and data collection.
The principle also holds with power plant combustion fundamentals. Today's coal-fired boilers continue to make similar progress with parallel commercialization of low-NOx burner technology, control systems, smart closed-loop systems, and neural networks. Improvements in pulverized coal combustion with solid fuel injection systems are in our future; they closely parallel the product development trajectory that resulted in advanced performance and emission controls for automotive internal combustion engines.
The solid fuel injection system approach for a natural gas–like rapid response to load changes can be mimicked by improving airflow measurement and control.
Primary airflow in a PC-fired boiler is akin to the gas valve of a gas-fired boiler. Achieving rapid response of pulverized coal to the furnace requires accurate and responsive changes in the transport or primary airflow. In-furnace NOx reduction by staged combustion or "lean burn" is possible by using measured and controlled overfire airflows. This can be effective when pulverizers are optimized for the best fineness and distribution as a complement to the optimum proportioning of total air and fuel delivered to the furnace.
The question is, How do we get there from here?
Email
Comments
Print
Reuse
