Loc Ho

Alternative fuel reburning

Abstract Advanced reburning is a NO x control technology that couples basic reburning with the injection of nitrogen agents and promoter compounds. Pilot scale experiments were conducted in which efficiency of basic and advanced reburning processes were characterized with a wide range of reburn fuels. Test fuels included natural gas, pulverized coal, coal pond fines, biomass, refuse derived fuel, and Orimulsion. Process variables that were studied included reburn fuel type, reburn fuel heat input, reburn zone residence time, initial NO x concentration, nitrogen agent injection temperature, and promoter type and amount. Reburn fuel properties found to affect the performance most significantly include fuel nitrogen content, volatiles, and ash constituents. Basic reburning performance for the tested solid fuels was found to approach that of natural gas reburning, with over 70% NO x reduction achievable at reburn heat inputs above 20%. Advanced reburn tests were conducted in which reburning was coupled with injection of nitrogen agents and promoters. The most effective promoter compounds were found to be alkalis, most notably sodium compounds. At reburn heat input of 10%, NO x reductions in the range of 85%–95% were achieved with natural gas and biomass advanced reburning.

Reactions of sodium species in the promoted SNCR process

Abstract Selective Non-Catalytic Reduction (SNCR) is a well-known commercial NOx control process based on injecting a nitrogen agent into combustion products containing NO at temperatures near 1250 K. A serious limitation of the SNCR processes is that the temperature range over which nitrogen agents are effective is relatively narrow. In this work, we show that adding small amounts of sodium salts significantly improves the performance of the SNCR process. Parts per million levels of sodium compounds enhance NO removal and extend the effective SNCR temperature range in comparison with use of a nitrogen agent alone. When added in the same sodium atom amounts, the efficiencies of different sodium compounds are similar. Kinetic modeling suggests that the performance improvement can be explained as a homogeneous chain reaction ensuing after the sodium compounds are converted into NaOH. The overall result of introducing sodium compounds is conversion of H2O and inactive HO2 radicals into reactive OH radicals, with the effective stoichiometry H2O + HO2 → 3 OH, which enhances the SNCR performance of nitrogen agents by increasing the production rate of NH2 radicals.

Reburning promoted by nitrogen-and sodium-containing compounds

Experiments in a 300-kW combustor and kinetic modeling demonstrate a strong effect of N-agents (ammonia and urea) and sodium salts on NO removal in the reburning process. The NO reduction process is more effective if the N-agents appear in the gas mixture with a delay time (0.1–0.5 s) after injection of the reburning fuel. By this time, the concentration of oxygen from the main combustion zone has been significantly depleted by the reburning fuel, thus preventing oxidation of the N-agents into NO. Sodium compounds, such as sodium carbonate, do not affect NO concentrations without N-agents but do significantly promote the effect of N-agents. The deeper NO reduction in the presence of promoters can be explained by reactions of additional active radicals formed in the reburning zone via interaction of sodium compounds with water molecules. About 80–90% NOx control was achieved by 10–20% natural-gas reburning in the presence of ammonia or urea and sodium carbonate in comparison with 45–65% NOx reduction by 10–20% reburning only. Kinetic modeling qualitatively describes the chemical processes responsible for NOx reduction. The following factors primarily control process efficiency: stoichiometric ratio in the reburning zone. delay time between injection of the reburning fuel and formation of NH1 radicals in the reburning zone, oxygen concentration at the time of N-agent injection, concentration of N-agent, and overfire air (OFA) injection location.

Oxidation of NO to NO2 by Hydrogen Peroxide and its Mixtures with Methanol in Natural Gas and Coal Combustion Gases

The CombiNOx process includes a family of NOx control technologies (reburning, urea injection, methanol injection, and wet scrubbing) capable of reducing NOx emissions from stationary combustion sources by about 90%. However, methanol forms CO in flue gas as a byproduct. Hydrogen peroxide and H2O2/CH3OH mixtures decrease the amount of CO formed from CH3OH and can substitute methanol in the CombiNOx process. This paper presents experimental and modeling results on H2O2 and H2O2/CH3OH reactions with NO in a 300 kW combustor firing natural gas and coal. Maximum NO oxidation was achieved at 750–820 K for injection of H2O2 and 1:1 H2O2/CH3OH mixture, and at 850–930 K. for CH3OH injection. NO-to-NO2 conversion of 90–98% and 64–76% was achieved at an additive NO molar ratio of 1.5 during natural gas and coal firing, respectively. Influence of initial NO concentrations, the additive/NO ratio, oxygen and SO2 concentrations, and the presence of fly ash on process performance is discussed. Experimental results are q...

Promotion of selective non-catalytic reduction of no by sodium carbonate

The selective noncatalytic reduction (SNCR) process is effective over a narrow “temperature window” centered at about 1250 K where the N agent (ammonia, urea, or cyanuric acid) forms NHi radicals that react with NO. Under ideal laboratory conditions, deep NOx control can be achieved: however, in practical full-scale installations, the nonuniformity of the temperature profile, difficulties of mixing the N agent across the boiler cross section, limited residence time for reactions, and ammonia slip limit SNCRs effectiveness to, typically, about 40–50%. These difficulties of SNCR can be alleviated by injection of inexpensive, nontoxic inorganic salts together with the N agent. Experiments using a laboratory-scale flow system and a pilot-scale burner showed that the temperature window of NO reduction via SNCR can be broadened and the efficiency of NH3 in reducing NO can be enhanced by adding parts per million levels of Na2CO3. Mass spectrometric measurements and thermodynamic calculations demonstrate that NaOH and Na are the products of Na2CO3 decomposition. Kinetic modeling showed that the promotional effect of sodium can be explained by a chain reaction mechanism in which Na2CO3 aerosol is rapidly converted to gasphase NaOH that enhances NO removal by increasing radical concentrations.

Gas phase reactions of hydrogen peroxide and hydrogen peroxide/methanol mixtures with air pollutants

Hydrogen peroxide and its mixtures with methanol are capable of removing multiple air pollutants, NO, SO3, and carbon-containing compounds from combustion-generated flue gas. Chemical kinetic calculations and experimental data demonstrate that hydrogen peroxide can be injected into flue gas at temperatures of 700–1100 K to convert unreactive NO and corrosive SO3 to NO2 and SO2. Nitrogen dioxide is much moure reactive than NO and can be removed downstream simultaneously with SO2 by wet or dry scrubbing systems that are currently required for SO2 control. Hydrogen peroxide also promotes the oxidation of organic compounds to CO and CO2 and the oxidation of CO to CO2. The H2O2 molecules dissociate into two OH radicals, and four chain reactions of different types are involved in the pollutant emission removal process with the following main reactions: NO-to-NO2 conversion: OH+H2O2→H2O+HO2: HO2+NO→NO2+OH SO3-to-SO2 conversion: OH+H2O2→H2O+HO2; HO2+SO3→HSO3+O2; HSO3+M→SO2+OH+M RH oxidation: OH+RH→H2O+R; R+O2→ oxidation CO oxidation: OH+CO→CO2+H; H+O2→OH+O Methanol is also capable of converting NO and SO3 to NO2 and SO2 respectively. However, injection of methanol generates CO emissions. Therefore, H2O2/CH3OH mixtures might be injected into combustion flue gas with the amount of CH3OH controlled to satisfy CO requirements and with the H2O2 concentration controlled to obtain target NO conversion. Experimental data describing chemical reactions of H2O2 and H2O2/CH3OH mixtures with NO, SO3, CO, CH4, C6H6, and CH2Cl2 were obtained in a 300-kW combustor firing natural gas. Up to 98,85, and 80% removal efficiencies were achieved for NO, SO3, and CH4, respectively. The results are explained by kinetic modeling.

MINIMIZATION OF CARBON LOSS IN COAL REBURNING

This project develops Fuel-Flexible Reburning (FFR), which combines conventional reburning and Advanced Reburning (AR) technologies with an innovative method of delivering coal as the reburning fuel. The FFR can be retrofit to existing boilers and can be configured in several ways depending on the boiler, coal characteristics, and NO{sub x} control requirements. Fly ash generated by the technology will be a saleable byproduct for use in the cement and construction industries. FFR can also reduce NO{sub x} by 60%-70%, achieving an emissions level of 0.15 lb/10{sup 6} Btu in many coal-fired boilers equipped with Low NO{sub x} Burners. Total process cost is expected to be one third to one half of that for Selective Catalytic Reduction (SCR). Activities during reporting period included design, manufacture, assembly, and shake down of the coal gasifier and pilot-scale testing of the efficiency of coal gasification products in FFR. Tests were performed in a 300 kW Boiler Simulator Facility. Several coals with different volatiles content were tested. Data suggested that incremental increase in the efficiency of NO{sub x} reduction due to the gasification was more significant for less reactive coals with low volatiles content. Experimental results also suggested that the efficiency of NO{sub x} reduction in FFR was higher when air was used as a transport media. Up to 14% increase in the efficiency of NO{sub x} reduction in comparison with that of basic reburning was achieved with air transport. Temperature and residence time in the gasification zone also affected the efficiency of NO{sub x} reduction.