Multifunction Catalytic Element for Exhaust Gas Treatment
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Solution Overview
Problem
Current post-combustion exhaust gas treatment systems face challenges in achieving sub-2 ppm levels of NOx, hydrocarbons, and carbon monoxide while minimizing ammonia injection and ammonia slip, especially when dealing with high oxygen levels in flue gases from power generation.
Innovation Solution
A multifunction catalytic element with both reducing and oxidizing capabilities is introduced, allowing for selective catalytic reduction of NOx with excess ammonia and subsequent decomposition of ammonia to minimize NOx production, combined with downstream oxidation of CO and HC pollutants to reduce ammonia requirements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If selective catalytic reduction is used to reduce NOx, then NOx reduction efficiency is improved, but ammonia slip and additional NOx production occur due to excess ammonia oxidation
Solution Approach 1:
The catalytic converter is divided into two distinct sections: a first section with oxidizing catalyst for CO and HC oxidation, and a second section with reducing catalyst for NOx reduction. This segmentation allows independent optimization of each function, preventing ammonia oxidation in the reducing section while maintaining effective NOx reduction.
Solution Approach 2:
Different catalyst properties are applied to different sections of the converter. The first section uses oxidizing catalyst (Pt, Pd, Rh) for CO and HC oxidation, while the second section uses reducing catalyst (Fe, Co, Ni) for NOx reduction. This local differentiation of catalyst quality enables simultaneous achievement of oxidation and reduction functions without interference.
2Reliability
If oxidizing catalyst is placed upstream of ammonia injection, then CO and HC oxidation is improved, but ammonia is oxidized to NOx compounds
Solution Approach 1:
The system separates the oxidizing function (first section) and reducing function (second section) into distinct zones. Ammonia is injected between the sections, allowing CO and HC oxidation in the first section without exposing ammonia to the oxidizing catalyst, thus preventing ammonia oxidation to NOx.
Solution Approach 2:
The first catalytic section acts as an intermediary zone that processes CO and HC oxidation before the ammonia injection point. This intermediary positioning allows oxidation of harmful gases while protecting subsequent ammonia from oxidation, as the reducing catalyst in the second section prevents ammonia conversion to NOx.
3Reliability
If stoichiometric ammonia injection is used, then NOx reduction is optimized, but sub-2 ppm NOx levels cannot be achieved
Solution Approach 1:
The system dynamically adjusts the ammonia injection ratio based on actual emission conditions. By allowing excess ammonia (ratio greater than 1.05:1) and using the reducing catalyst to convert excess ammonia to nitrogen, the system can achieve precise sub-2 ppm NOx control, adapting to varying operational conditions rather than relying on fixed stoichiometric ratios.
Solution Approach 2:
The invention changes the ammonia-to-NOx ratio parameter from strict stoichiometric (1:1) to excess ammonia ratios (greater than 1.05:1). This parameter change, combined with the reducing catalyst that converts excess ammonia to nitrogen, enables achieving sub-2 ppm NOx levels that cannot be obtained with stoichiometric injection alone.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The system effectively reduces NOx, hydrocarbons, and carbon monoxide to sub-2 ppm levels with reduced ammonia injection and minimal ammonia slip, maintaining efficiency over a wide range of NH3 to NOx ratios and temperatures, while avoiding the production of additional NOx compounds.
Implementation Method 1
the flue gas is mixed with anhydrous ammonia and is passed over a suitable reduction catalyst at temperatures between about 150-550° C., and preferably between 300-550° C.
Implementation Method 2
One technology for the control of oxides of nitrogen that is currently being used commercially at large land-based electrical power generating stations is selective catalytic reduction (SCR).
Implementation Method 3
It is also known to combine an SCR process with a catalytic oxidizing process to treat an exhaust gas flow by oxidizing carbon monoxide to carbon dioxide and by oxidizing hydrocarbons to carbon dioxide and water.
Implementation Method 4
oxidizing carbon monoxide to carbon dioxide and by oxidizing hydrocarbons to carbon dioxide and water
Implementation Method 5
the oxidizing catalyst will also function to oxidize ammonia, which is undesirable when it decreases the amount of ammonia available for reduction of the NOx
Implementation Method 6
oxidize ammonia
Data Source
AI summary
An exhaust gas treatment apparatus (20) for reducing the concentration of NOx, HC and CO in an exhaust gas stream (18) such as produced by a gas turbine engine (12) of a power generating station (10). The treatment apparatus includes a multifunction catalytic element (26) having an upstream reducing-only portion (28) and a downstream reducing-plus-oxidizing portion (30) that is located downstream of an ammonia injection apparatus (24). The selective catalytic reduction (SCR) of NOx is promoted in the upstream portion of the catalytic element by the injection of ammonia in excess of the stoichiometric concentration, with the resulting ammonia slip being oxidized in the downstream portion of the catalytic element. Any additional NOx generated by the oxidation of the ammonia is further reduced in the downstream portion before being passed to the atmosphere (22).