Removal of NOx: Past and Future Methods
Predominant stationary sources of man-made NOx include energy-producing combustion processes, such as power plants, and chemical and manufacturing processes. Combustion sources of NOx primarily produce nitric oxide (NO) and smaller portions of nitrogen dioxide (NO2).
Certain combustion processes and conditions require the use of post-combustion NOx-control techniques to meet targeted NOx-emission goals. Commonly used post-combustion NOx control methods for stationary sources include selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), and wet or dry sorption.
The goal of SCR and SNCR is to reduce NOx (using a reducing gas) to molecular nitrogen and other inert species.
Catalytic NOx reduction requires steady-state combustion conditions to perform efficiently and thus has limited applicability. Non-stoichiometric proportions or incomplete mixing of the reducing gas and flue gas can result in the emission of reducing gas. Other issues associated with catalytic reduction include a limited functional temperature range, catalyst deactivation, and the cost of installation and maintenance.
Non-catalytic reduction typically requires a higher and a narrower temperature range than does catalytic reduction. Plus, it is susceptible to many of the disadvantages of catalytic reduction.
NO's insolubility in H2O makes it difficult to effectively absorb from a flue-gas stream. Wet sorption has been attempted in combination with oxidizing techniques to selectively transform NO into NO2, thereby increasing the efficiency of aqueous absorption of NOx. However, wet sorption/oxidizing has its drawbacks, including difficulty in handling and disposing of wet spent material, the technique's continuous need for an oxidizing material, and its cost.
Hybrid systems, incorporating the joint application of several techniques to collectively reduce NOx emissions also have their limitations.
Alternate control techniques
Based on laboratory and pilot-scale studies, dry-sorption methods to eliminate NOx have several common advantages, including ease of material handling, simple system design, and maintenance.
Several dry-sorption materials have been found to remove NOx over a greater range of operating conditions than catalytic reduction methods without causing adverse emissions or necessitating monitoring controls.
The U.S. Air Force has funded several studies to control NOx emissions from jet engine test cells (JETCs) where conventional NOx-control methods were found inapplicable due largely to operating-condition variations.
The most promising dry-sorbent material resulting from these studies was vermiculite coated with magnesium oxide (MgO/v). MgO/v was pilot-tested and performed with variable efficiency (30-70%) over the range of operating conditions and allowable pressure drop.
However, the vermiculite, when heated, expands and has very poor compressive strength and is brittle.
More recently tested materials show better promise. The material gamma-alumina, a high-surface-area sorbent, obtainable in variously sized pellets, has proved to be a durable catalytic support. Researchers at the U.S. Bureau of Mines studied removal of SO2 by Na2CO3-impregnated alumina. Other laboratory and pilot-scale studies have shown that Na2CO3-coated alumina provides for up to 90% removal of NOx and SO2 from flue gas.
Other coatings, such as potassium hydroxide (KOH), are being examined.
Treated samples saturated with NO and NO2 show that KOH and K2CO3 provide the most NO2 removal. Comparison between molar and mass quantities of potassium and sodium compounds indicates that potassium compounds have better NO2 removal efficiencies, whereas carbonate compounds have better NO2 removal efficiencies on a molar basis than hydroxides.
Treatment with hydroxides rather than carbonates increases the response of NO removal to coating mass. Carbonate treatments indicated that NO2 removal is a maximum for intermediate quantities of coatings, and NO formation appeared to be directly related to NO2 removal.
Tests of the effects of varied NO concentrations indicate that the sorption of NO and NO2 depend on their ratio in the feed gas for both hydroxide- and carbonate-compound sorption. The effect of increasing the sorption bed temperature from 100º to 200ºC produced a small reduction in NO2 uptake for all treated aluminas.
Wrap-up
Further studies of the effects of NO on NO2 sorption by treated alumina should provide insight into the nature of NOx sorption to these materials.
An inexpensive, widely available sorbent with known sorptive characteristics could find application for many minor NOx emission sources as a low-capital-cost control method.
The previous article was adapted from paper 98-RA93A.03, "Evaluation of Coated Alumina for the Removal of Nitrogen Oxides (NOx)," presented at the Air & Waste Management Association's 91st Annual Meeting & Exhibition, June 14-18, 1998, San Diego, CA.