OCR Text |
Show fuel and air are fully mixed prior to combustion, increasing excess air greatly lowers NOx emission. With nozzle-mix burners, the fuel and air are mixing during combustion. In this case, increasing excess air usually increases NOx emission. 3. Dilution of fuel or air with recirculated flue gases. 4. Staging the combustion. The fuel is burned with insufficient air while releasing some of its heat to the process before additional air is supplied to release the remaining heat. 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 average composition. The peak combustion temperature is reduced, resulting in a lowered NOx emission. For the first three methods, any lowering of the flame temperature that results in significant reductions in NOx emission is accompanied by significant reductions in 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 rich in the higher-temperature 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 lean. 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 stearn generators to 0.2 Ib/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 is 100 mg/Nm3 , which translates to 0.057 Ib/MMBtu for natural gas. In Los Angeles, it will be 0.03 Ib/MMBtu, or 25 ppm for natural gas. Experimental Equipment The NOx emission measurements were made in two test furnaces. Water-cooled, stainless-steel, bayonet heat exchangers were used to control the furnace temperature independently of the firing rate. The furnace was lined with 11 inches of insulating firebrick and fiber insulation. The outer steel casing was -2- |