OCR Text |
Show 1. Introduction Environmental regulations in all industrialised countries are becoming more and more stringent. For this reason, engineers and researchers continue their effort in developing nev combustion techniques and/or modifying established ones to lover pollutant emissions. Among the less desirable species, generally associated to acid rain and the greenhouse effect, are the nitrogen oxides (NOx). The portion of NOx generated by industry comes mainly from high temperature processes such as glass melting. In such plants, reductions of NOx emissions can be achieved by tvo different approaches : primary methods to prevent generation of NOx, and secondary methods, vhich are aimed to destroy the undesirable species before their release to atmosphere. Secondary methods involve the postreatment of flue gases (SNCR, SCR, steam injection ... ). This type of option is not only capital intensive but it may demand a continuing effort in equipment maintenance and reagent cost during operation. From both the economical and the environmental perspectives, every effort should thus be made in order to avoid generating the NOx species in the first place. Primary methods such as recirculation and staged combustion address the issue of avoiding excessively high flame temperatures by inducing specific turbulence patterns and are generally based on burner design. CHARON et al.(1990) have shovn in a recent publication that a pure oxygen/natural gas flame could, if properly managed, be by itself a primary-type of 10v-NOx combustion technique. To further improve the environmental performance of oxygen-based combustion, a nev technique has been developed at the AIR LIQUIDE Research Center. The technique is based on the principle of Pulsating Combustion, and results indicated that NOx reductions betveen 40 and 90% can be obtained relative to their non-pulsated equivalents . A number of studies on pulsed combustors are mentionned in the scientifi c literature : BRIFFA and STADDON (1982), PUTNAM et al e (1986), AL-HADDAD and AL-BINALLY (1989) and KELLER et ale (1990). These studies are mainl ' related to air-based flames and their interes t focuses on heat transfer research. The present vork deals specifi cally with flames involving pure oxygen and natural gas, the selective pulsat ion of either one or both of the gaseous streams, and the impact of the pulsation phenomena on the generation of NOx. In this paper ve introduce the princ iples of pulsating combustion, describe the pulse generating equipment and discuss the influence of operat ing parameters on NOx emissi ons. 2. Principles The nitrogen for NOx formation may come from either natural gas (0 to 20%) or oxygen (vhen produced through non-cryogeni c method s) . For these experiments, O2 came from cryogenic sources and vas thus 99.999% pure. The N2 content of the available natural gas varied from .5 to 4%. NOx. high A typical natural gas/pure oxyg n flame produces "prompt"and "thermal " Prompt NOx is generat ed at an early s t age in the flame , i nduced by temperature gradients and high oxygen concentrati ons near the burne r 4 |