{"title":"Effects of sub-atmospheric pressure on appearance and pollutant formation of inverse diffusion flame within a confined space","authors":"","doi":"10.1016/j.expthermflusci.2024.111340","DOIUrl":null,"url":null,"abstract":"<div><div>Gas-fired boilers operating at high-altitude regions often suffer from inadequate output, decreased thermal efficiency, and excessive NO<em><sub>x</sub></em> emissions. The effect of sub-atmospheric pressure on flame appearance and pollutant formation is the main reason for those problems, and thus needs to be clarified particularly under furnace combustion conditions with a fixed excess air coefficient. Inverse diffusion is a widely employed fuel–air configuration in burners of gas-fired boilers, and therefore the flame appearance, CO generation, and NO generation were experimentally investigated in this paper by adopting a low-pressure quartz tube reactor. Results show that the flame is elongated from reducing pressure under fuel-lean conditions, mainly due to the reduced oxygen mass concentration and the elevated jet velocity. Under fuel-rich combustion conditions, however, the flame is shorted at sub-atmospheric pressure from the suppressed soot formation. The reduced pressure leads to an increase in the global strain rate, making the flame more prone to uplift. With decreasing pressure, the increased air–fuel mixing and flame length facilitate the gas burnout, thus decreasing CO generation. The sub-atmospheric pressure could evidently reduce the NO generation under fuel-rich conditions, but slightly increase it under fuel-lean conditions. Under fuel-lean conditions, the NO major pathways (prompt, thermal, NNH, and N<sub>2</sub>O) are promoted which leads to an increase in NO generation with decreasing pressure. Under fuel-rich conditions, however, NO formation is suppressed from the decreased rate of reaction <span><math><mrow><mtext>N2</mtext><mo>+</mo><mtext>CH</mtext><mo>↔</mo><mtext>HCN</mtext><mo>+</mo><mtext>N</mtext></mrow></math></span>.</div></div>","PeriodicalId":12294,"journal":{"name":"Experimental Thermal and Fluid Science","volume":null,"pages":null},"PeriodicalIF":2.8000,"publicationDate":"2024-10-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Experimental Thermal and Fluid Science","FirstCategoryId":"5","ListUrlMain":"https://www.sciencedirect.com/science/article/pii/S0894177724002097","RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q2","JCRName":"ENGINEERING, MECHANICAL","Score":null,"Total":0}
引用次数: 0
Abstract
Gas-fired boilers operating at high-altitude regions often suffer from inadequate output, decreased thermal efficiency, and excessive NOx emissions. The effect of sub-atmospheric pressure on flame appearance and pollutant formation is the main reason for those problems, and thus needs to be clarified particularly under furnace combustion conditions with a fixed excess air coefficient. Inverse diffusion is a widely employed fuel–air configuration in burners of gas-fired boilers, and therefore the flame appearance, CO generation, and NO generation were experimentally investigated in this paper by adopting a low-pressure quartz tube reactor. Results show that the flame is elongated from reducing pressure under fuel-lean conditions, mainly due to the reduced oxygen mass concentration and the elevated jet velocity. Under fuel-rich combustion conditions, however, the flame is shorted at sub-atmospheric pressure from the suppressed soot formation. The reduced pressure leads to an increase in the global strain rate, making the flame more prone to uplift. With decreasing pressure, the increased air–fuel mixing and flame length facilitate the gas burnout, thus decreasing CO generation. The sub-atmospheric pressure could evidently reduce the NO generation under fuel-rich conditions, but slightly increase it under fuel-lean conditions. Under fuel-lean conditions, the NO major pathways (prompt, thermal, NNH, and N2O) are promoted which leads to an increase in NO generation with decreasing pressure. Under fuel-rich conditions, however, NO formation is suppressed from the decreased rate of reaction .
在高海拔地区运行的燃气锅炉经常会出现出力不足、热效率降低和氮氧化物排放过多的问题。亚大气压对火焰外观和污染物形成的影响是造成这些问题的主要原因,因此需要对其进行澄清,尤其是在过剩空气系数固定的炉膛燃烧条件下。反向扩散是燃气锅炉燃烧器中广泛采用的一种燃料-空气配置,因此本文采用低压石英管反应器对火焰外观、CO 生成量和 NO 生成量进行了实验研究。结果表明,在燃料贫乏的条件下,火焰因压力降低而伸长,这主要是由于氧气质量浓度降低和喷射速度提高所致。然而,在燃料丰富的燃烧条件下,由于烟尘的形成受到抑制,火焰在亚大气压下被缩短。压力降低导致整体应变率增加,使火焰更容易上浮。随着压力的降低,空气-燃料混合和火焰长度的增加会促进气体燃烧,从而减少 CO 的生成。在燃料丰富的条件下,亚大气压可以明显减少氮氧化物的生成,但在燃料贫乏的条件下,氮氧化物的生成会略有增加。在燃料贫乏的条件下,促进了 NO 的主要生成途径(瞬时、热、NNH 和 N2O),从而导致 NO 生成量随着压力的降低而增加。然而,在燃料丰富的条件下,NO2+CH═HCN+N 的反应速率降低,从而抑制了 NO 的生成。
期刊介绍:
Experimental Thermal and Fluid Science provides a forum for research emphasizing experimental work that enhances fundamental understanding of heat transfer, thermodynamics, and fluid mechanics. In addition to the principal areas of research, the journal covers research results in related fields, including combined heat and mass transfer, flows with phase transition, micro- and nano-scale systems, multiphase flow, combustion, radiative transfer, porous media, cryogenics, turbulence, and novel experimental techniques.