NOx Removal System for Treatment of Stationary Diesel Engine Exhaust Demonstration

This page updated December 5, 2005.

CHA Corporation

NOx Removal System for Treatment of Stationary
Diesel Engine Exhaust

CARB Grant Number 95-348


The statements and conclusions in this Report are those of the grantee and not necessarily those of the California Air Resources Board. The mention of commercial products, their source, or their use in connection with material reported herein is not to be construed as actual or implied endorsement of such products.

Introduction
The CHA Corporation has developed a new process for removing and destroying pollutants from combustion flue gases. The primary pollutants of interest are nitrogen oxides (NOx) and sulfur oxides (SOx) which are commonly present in combustion flue gases and are strictly regulated by the Clean Air Act. The overall objective of this project was to design, build and demonstrate a prototype devise utilizing the CHA process to remove NOx and other pollutants from the exhaust gases emitted by a 58-hp diesel engine.
Currently, there is no single device available for the simultaneous removal of NOx, CO, VOCs and PM10 from oxygen-rich exhaust gases. Two methods for NOx reduction are currently available: selective non-catalytic reduction (SNCR) and selective catalytic reduction (SCR). In SNCR and SCR (processes originally developed for large, coal-fired power plants), a chemical reducing agent, usually anhydrous ammonia or urea, is injected into the exhaust gas stream as it exits the combustor. In the SCR process, impregnated gases pass through a catalytic element that promotes reaction of NOx with the reducing agent to form nitrogen and water. The SCNR process has a much lower NOx-removal efficiency and requires higher temperatures than SCR. SCR vendors report that a 90 percent reduction in NOx levels is achievable if the catalyst bed can be operated at temperatures between 400°C and 500°C. However, the costs of implementing and operating the SCR process are very high, and emissions of small amounts of ammonia are very likely. Currently, a low-temperature (ambient to 125°C) NOx-removal technology is not available. The CHA process is intended to fill this need.
The emission of CO and VOCs can be reduced by a catalytic converter very similar to that found in automobiles. A catalytic converter is a ceramic honeycomb structure that is impregnated with oxidation catalysts, usually platinum and / or palladium. When a hot exhaust stream passes through the ceramic structure, the catalyst promotes the reaction of CO and VOCs with oxygen in the exhaust gases. Catalytic converters are most effective when the catalyst element is about 260°C with optimal performance near 400°C. We assume PM10 is mainly carbonous material that is produced by internal combustion engines, especially diesel engines. The soot can be either oxidized or removed by a filter. In catalytic oxidation, the exhaust gas is sent through a catalyst element, which traps and oxides the soot. Removal efficiencies of up to 50 percent are achievable with this process the average exhaust temperature can be kept near 400°C. Generally, higher removal efficiencies are not possible due to the temperature limitation of the exhaust stream. Very high removal efficiencies are achievable with soot filters. These paper filters for low-temperature service or ceramic fiber filters for high-temperature service remove soot from the exhaust stream as it flows through the filter. Typical service lifetimes for these filters are short, usually less than 50 hours. The process developed and reported here fills the need for a new process to deal with the harmful emissions from diesel engines.
The process can be utilized on gas streams that contain NOx, SOx or both NOx and SOx. The pollutants are first removed from the flue gases by passing the gases through a bed of relatively inexpensive carbon based adsorption catalyst. The NOx and SOx gases are adsorbed and stored on the adsorption catalyst during this step. The pollutants are then destroyed during regeneration of the catalyst with microwave energy. The microwave energy decomposes NOx to nitrogen and carbon dioxide and decomposes SOx to elemental sulfur and carbon dioxide.
Laboratory results indicate that this catalyst shifts the equilibrium of the exhaust gases so that species that are not normally well adsorbed by the high surface area carbon adsorbent, such as NOx, are readily adsorbed while the carbon's natural affinity for substances such as SOx, soot, and VOC's is left intact.
Regeneration can be conducted either in place or in a separate vessel, depending upon the overall system requirements. Microwave energy is introduced during regeneration through a simple system that is conceptually identical to a home microwave oven. This energy is readily adsorbed by the pollutant-laden adsorbent and promotes NOx and SOx reactions with carbon to decompose NOx into nitrogen and carbon dioxide and SOx into elemental sulfur and carbon dioxide.
Because the energy transfer is through electromagnetic means rather than bulk conventional heating, we have consistently observed that the bulk gas temperature remains remarkable low throughout regeneration, usually less than 300°F (149°C). Once regeneration is completed, the adsorbent is ready for reuse. Total removal efficiencies of 98 percent and greater have been experimentally observed for NOx removal, SOx removal, and combined NOx and SOx removal.

Funding Source

Funding Amount


ICAT

$212,676

Grantee

$112,678

Sacramento Municipal Utility District

$100,000


Click here for the entire final report.

 Click here for final report appendices.




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