OCR Text |
Show is 100 mg/Nm3, which translates to 0.057 lb/MMBtu for natural gas. In Los Angeles, it will be 0.03 lb/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 welded to minimize leakage of air into it or combustion gases out of it. The furnace's low leakage permitted the air/fuel ratio to be controlled and the influence of tramp air to be measured. Natural gas and air flows were measured by variable area flow meters. The oxygen and combustibles in the flue gas were measured by means of oxygen and combustibles analyzers. The firebox temperature was measured with a Type K thermocouple in a conventional protection tube. Recent measurements have been made with a high-velocity thermocouple, also. Measurements of NOx were made with a chemiluminescence analyzer or with an electrochemical sensor. Carbon monoxide was measured with an infra-red, gas-filter-correlation analyzer. A few measurements of the combustion products have been made with a volatile organic compound analyzer. Types of Burners Tested Wall-mounted burners were tested. One of these, the K976, was an inspirator burner developed in the late 60's. Two, the K988 and K988M, are different sizes of fuel-staged burners developed recently. The K-976 burner (Figure 1) uses a high-velocity jet of gaseous fuel to entrain primary combustion air and furnace draft to induce the flow of secondary combustion air. The primary air and fuel are completely mixed by this entrainment process before being burned. The secondary air does not mix with the fuel until combustion has begun. The version tested had a burner block with a cup-shaped cavity. It was tested with and without secondary air. The staged-combustion K988 and the K988M burners (Figure 2) also use the fuel to entrain primary air. About half of the fuel is premixed in the first stage with all of the combustion air and then burned. In the second stage, the remaining fuel is mixed with combustion products from the first stage. The burner block can have either a cup or flat contour. Secondary air is used for increased capacity or fuel flexibility if needed. Field tests with small numbers of burners have shown that the design produces a stable, wall-hugging flame whether a flat or cup-type -4- IV-23 |