OCR Text |
Show Other techniques modify the combustion process to reduce the amount of NOx that is formed. Five methods to do this that have been proposed and investigated are: 1. Mixing steam with either the fuel or the combustion air ahead of the burner. As steam is a heat-absorbing diluent, it lowers the flame temperature and, consequently, the NOx emission. 2. Dilution of fuel or air with recirculated flue gases acts much like steam dilution. 3. Controlling excess air. Its effectiveness depends on the type of burner. With premix burners, i.e., the fuel and air are fully mixed prior to combustion, increasing excess air lowers NOx emission. With nozzle-mix burners, i.e., the fuel and air are mixed during combustion, increasing excess air increases NOx emission. 4. Staging the combustion. A part of the fuel is burned with all of the air before the remainder of the fuel is supplied to bring the combustion products back to a normal excess air level. 5. Mixed ratio combustion. In this method, some of the burners within a furnace are fired with an excess of air and some with a deficiency. The combustion products from all of the burners mix together in the furnace to complete combustion and to give the desired excess air level. The peak combustion temperature is reduced, resulting in lower NOx emission. For the first three methods, any lowering of the flame temperature that results in significantly reduced NOx emission produces lower thermal efficiency. There are some processes in which mixed-ratio combustion is used to meet a process requirement. One type of continuous annealing furnace for processing strip steel is an example of this. The burners are fired with an air deficiency in the hotter zones to clean oxides from the strip. The resulting combustion gases flow into the cooler zones where they mix with the products from burners firing with excess air. As the two streams mix, the combustion is completed. Since mixed-ratio combustion was required by the process, lowered NOx emission results as a side benefit. In the late 70s, the U.S. Environmental Protection Agency set New Source Performance Standards limiting the emission of NOx from gas-fired burners in large steam generators to 0.2 lb/MMBtu, based on the gross heating value of the fuel. For natural gas burned with 15% excess air (3% oxygen in dry flue gas), this corresponds to 168 ppm of NOx. Since then, allowed values for NOx emission have been falling. In Germany the limit -3- IV-23 |