In-Furnace Reduction Of Nitrogen Oxide By Mixed Fuels Involving A Biomass Derivative

Abstract

A method of reducing nitrogen oxide emissions formed during fuel combustion by introducing biomass ash into a combustion chamber.

Description

PRIORITY INFORMATION [0001] This application claims priority to U.S. Patent Application No. 60 / 694,181, filed Jun. 27, 2005, the contents of which are incorporated herein by reference in their entirety.GOVERNMENT SUPPORT [0002] This invention was made with support from United States Department of Energy Grant Nos. DE-FG26-02NT41552 and DE-FG-04-NT42183. The United States government has certain rights to this inventionFIELD OF THE INVENTION [0003] The present invention relates generally to the field of reduction of nitrogen oxide emissions. More specifically one embodiment of the present invention relates to the reduction of nitrogen oxide emissions by introducing a reburn fuel with biomass-derived products, such as biomass char and biomass ashes. SUMMARY AND BACKGROUND OF THE INVENTION [0004] Reburning is a three-stage, in-furnace combustion technology designed for the reduction of nitrogen oxide (NO) by introducing a supplemental fuel above the primary combustion zone, where the ma...

AI Technical Summary

Benefits of technology

[0005] It is known that the use of a dual-function fuel for reburning can effectively reduce NO in the coal-fired utility furnaces. One of the components natural gas, reduces NO, and the other, a small amount of lignite ash reduces one of its major reaction intermediates hydrogen cyanide (HCN). HCN is a precursor to NO formation under oxidizing conditions.

[0009] Embodiments of the present invention revealed that a mixed fuel containing a biomass ash is likely to achieve about a 85% NO reduction. Additionally, the cost of embodiments of the present invention are expected to be low

[0012] Introduction of ignite ash or biomass derivatives into the fuel-rich zones can catalyze the reduction of NO and decomposition of HCN (a NO formation precursor). This can be accomplished with at least two embodiments of the present invention. In staged operations, where part of the combustion air is supplied through the overfire air (OFA) ports above the substoichiometric (fuel-rich) combustion zone for NO reduction, the lignite ash or biomass derivatives can be injected anywhere between the burner level and OFA ports. To maximize the NO removal efficiency, it's preferable to introduce the compounds closer to the burner level. In the absence of OFA ports (unstaged combustion), the lignite ash or biomass derivatives can be strategically introduced into locally fuel-rich zones of the low-NOx burner flame via especially, designed nozzles for effective NO reduction.

[0014] One of the present inventors has described variables, kinetics and mechanisms of heterogeneous reburning, i.e., reburning involving a coal-derived char (Chen and Ma, 1996; Chen and Tang, 2001). Young chars derived from low rank coals, lignite ant sub-bituminous coal, contain catalysts that effectively reduce not only NO, but also HCN. Moreover, minerals in the young chars effectively catalyze the gasification of carbon in the chars by CO2 and O2 for production of CO. Gaseous CO, a rich product of incomplete combustion in a reburning stage, effectively scavenges surface oxides and regenerates the carbon active sites on the char surface. The mechanistic information renders it possible to design a highly efficient reburning fuel.

[0015] Mixed fuels containing these multiple functions have been designed and tested in a simulated, bench-scale reburning apparatus a 1100° C. with a 0.2 s residence time. Reburning experiments were carried out in a ceramic flow reactor with a simulated flue gas comprising about 16.8% CO2, about 1.95% O2, and about 0.1% NO in a helium base. These concentrations of CO2, O2, and NO were chosen to be consistent with those of a coal primary flame operated at a stoichiometric ratio of 1.1. Methane is usually used as one of the reburning, fuel components for its capability of converting NO to HCN and for the speedy production of CO for scavenging surface oxides, if char is present in the system. A second component is chosen mainly for the effective reduction of HCN; they include lignite ashes, ashes of sub-bituminous coals ashes from utility boilers, ashes from Bunsen burners, and a biomass fly ash produced from a paper mill.

[0018] In a time when the price of natural gas experiences large fluctuations, embodiments of the present invention provide cost savings by the use of a combination of biomass fly ash and natural gas for reburning for even greater NOx reduction efficiency than gas reburning alone. Mixed-fuel reburning involving biomass ash also appears to be more coast effective than SCR white achieving the same level to NOx reduction efficiency. The minerals in biomass have high volatility and they form deposits in the downstream segment of our stimulated reburning reactor where the temperature starts to drop. Nevertheless, due to its relatively small quantity in the practical boilers, calculations suggest the ash-fouling tendency, index remains to be low to medium. Moreover, calculations suggested that T250poise, the temperature that causes the viscosity of softened ash to reach the 250 poises threshold, decreases from 1232 to 1149° C. after the heterogeneous reburning with biomass ash is introduced into a bituminous-coal fired boiler.

Problems solved by technology

Thus, reburning technology on its own, has not been sufficient to meet the stringent regulations established by laws.

Ashes from lignite-fired power plants are geographically limited to the Northern Great Plains (North and South Dakota and Montana), and Southern United States (Texas, Louisiana, and Mississippi).

Transportation of lignite ashes to boilers in other parts of the United States poses a cost constraint.

During natural gas reburning, hydrocarbon free radicals including C, CH— and CH2, chemically reduce NO to HCN, a major reaction product in the reburning zone, A significant portion of HCN oxidizes to NO in the burnout zone that limits the overall NO reduction efficiency.

Ashes collected from utilities are less effective than those from laboratory by a Bunsen burner probably due to the differences in their temperature history and level of sintering.

Nevertheless, due to its relatively small quantity in the practical boilers, calculations suggest the ash-fouling tendency, index remains to be low to medium.

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