NO_x control through reburning¹

Abstract

Reburning is a process whereby a hydrocarbon fuel is injected immediately downstream of the combustion zone to establish a fuel-rich zone in order to convert nitric oxide to HCN. The reburning fuel can be gaseous (e.g., natural gas), solid (e.g., coal char or wood) or liquid (e.g., residual oil). Typically, the amount of reburning fuel used is 10–30% of the total fuel. This technology is practiced commercially with nitric oxide reduction levels of 35–65%, depending on the type and scale of the boiler or combustion, the primary and reburning fuels and other variables. Current research and development are suggesting several advanced reburning concepts including injection of ammonia or urea aft of the reburning fuel injection. Nitric oxide reductions of over 90% are anticipated. In this mini-review, a review of reburning technologies, measurements and mechanisms is presented. Predictive methods for reburning are also discussed. Recent work on reburning, including development of a global reburning reaction rate, is summarized, and results of application of a comprehensive combustion model to reburning measurements are summarized.

Introduction

Nitrogen oxides (NO_x) have been recognized as acid rain precursors that impose a significant threat to the environment. Coal combustion is a major anthropogenic source of NO_x. In coal combustion, NO_x originates mostly from nitrogen bound in the coal matrix, namely, fuel-NO_x[1]. Molecular nitrogen in air typically contributes less than 10% of overall NO_x emissions from coal combustion[2]. Several technologies have been demonstrated to reduce NO_x emissions during fossil fuel combustion, as summarized by Boardman and Smoot[3]and others. During the past decade, work on NO_x formation and control has been substantial, including gas-phase and coal-phase NO_x formation modeling and measurement, demonstration of very low NO_x burner technologies, and prediction and measurement of NO_x control through reburning. International work in NO_x formation and control has been particularly extensive during this decade, as noted in reviews by Miller and Bowman[4], Hayhurst and Lawrence[5], Bowman[6], and Kramlich and Linak[7]. Among the most recent developments for reducing NO_x emissions from coal systems are the reburning technologies, wherein gaseous, liquid or solid hydrocarbon fuels are injected downstream of the main combustion zone with NO and produce HCN3, 8, 9. Advanced reburning is an even more recent technology whereby ammonia, urea or similar substances are injected aft of hydrocarbon injection for reburning to further reduce NO_x species[10]. Reburning and advanced reburning technologies are commercially available retrofit technologies, capable of providing 50%–85% NO_x reduction beyond other combustion modifications. This paper provides a review of reburning work, including work at ACERC in the area of reburning.

No published review of reburning was located. However, a review chapter on reburning is reported to be forthcoming in a new encyclopedia[11]. To meet the requirements of the Clean Air Act Amendments of 1990 (CAA) within budget limitations and scheduling constraints, electric utilities have considered a variety of novel approaches to reduce NO_x emissions from powerplants. Categories of NO_x control options include burner replacement, combustion modifications, fuel staging, fuel reburning, steam or water injection, selective catalytic reduction (SCR), and selective non-catalytic reduction (SNCR)[12]. The efficiencies and limitations of various methods have been summarized by Boardman and Smoot[3]. Reburning technology is not fully understood theoretically, but it is a successful and commercially available retrofit technology, capable of providing 50%–70% NO_x reduction beyond other combustion modifications[13]. Advanced reburning technologies are reportedly capable of greater reductions.

The reduction of NO by hydrocarbons was observed nearly half a century ago by Patry and Engel[14], and subsequently by Drummond[15]. This concept, and the term reburning, were first proposed by Wendt et al.[16]who noticed that, with the injection of CH₄ just downstream of the primary flame zone, up to 50% of NO was reduced, as shown in Fig. 1. Myerson[17]proposed that the overall reactions for NO reduction by methane (and other hydrocarbons) could be written as:

$$\text{2NO+2CH}{}_4\text{=2HCN+2H}{}_2\text{O+H}{}_2\text{+88}\text{kcal}$$

$$\text{6}\text{NO+2CH}{}_4\text{=2CO+4H}{}_2\text{O+3N}{}_2\text{+428}\text{kcal}$$

Since this earlier work, much recent work has been completed in several countries. This work has included commercial demonstration, detailed laboratory measurement, and demonstration of improved and advanced reburning technologies. Also, work on understanding the kinetics of reburning and on predicting reburning effectiveness during turbulent fuel combustion has been studied. What follows is a brief review of this recent work, including work in process.