1. Field of the Invention
This invention relates to a method that provides for reduction of nitrogen oxides from the combustion of carbonaceous fuels. More particularly, it refers to a combustion technique that uses two sequential stages of partial oxidation followed by a final stage of complete oxidation that can be easily retrofitted to existing pulverized coal-fired and oil-fired utility boilers.
2. Description of the Prior Art
There are several patents that describe staged combustion techniques to reduce nitrogen oxides emissions from the combustion of fuels containing nitrogen. U.S. Pat. No. 3,727,562 describes a three stage process for reducing nitrogen oxides (NO.sub.x) emissions wherein the first stage of combustion is operated with a deficiency of air and the unburned fuel from this stage is separated and burned in a second zone with excess air and then the second stage gases are burned in a third excess air stage. U.S. Pat. No. 4,343,606 describes a multi-stage combustion process wherein fuel gas produced in a first stage partial oxidation zone, operated at a stoichiometric air to fuel ratio of 0.50 to 0.625, followed by a second stage of oxidation wherein it is operated at an air to fuel stoichiometric ratio of 1.0 or slightly greater. Following this, additional air is added to insure that the fuel is completely oxidized. While these methods accomplish their intended purposes, they do not provide the NO.sub.x reduction required under current regulations, which reductions are achievable with the present invention.
The Clean Air Act Amendments of 1990 set NO.sub.x emission limits for coal-fired utility boilers, to be met in the year 2000, that range from 0.40 to 0.86 lb NO.sub.x /10.sup.6 Btu depending on boiler type. However, the U.S. EPA has promulgated more stringent regulations for Eastern and Mid-Western States that will limit nitrogen oxides emissions for all types of coal-fired boilers during the ozone season (May through September) to 0.15 lb NO.sub.x /10.sup.6 Btu. The combustion technologies commercially available today cannot meet this limit.
The only technology available to the carbonaceous fuel fired utility boiler industry that will guarantee this low level of NO.sub.x emissions is the Selective Catalytic Reduction (SCR) technology. The SCR method uses ammonia addition and a downstream catalyst to destroy the NO.sub.x produced in the coal combustion process. This approach is expensive both from capital and operating cost perspectives.
Further, arsenic concentrations (&gt;10 ppmw) in the coal can also poison the catalyst, shortening its life. In addition, ammonium sulfites/sulfates and calcium sulfates from the combustion process can blind the catalyst, reducing catalyst effectiveness. Still further any ammonia that passes through the catalyst unconverted (ammonia slip) will enter the atmosphere, react with air born sulfur dioxide and nitrogen dioxide to form fine particulate (PM.sub.2.5). This is an environmental debit for SCR technology because the U.S. EPA regulates emissions of fine particulate. Very fine particles are not filtered out by nose hairs (particles less than 6 microns) and enter the lungs.
It would therefore be very advantageous to have an improved combustion process that will yield nitrogen oxide emissions, when firing carbonaceous nitrogen containing fuels, of 0.15 lb NO.sub.x /10.sup.6 Btu or less. In such a system, catalyst is not used; therefore, coal products of combustion have no effect on the process other than that of normal combustion processes, making staged combustion is a more reliable technology than SCR. Further, ammonia is not used and fine particulate emissions will not increase.
Such a system will also provide a much lower cost per ton of NO.sub.x reduced compared to SCR, providing the electric utility industry with an economical technology to meet the level of nitrogen oxides emissions to be imposed on Eastern and Mid-Western utilities in the year 2003.