Emission Control Concepts

About five basic catalytic concepts have been used in the history of catalytic emission control (Fig. 20(a)). 

The first concept is the closed-loop-controlled three-way catalyst. In this, one type of catalyst, which is placed in the exhaust gas stream, is able to promote all the main reactions that lead to the simultaneous removal of carbon monoxide, hydrocarbons and nitrogen oxides. To balance the extent of the oxidation and the reduction reactions, the composition of the engine-out exhaust gas is maintained at or around stoichiometry. This is achieved by a closed-loop engine operation control, in which the oxygen content of the engine-out exhaust gas is measured up-stream of the catalyst with an electrochemical oxygen sensor, also called lambda sensor. 

There exist a multitude of engine management systems with various degrees of 

The refinement of the engine management system affects both the performance and the durability of the emission control catalyst. 

Because of its performance in promoting the main reactions to reach completion and at the same time minimizing the extent of the secondary reactions, the closed-loop-controlled three-way catalyst has become the most widely applied technique for catalytic emission control. 

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The fourth emission control concept is the oxidation catalyst. Secondary air is added to the exhaust gas to assure a lean composition, independent on the engine operation condition. The catalyst is designed to promote reactions between oxygen and both carbon monoxide and hydrocarbons, which can be removed to a high extent. However, nitrogen oxides cannot be removed in this manner. 

Fig. 25-3. Bubble concept. This pollution control concept places an imaginary bubble over an entire industrial plant, evaluating emissions from the facility as a whole instead of requiring control point-by-point on emission sources. Numbers represent emissions from individual sources, some of which can be fugitive sources, and from the entire industrial plant. Source Drawing courtesy of the Chemical Manufacturers Association.

Control of stationary sources of air pollution requires the application of either the control concepts mentioned in Chapter 28 of the control devices mentioned in Chapter 29. In some cases, more than one system or device must be used to achieve satisfactory control. The three general methods of control are (1) process change to a less polluting process or to a lowered emission from the existing process through modification of the operation,... 

Society of Automotive Engineers. (1998). Advanced Converter Concepts for Emission Control (SP-ldd2). Warrendale, PA Author. 

There are, however, some distinctive differences between the environmental and the other aspects of catalysis. Fust, the feed and operation conditions of environmental catalysts cannot be changed in order to increase conversion or selectivity, as commonly done for chemical production catalysts. Second, environmental catalysis has a role to play not only in industrial processes, but also in emission control (auto, ship, and flight emissions), and even in our daily life (water purifiers). Consequently, the concept of environmental catalysis is vital for a sustainable future. Last but not least, environmental catalysts often operate in more extreme conditions than catalysts in chemical production. There are also cases, such as automotive vehicles, where they have to operate efficiently for a continuously varying feed flow rate and composition. 

Different design concepts of catalytic combustors were proposed in order to meet the stringent requirements on emission control and on durability under the severe... 

Also, there is serious question as to whether such a system would be capable of obtaining a RCRA Part B Permit to operate without formal air emission control systems. These open tank systems are designed to be crude but effective. When one begins to collect flammable and toxic gases over such open tanks and to allow access of fork lift trucks to deliver and retrieve hoppers of slag, the logic of the system falls apart very quickly and one returns to the reactor concept or other options. 

The physical and chemical properties of synthetic crudes are different from those of petroleum. Increased NO and soot production are the principal problems of the combustion of synthetic fuels, and control concepts for these two problems are in conflict. Fuel-rich combustion decreases NO but augments soot production, while fuel-lean combustion decreases (and can eliminate) soot production but augments NO emissions. Moreover, control procedures can affect combustion efficiency and heat-transfer distribution to the chamber surfaces. Table I, taken from Grumer (6), illustrates some specific relevant properties of synthetic liquid fuels and petroleum-based fuels. The principal differences between these fuels as related to their combustion behavior are summarized in Table II. 

Having just worked through the industrial yield example allows one to visualize another important aspect. It tells us the fraction of the converted starting material that is product, a sort of efficiency term for avoidance of byproducts. For this reason, selectivity or efficiency are often used synonymously with industrial yield. From sustainability, economic, and emission control points of view, the selectivity of a process is a very important concept, and well worth the research effort toward maximization. 

The third concept is the dual-bed emission control catalyst. In this, the catalytic converter is made of two different types of catalyst. The first is either a multifunctional catalyst or at least one capable of promoting NO.v reduction reactions. The engine is calibrated so as to guarantee a net reducing exhaust gas composition. Under these conditions, the first catalyst will lead to an elimination of the nitrogen oxides. The second catalyst is an oxidation catalyst. Extra air is injected in front of the second catalyst to assist the removal of carbon monoxide and hydrocarbons. The secondary air can be added either by mechanically or by electrically driven air pumps. 

As part of an interdisciplinary study we are working on emission control catalysis for an extended parallel hybrid concept [1-5]. The main elements of the experimental car developed at the Swiss Federal Institute of Technology are the combustion engine, operating in a fixed cycle mode, a flywheel as a short term energy storage device, an electrical machine and a continuous variable transmission. One of the features of this hybrid vehicle is the so called intermittent... 

WFD/EAE (2004) Expert Advisory Forum (EAF) on Priority Substances and Pollution Control Concept Paper on Emission Control from 8 lune 2004, Common Implementation Strategy (CIS) for the Water Framework Directive (2000/60/EC), 7th EAF Meeting at Brussels, 14-15 June 2004. 

With their State Implementation Plans (SIP), EPA has mandated that each state submit pollution control plans for their districts by 1982 (with probable extension to 1987). EPA has also established an equivalence or "bubble" concept, which provides for umbrella control or grouping of Individual emission sites in one area for the purpose of overall emission control. 

H.D. Schuster, J. Abthoff and C. Noller, Concepts of Catalyst Exhaust Emission Control for Europe, SAE paper 852095. 

As a part of an interdisciplinary study [2] we are working on emission control catalysis for an extended hybrid concept (fig. 1). The heart of this hybrid system is a flywheel, equipped with a continuous variable transmission. This permits the flywheel to be accelerated with the intermittently operated combustion engine or the electrical engine independently of the speed of the vehicle. The flywheel is then used to accelerate the car and to store braking energy. The fl)nvheel concept allows the recuperation of... 

Necessarily then, the Clean Air Act apportions remaining emissions around the country. The Clean Air Act apportions remaining amounts of clean air by setting up the NAAQS as a constraint over additions to the air pollution baseline in areas where the NAAQS are not yet violated. It is necessarily a quota on new polluting industrial development, applied to air quality control regions. The emission quota concept simply makes explicit the air pollution quota assignment implicit in the Clean Air Act. 

Research activities in past decades have produced new reactor concepts. For example, hybrid (multifunctional) reactors combining reaction and heat or mass transfer are an interesting new option. Progress has also been achieved with monolithic reactors, which are already used commercially in emission control systems. An overview of these developments is given by Moulijn, Makkee, and Van Diepen (2004). Details are given by Westerterp (1992) and Cybyulski and MouUjn (1997). 

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