Mechanism Of N₂O Emission Reduction With Biomass Gas Reburning In A Coal Fired Fluidized Bed

Nitrogen oxides (NO and NO₂) and SO₂ emission could be controlled by a coal fired fluidized-bed combustion technology. N₂O emission produced in the technology is approximately 30～360mg/Nm3, which are times higher than that in pulverized coal-fired power plant (less than 30 mg/Nm3). N₂O, a kind of greenhouse gas, has a strong destruction effect on the ozonosphere in aerosphere. Therefore, it is necessary to study further and find effective measures of controlling N₂O.Biomass has some perfect advantages for developing biomass technology, it is renewable and has zero net CO₂ emissions. Biomass was gasified and changed into a mixing gas, which was consisted of some combustible gases, such as CO, H₂ and CH₄. Because of reduction action characters, N₂O was conversed by them. The method of reducing N₂O emissions with biomass gasification gas reburning (co-firing indirectly with coal) in the coal fired furnace was proposed in this paper. Comparing with co-firing with coal directly, the way of co-firing indirectly, could not only reduce N₂O emissions, but also avoid fouling and corrosion caused by the process of co-firing directly. The reburning technology could guarantee safety, economy and stable operation of coal-fired fluidized bed boiler. Revolving around the subject of N₂O emissions reduction with biomass gasification gas reburning, combining experimental research and theoretical analysis, influence of biomass gasification gas and its components on homogenous N₂O decomposition and heterogenous N₂O decomposition was researched in this paper.Influence of biomass gas and its main components CO, H₂ and CH₄ on homogenous N₂O decomposition was researched in a small scale vertical fluidized bed by varying reburning gas and oxygen content in flue gas. At a reaction temperature of 900℃, the promoting effect order of biomass gas components on N₂O were:H₂>CO>CH₄. N₂O decomposition was increased with the increase of H₂, CO and CH₄, while the oxygen content in flue gas was constant. N₂O decomposition was decreased with the increase of oxygen content in flue gas, when the content of H₂ or CO was invariable, it was also increased with the increase of oxygen content in flue gas with CH₄ remaining the same.Based on the combustion condition of CFB power plant, influence of CFB ash and its four typical oxide compounds (CaO, Fe₂O₃, Al₂O₃ and SiO₂) together with biomass gas on heterogeneous N₂O decomposition. The experimental results showed that:when four typical oxide compounds was bed material, N₂O decomposition was promoted most effectively by CaO; when oxygen content in flue gas was 0%, with the effect of circulating ash and bottom ash in CFB, heterogeneous N₂O decomposition with biomass gas content of 0.2% was 10% higher than that without biomass gas reburning; the positive effect of H₂ was more than CH₄ and CO on heterogeneous N₂O decomposition with circulating ash as bed material and oxygen content was 0%; while effect of CH₄ was more than H₂ and CO on heterogeneous N₂O decomposition with circulating ash as bed material and a certain amount of oxygen content.Based on chemical reaction kinetics, plug flow reactor model was used to reveal the influence mechanism on homogenous N₂O decomposition with different combustion conditions. The results showed that:the effect of reaction N₂O(+M)<=>N₂+O(+M) on N₂O decomposition were increased with the increase of temperature from 800℃to 1000℃, where N₂O+H<=>N₂+OH was the most important reaction with a maximum effect; N₂O decomposition was decreased with oxygen content or steam content increased in flue gas, reaction N₂O+H<=>N₂+OH has a most positive effect on N₂O decomposition with a low value of oxygen content or steam content, while reaction N₂O(+M)<=>N₂+O(+M) has a most positive effect on N₂O decomposition with a high value of oxygen content or steam content; N₂O decomposition was decreased with the increase of ratio of NH3 to N₂O, where reaction N₂O(+M)<=>N₂+O(+M) has a most important effect.Based on the quantum chemistry theory, through Density Function Theory and Cycle Layer Model, the mechanism of heterogeneous N₂O decomposition at the surface of CaO was researched. It showed that:N₂O molecules adsorbed at O atom of CaO (100) surface and (110) surface and decomposed to form O atom and N₂ molecule, afterwards O atom and N₂O molecule reacted and O₂ molecule and N₂ molecule were generated. Results of electronic characteristic analysis showed that CaO surface has high chemical activity effect on N₂O molecular decomposition, which indicates that the reaction mechanism analysis has rationality.