METHOD FOR REDUCING NOx DURING COMBUSTION OF COAL IN A BURNER BY OPTIMIZING COMBUSTION AIR FLOW

Abstract

An initial coal feedstock comprised of primary or second coal is cleaned so as to reduce its ash content by at least about 20% in order to yield a refined coal product that produces fewer NOx emissions. The reduced NOx emissions result primarily from the ability to use less primary combustion air in order to maintain the pulverized refined coal in a suspended condition within the feeder pipes of a coal burner compared to the minimum quantity of air required to maintain pulverized coal from the initial coal feedstock in a suspended condition within the feeder pipes. Reducing the primary combustion air reduces the amount of oxygen in the primary combustion zone that would otherwise be available for converting fuel nitrogen into NOx. Instead, more of the fuel nitrogen is converted into N2. Reducing the primary combustion air also reduces the temperature of the core flame, reducing thermal NOx formation. Increasing the amount of secondary and / or tertiary combustion air to compensate for the reduced primary combustion air results in an overall decrease in NOx formation, as thermal NOx formation is reduced as combustion is completed in the cooler secondary and / or tertiary combustion zones.

Description

CROSS-REFERENCE TO RELATED APPLICATION[0001]None.BACKGROUND OF THE INVENTION[0002]1. The Field of the Invention[0003]The present invention relates to a method for reducing NOx emissions during combustion of coal, more particularly to methods that utilize coal that has been cleaned to lower the ash while reducing the ratio of primary combustion air to fuel and compensating for such reduction with increased secondary and / or tertiary (or overfire) air.[0004]2. Related Technology[0005]Coal combustion is a major source of energy for the production of electricity throughout the world. Coal is a good source of energy because of its high energy to weight ratio and its great abundance. The use of coal, however, is increasingly under scrutiny because of environmental concerns. Among the known environmental difficulties with coal combustion is the production and emission of NOx compounds, such as NO, N2O, and NO2. NOx compounds can be very harmful to human health and are known to produce undes...

AI Technical Summary

Benefits of technology

[0013]Embodiments of the present invention provides methods for reducing the output of NOx during coal combustion by (1) cleaning an initial coal feedstock to yield a refined coal product having reduced ash content and density, (2) pulverizing the refined coal product, (3) feeding the pulverized refined coal product of lower ash content and density into the primary combustion zone of a coal burner while reducing the amount of primary combustion air used to convey the coal particles in suspended condition, which reduces the air / fuel ratio compared to the minimum amount of primary air required to keep the initial coal feedstock from settling, and (4) compensating for the reduction in primary combustion air by introducing more secondary air and / or tertiary (overfire) air. In many cases the total amount of combustor exit oxygen passing out with the flue gas remains the same (e.g., between about 2-3%) to provide the same total burn efficiency.

[0014]Cleaning the coal to reduce its ash content was found to greatly reduce the formation of NOx because coal containing reduced ash yields pulverized coal having significantly lower average particle density. Reducing the particle density of the pulverized coal reduces the quantity of primary combustion air that is required to maintain the pulverized coal in a suspended condition while traveling through the feeder pipes that introduce the coal into the primary combustion zone of the coal burner. Lowering the amount of primary combustion air in turn reduces NOx formation.

[0015]To prevent settling of the pulverized coal particles, the primary air velocity must exceed the settling velocity of the coal particles. The settling velocity is a function of the air conditions (e.g, temperature, density, velocity and fluid viscosity) and coal properties (e.g., particle size, density and particle morphology). Providing insufficient air velocity in the pipes results in settling out of the heavier particulate fractions. This temporarily increases the velocity of the air at the point of coal lay-down. When enough of the coal has settled out, the air velocity becomes high enough to pick up the laid-out coal, causing a slug of fuel to suddenly exit the pipe and enter the burner, which causes flame instabilities and less efficient combustion. For any given type of coal, there will generally be a minimum air velocity that is required to prevent settling out. Variations in the quality of coal can create variations in the amount of air required to keep the pulverized coal in suspension, thus requiring either constant adjustment or excess primary combustion air to maintain the pulverized coal in suspension in spite of fluctuating coal quality. Cleaning coal to reduce its ash content and particle density allows for a significant reduction of primary combustion air. It also improves the consistency of the coal, which permits the coal burner to be operated with a lower minimum primary air velocity required to prevent settling.

[0017]The oxygen deficit that results from reducing the quantity of primary combustion air introduced into the primary combustion zone may be compensated for by increasing the quantity of secondary and / or tertiary air in order to complete combustion. One way that reducing primary air reduces NOx formation is described above (i.e., substantially more of the fuel nitrogen is converted into inert N2 gas rather than NOx). Another way is that reducing primary are reduces the flame temperature, which reduces thermal NOx formation (i.e., NOx formed as a result of the thermally induced reaction between N2 and O2 in the combustion air, which is reduced with decreasing temperature). An advantage of increasing secondary and / or tertiary combustion air is that the secondary and tertiary combustion zones are significantly cooler than the primary combustion zone by several hundred degrees, which further reduces thermal NOx formation when completing combustion of the fuel.

[0018]The ash content of coal can be reduced using mineral processing techniques known in the art. High ash run-of-mine coal or waste coal recovered from impoundments and piles can be upgraded to cleaner burning low ash coal utilizing dry jigging and / or wet processing to separate rock, clays and other inert mineral impurities from the waste coal and convert it into a higher value refined coal product. In addition to reducing the sulfur and mercury present in the feedstock mineral component, the refined coal exhibits cleaner burn characteristics relative to NOx emissions as compared to the initial coal feedstock.

[0020]It is currently believed, based on pilot scale testing, that reducing the ash content of a relatively dirty coal feedstock from approximately 30-40% by weight to less than 15% can result in NOx reductions of over 20% and as high as 50%. According to one embodiment, the dry basis ash content of an initial coal feedstock is reduced by at least about 20% to yield the refined coal product that is introduced into the coal burner using less primary combustion air to reduce NOx emisions. According to another embodiment, the dry basis ash content of an initial coal feedstock is reduced by at least about 30% to yield the refined coal product. According to yet another embodiment, the dry basis ash content of the initial coal feedstock is reduced by at least about 40% to yield the refined coal product. The dry basis ash content of the initial coal feedstock can be reduced by as much as about 60% or more for secondary coal feedstocks that are considered to be waste, low value and / or non-usable coal.

Problems solved by technology

Providing insufficient air velocity in the pipes results in settling out of the heavier particulate fractions.

When enough of the coal has settled out, the air velocity becomes high enough to pick up the laid-out coal, causing a slug of fuel to suddenly exit the pipe and enter the burner, which causes flame instabilities and less efficient combustion.

However, if there is insufficient available oxygen (i.e., because it is being consumed in the main combustion reactions involving elemental solid carbon and volatile hydrocarbons), the nitrogen will instead typically react with carbon and hydrogen atoms to form CN− and NH− radicals.

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