Although fume regenerator or thermal oxidizer systems for removing volatile organic compounds, VOC's, are highly developed and relatively efficient, it is desirable to further improve such systems as to fuel consumption and generation of nitric oxide. Present regenerators can achieve a 99% destruction efficiency, if the regenerator bed is effectively purged. Also, present regenerators can be as much as 95% thermally efficient and can be self-sustaining at 5% of the lower explosive limit, LEL. This means that if the incoming contaminated air contains enough fuel values, such as methane, CH4, or the like, to be 5% of the lower explosive limit, then as this fuel is burned in the regenerative incinerator, enough heat is generated to make the process self-sustaining. If the fuel is methane, for example, the lower explosive limit is 5.3%. 5% of 5.3% is 0.265% methane or 2650 parts per million (ppm) by volume.
A 95% efficiency is a good result, however, it is not always achieved. Usually the contaminated air contains less than 5% of the LEL and additional fuel is supplied to the burners in the hot zone (combustion zone) of the regenerative incinerator. Typically, natural gas and combustion air are added to the burners. However, the fuel and air are not preheated because preheating would increase the amount of NOx produced by the burners. If all the fuel and corresponding air is supplied to the burners without preheating, the thermal efficiency drops from 95% to 93%. This drop in efficiency represents substantial operating expense. For instance, in a typical regenerative incineration system processing 100,000 standard cubic feet per minute at 93% thermal efficiency, the fuel cost per year, with natural gas priced at $4 per 1000 cubic feet, is about $780,000. At 95% thermal efficiency, the annual cost for fuel is $557,136. Thus in this typical system the annual fuel savings is $222,864.
It is known to process contaminated air containing about 2650 ppm of methane or equilavent fuel. There is, however, a big disadvantage in processing heavily contaminated air, namely destruction efficiency is adversely affected. For instance, in a three bed regenerative incinerator, such as disclosed in my U.S. Pat. No. 3,870,474, one bed is processing impure air, a second bed is processing purified air from the furnace and a third bed is being purged of impure feed gas before being used for processing purified air. However, purging is not perfect. Incomplete purging allows some impure feed gas to remain in the regenerator bed of porous heat retaining material (typically fire brick), thus permitting this retained impure gas to mix with purified gas, which lowers the destruction efficiency.
A notable disadvantage of the use of burners in the combustion zone is that the products of combustion are hot enough (over 1800.degree. F.) to generate NOx. Even the low NOx burners, available at extra cost, only cut:the NOx production in half, which is still an undesirable amount of NOx in the purified gas.