Utilization of Iron Additives for Advanced Control of NO_x Emissions from Stationary Combustion Sources

Abstract

A novel NO x control technique based on the utilization of iron-containing compounds was studied. Iron-containing additives can be injected in small amounts either into the combustion zone or, if reburning is used, into the reburning zone to reduce NO x emissions. Tests in a 300-kW Boiler Simulator Facility demonstrate that iron additives achieve 20−30% NO x reduction without reburning, whereas in conjunction with reburning, they increase NO x reduction from about 60%, typical for conventional reburning, to about 80−85%. The additives tested included powdered metallic iron, iron oxides, iron waste, Fe(CO)5, char, and ash. Up to 1000 ppm (atomic Fe basis) was added to the flue gas. Experimental variables studied include additive composition and amount, injector location, and reburning heat input. Depending on the additive, the effect is dominated by either heterogeneous or homogeneous reactions.

Investigations of the reduction of NO to N2 by reaction with Fe under fuel-rich and oxidative atmosphere

“…4, favorable conditions for the utilization of iron in the de-NO x process are created in fuel-rich atmospheres. Lissianski et al [9] reported a similar trend using a pilot-scale boiler system. The pulverized coal and natural gas were used as the main fuel.…”

“…The influence of the aforementioned reaction on the NO conversion was intentionally omitted to emphasize the role of iron as an additive that supports the reburning process. The experiments were performed this way because the influence of temperature, reburning fuel type, reburning fuel heat input and other process parameters on the efficiency of NO reduction has already been intensively investigated [30][31][32][33], whereas relatively few studies on the influence of iron on the reburning efficiency have been published [9,14,23,24]. The efficiency of NO x reduction in the presence of iron as a function of the excess air numbers was calculated according to Eq.…”

“…The de-NO x efficiency can be increased through the use of additional compounds, such as ammonia, urea or metals, that can be introduced simultaneously with the main reburning fuel. This modification is known in the literature as advanced reburning [9][10][11]. An application of ammonia can be risky because of ammonia slip, the build-up of ammonium salts on heat-transfer surfaces and the creation of ammonium chloride [1]; consequently, metallic iron has been proposed as an alternative additive in advanced reburning [9].…”

“…This modification is known in the literature as advanced reburning [9][10][11]. An application of ammonia can be risky because of ammonia slip, the build-up of ammonium salts on heat-transfer surfaces and the creation of ammonium chloride [1]; consequently, metallic iron has been proposed as an alternative additive in advanced reburning [9]. This low-cost compound is readily available; moreover, it does not negatively influence the combustion process.…”

“…Additionally, the kinetic coefficients of the chemical reaction between NO and Fe have been determined as a function of temperature [9,13,15,17,19,23]. Recently, Fennell et al [12] reported a study of the latter stages of reaction (1), when the process has been controlled by the diffusion of iron ions through an oxide layer around the iron particle.…”