US20100024400A1

US20100024400A1 - Emission control system and method

Info

Publication number: US20100024400A1
Application number: US12/184,274
Authority: US
Inventors: Ke Liu; Gregg Anthony Deluga; Dan Hancu; Daniel George Norton; Frederic Vitse; Benjamin Hale Winkler
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 2008-08-01
Filing date: 2008-08-01
Publication date: 2010-02-04

Abstract

A system includes an exhaust conduit configured to conduct a stream of exhaust gas, wherein the exhaust conduit comprises a selective catalytic reduction catalyst reactor comprising a first catalyst composition; an fuel source configured to introduce a fuel into the exhaust gas stream within the exhaust conduit upstream of the selective catalytic reduction catalyst reactor; a catalytic partial oxidation reformer in fluid communication with the exhaust gas stream and upstream from the selective catalytic reduction catalyst reactor, wherein the catalytic partial oxidation reformer can introduce a hydrogen-rich syngas co-reductant into the exhaust gas stream, when a temperature of the exhaust fluid is less than a determined threshold temperature.

Description

BACKGROUND

1. Technical Field
The present disclosure includes embodiments that relate to a system for controlling emissions. The disclosure includes embodiments that relate to a method for controlling emissions.
2. Discussion of Art
Some vehicles may emit nitrogen oxides (NOx) during use. Such emissions may be undesirable.
Emission controls have included engine modification and exhaust gas treatment. It may be desirable to have a system for emissions control that differs from those systems currently available. It may be desirable to have a method of controlling emissions that differs from those methods that are currently available.

BRIEF DISCUSSION

In one embodiment, a system for controlling emissions includes an exhaust conduit that conducts a stream of exhaust gas. The exhaust conduit includes a selective catalytic reduction catalyst including a first catalyst composition. A fuel can be introduced into the exhaust gas stream in the exhaust conduit upstream of the first catalyst composition. A catalytic partial oxidation reformer includes a second catalyst composition. The reformer can produce a hydrogen-rich syngas co-reductant from a primary fuel and can introduce the hydrogen-gas co-reductant into the exhaust gas stream in the exhaust conduit upstream from the first catalyst composition when a temperature of the exhaust fluid is greater than a determined temperature set point.
Optionally, a fuel source can hold the primary fuel or a secondary fuel. The catalytic partial oxidation reformer can be included in a fuel converter.
A method of controlling emissions includes determining a condition of an exhaust gas stream relative to a determined threshold value; responding to the exhaust gas stream condition being at or above the threshold value by flowing a co-reductant into the exhaust gas stream prior to the exhaust gas stream contacting a selective catalytic reduction catalyst reactor; and contacting the co-reductant, exhaust gas stream and a reductant with the selective catalytic reduction catalyst reactor to control a concentration of one or more components of the exhaust gas stream.
The above-described disclosure, and other features, are exemplified by the following Figures and detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the exemplary drawings wherein like elements are numbered alike in the several Figures:

FIG. 1 is a schematic illustration of an embodiment of a system for controlling emissions.

FIG. 2 is a flow chart illustrating a method according to the invention.

DETAILED DESCRIPTION

The present disclosure includes embodiments that relate to a system for controlling emissions. The disclosure includes embodiments that relate to a method for controlling emissions. A system or a method for controlling emissions may reduce the nitrogen oxides (NOx) emissions from the exhaust gas stream of a vehicle or a stationary source. Vehicles may include locomotives, marine vessels, off-highway vehicles, tractor-trailer rigs, and passenger vehicles. Emissions control refers to the ability to affect the compositional make up of an exhaust gas stream. As exhaust gas is a mixture of components, the reduction of one component almost invariably increases the presence of another component. For clarity of discussion, the chemical reduction of NOx is used as a non-limiting example of emission reduction insofar as the concentration of a determined species within the exhaust gas stream is controlled.
In one embodiment, the system may utilize a fuel source, as well as hydrogen gas (H2) obtained from a conversion reactor, to reduce emissions. The conversion reactor converts some of the fuel used by the engine (e.g., locomotive diesel fuel) into hydrogen. The fuel and/or hydrogen can be mixed with the exhaust stream and facilitate a reduction of NOx emissions in the presence of a hydrocarbon based selective catalytic reduction (SCR) catalyst bed. The fuel reductant, along, optionally, with the hydrogen-rich syngas co-reductant generated in-situ from the fuel (e.g., diesel) converter, will react with the NOx in the exhaust stream and reduce NOx to nitrogen at the surface of the SCR catalyst bed. The NOx concentration reduced in the exhaust gas stream.
The fuel can be the primary fuel, which is also utilized by the vehicle engine. The fuel source, therefore, can be the primary fuel source (e.g., the primary fuel tank). In another embodiment, the fuel can be a secondary fuel (i.e., a fuel other than the primary fuel burned by the engine). When a secondary fuel is used as a reductant it can be stored in a secondary fuel source, separate from the primary fuel used by the fuel converter.
The system can be used in a vehicle that employs an internal combustion or compression engines powered by a hydrocarbon-based fossil fuel. The hydrocarbon-based fossil fuels can be a liquid (e.g., diesel) or a gas (e.g., propane).
Hydrogen gas can be derived from the primary fuel. The hydrogen-rich syngas can be used as a co-reductant with the fuel, particularly the partially oxidized main reductant derived from the secondary fuel, so that the HC-SCR bed can be used in the presence of exhaust gases without sacrificing the operable temperature range. For example, the system disclosed herein may be effective for NOx reductions at temperatures from about 150 degrees Celsius to about 600 degrees Celsius. Moreover, the use of hydrogen-rich syngas in the HC-SCR optimized bed permits lower levels of hydrocarbon to be used for efficient catalytic reduction of the nitrogen oxides
As used herein, the term “primary fuel” is a fuel being combusted by the engine of the vehicle, locomotive, generator, or the like. Exemplary primary fuels include one or more of diesel, jet-fuel, logistic fuel (JP-8), kerosene, fuel oil, bio-diesel, and the like. As used herein, the term “secondary fuel” is a fuel other than the fuel being combusted by the engine. Suitable secondary fuels include gasoline and short chain alcohol. Short chain alcohol includes those molecules having one or more hydroxyl groups and less than about 10 carbon atoms. One short chain alcohol includes ethanol. Suitable combinations of ethanol-containing gasoline include E-10, E-85, E-90, and E-95, and the like. Also, in the following description, an “upstream” direction refers to the direction from which the local flow is coming, while a “downstream” direction refers to the direction in which the local flow is traveling. In the most general sense, flow through the system tends to be from front to back, so the “upstream direction” refers to a forward direction, while a “downstream direction” will refer to a rearward direction. The terms reducing agent and reductant are used interchangeably throughout this disclosure except where language or content indicates otherwise.
The term “fluid communication” is the containment and/or transfer of compressible and/or incompressible fluids between two or more points in the system. Examples of suitable fluids are gases, liquids, combinations of gases and liquids, or the like. The use of pressure transducers, thermocouples, injectors, flow, hydrocarbon, and sensors aid in communication and control. In one embodiment, a computer and a computer implemented controller can control and aid in the flow of fluids in the system. The term “on-board” refers to the ability of a vehicle or locomotive to host the system in its entirety aboard the vehicle or locomotive.
Referring to FIG. 1, an exemplary embodiment of the system 10 for reducing determined emission species is illustrated. The system includes a primary fuel source 12, a fuel converter 14, a SCR catalyst reactor 16 that includes an SCR catalyst, an engine 18, and a secondary fuel source 20. The primary fuel source is upstream of the fuel converter and the SCR catalyst reactor. The primary fuel source, the fuel converter, and the SCR catalyst reactor are in fluid communication with one another. The fuel converter is located between the primary fuel tank and the SCR catalyst reactor, and is upstream of the SCR catalyst reactor.
The engine is located downstream of the primary fuel source and in fluid communication with the primary fuel source. The engine is located upstream of the fuel converter and the SCR catalyst reactor and is in fluid communication with both the fuel converter and the SCR catalyst reactor via an exhaust stream conduit 22. The secondary fuel source 20 contains the secondary fuel, and is upstream of, and in fluid communication with, SCR catalyst reactor. The secondary fuel enters the exhaust stream conduit and mixes (e.g. atomizes) with the exhaust upstream of the SCR catalyst reactor. Likewise, the hydrogen-rich syngas co-reductant generated in the fuel converter enters the exhaust stream conduit upstream of the SCR catalyst reactor. In an exemplary embodiment, the SCR catalyst reactor can be employed within the exhaust stream conduit.
As mentioned previously, the primary fuel or the secondary fuel can be used as a reductant, along with the hydrogen-rich syngas co-reductant, in the exhaust stream. In one embodiment, for example, the primary fuel source 12 will feed the engine 18, the fuel converter 14, and the exhaust conduit 22 (upstream of the SCR reactor 16). In another embodiment, the primary fuel source 12 will feed the engine 18 and the fuel converter 14, while the secondary fuel source 20 will feed the exhaust conduit 22, thereby providing the secondary fuel as a reductant to be combined with the hydrogen-rich syngas.
A variety of differing fuels may be stored in the primary fuel tank and may be used based on other components of the system. The primary fuel tank supplies fuel to the engine. As mentioned above, the engine can be a spark ignition engine or a compression ignition engine. Spark ignition engines include gasoline engines, and compression ignition engines include diesel engines. Other types of hydrocarbon-based fuels can be employed in the respective internal combustion engines. As mentioned, in an exemplary embodiment, the primary hydrocarbon-based fossil fuel is a liquid fuel. The fuel converter converts long chain and/or high molecular weight hydrocarbons of the primary fuel to hydrogen-rich syngas, which can then used to reduce NOx in the exhaust depending on the exhaust temperature. Long chain hydrocarbons are hydrocarbons that have 9 or more carbon atoms. In an exemplary embodiment, an exemplary long chain hydrocarbon primary fuel is diesel.
As shown, the exhaust stream from the engine is disposed into the exhaust stream conduit, which is in fluid communication with the SCR catalyst reactor. In the illustrated embodiment, the SCR catalyst reactor includes a selective catalyst reduction bed optimized for a HC-SCR process. The reduction bed includes a catalyst, which is placed at a location within the exhaust conduit where it will be exposed to the exhaust stream containing the NOx. The catalyst may be arranged as a packed or fluidized bed reactor, coated on a monolithic or membrane structure, or arranged in any other manner within the exhaust system such that the catalyst is in contact with the effluent gas.
The NOx emissions reduction can take place over a range of temperatures found in the exhaust stream. Various factors influence the placement, selection, and type of materials and structures in conjunction with the selected operating temperature to achieve a desired emission control level. The emission control can occur at a temperature that is less than about 150 degrees Celsius. The emission control can occur at a temperature that is less than about 600 degrees Celsius. In one embodiment, the exhaust stream temperature may be in a range of from about 150 degrees Celsius to about 300 degrees Celsius; from about 300 degrees Celsius to about 350 degrees Celsius; from about 350 degrees Celsius to about 450 degrees Celsius; from about 450 degrees Celsius to about 500 degrees Celsius; or from about 500 degrees Celsius to about 600 degrees Celsius.
The secondary fuel may be fed continuously to the exhaust conduit to mix with the exhaust fluid upstream of the SCR catalyst reactor to provide the reducing agent for the reduction of NOx in the reactor. Alternatively, the secondary fuel may be fed into the exhaust conduit in a pulsed manner, or in some non-continuous manner. The method of feeding the secondary fuel may affect system performance.
The emission control, such as the reduction of NOx by the secondary fuel reducing agent, occurs in contact with the selective catalytic reduction (SCR) catalyst. Examples of suitable selective catalytic reduction catalysts are metals such as silver, gallium, cobalt, molybdenum, tungsten, indium, bismuth, or vanadium. In one embodiment, the SCR catalyst is a combination of two or more of the foregoing metals in a binary, ternary or quaternary mixture. Oxides of one or more of the foregoing metals can be used as catalysts. Oxides of metals can also be used as catalyst supports. Examples of suitable metal oxide supports are alumina, titania, zirconia, or ceria. Other supports may include silicon carbide and aluminum nitride. The SCR catalyst may be disposed upon a suitable support.
The secondary fuel can be used to reduce NOx in the exhaust stream, according to the following overall reaction (1).
NO_x+O₂+organic reductant→N₂+CO₂+H₂O (1)
The exhaust stream can include air, water, CO, CO₂, NO_x, SO_x, H₂O, and may also include other impurities. Water contained in the exhaust stream can be steam. Additionally, uncombusted or incompletely combusted fuel may be present in the exhaust stream. The secondary fuel is fed into the exhaust stream to form a gas mixture, which is then fed through the selective catalytic reduction catalyst. Sufficient oxygen to support the NO_xreduction reaction may be present in the exhaust stream.
If the oxygen present in the exhaust stream is not sufficient for the NO_xreduction reaction, additional oxygen gas may be introduced into the exhaust stream in the form of air. In some embodiments, the gas mixture may include from about 1 mole percent (mole %) to about 21 mole % of oxygen gas. In some other embodiments, the gas mixture includes from about 1 mole % to about 15 mole % of oxygen gas.
For a standard locomotive diesel engine, the exhaust temperature may be less than 375 degrees Celsius during driving cycles. But, the exhaust stream can have a lower temperature. During operation at temperatures less than about 375 degrees Celsius, the effectiveness of the secondary fuel reductant on NOx emissions can be particularly enhanced by the addition of hydrogen-rich syngas can be used as an additional reductant. The hydrogen-rich syngas may reduce the NOx emissions when the exhaust temperature is in the lower operating range.
The fuel converter is a fixed bed reactor that includes a catalytic partial oxidation (CPO) reformer followed by a water-gas shift (WGS) reactor. A gas-assisted nozzle can be utilized to atomize the primary fuel at a low-pressure inlet into the fuel reactor. The atomized primary fuel is then converted via catalytic partial oxidation into a syngas comprising a mixture of hydrogen and carbon monoxide. The fuel converter has a catalyst that can convert the primary fuel (e.g. diesel) to the hydrogen-rich syngas co-reductant. In one embodiment, the catalyst operates effectively and without thermal degradation at a temperature in a range of from about 200 degrees Celsius to about 900 degrees Celsius; operates effectively in the presence of air, carbon monoxide, carbon dioxide, water, alkanes, alkenes, cyclic and linear compounds, aromatic hydrocarbons and sulfur-containing compounds; provides for low levels of coking; and is able to resist poisoning from common poisons such as sulfur and halogen compounds. Low level coking may be handled by preferentially catalyzing the reaction of carbon with water to form carbon monoxide and hydrogen to permit the formation of a low level of carbon on the surface of the catalyst.
The catalyst composition of the fuel converter can perform a catalytic partial oxidation function. The catalytic partial oxidation function involves the oxidation of the hydrocarbon molecules of the primary fuel into carbon monoxide and hydrogen. The catalytic partial oxidation reaction is an exothermic reaction. The use of a fuel converter that employs the catalytic composition allows a single fixed bed reactor to convert diesel fuel to a syngas. The syngas can include hydrogen-rich syngas and carbon monoxide. The hydrogen-rich syngas can be used as a co-reducing agent for NOx reduction in the diesel engine exhaust stream, along side the secondary fuel reductant.
The catalytic partial oxidation sites include noble metals that perform the catalytic partial oxidation function. The catalytic partial oxidation sites include one or more “platinum group” metal components. As used herein, the term “platinum group” metal is used to generally refer to the use of platinum, palladium, rhodium, iridium, osmium, ruthenium or mixtures thereof Exemplary platinum group metal components include, without limitation, rhodium, platinum, and iridium. The catalyst composition can include an amount of the platinum group metal that is in a range of from about 0.1 weight percent to about 20 weight percent.
The platinum group metal components optionally may be supplemented with one or more base metals, particularly base metals of Group III, Group IB, Group VB and Group VIB of the Periodic Table of Elements. Exemplary base metals may include one or more of iron, cobalt, nickel, copper, vanadium, and chromium. Selection of the base metal may be based on other system parameters and compositional makeup. The platinum group catalysts, along with other base metal catalysts, are washcoated onto the molecular sieves to form a catalytic composition. Suitable molecular sieves include, for example, zeolites.
A portion of the hot exhaust gas\can be used as a secondary gas for atomizing the primary fuel in the fuel converter. Air can be used as the secondary gas for atomizing the primary fuel. In an exemplary embodiment, a portion of the exhaust stream is combined with air to form the secondary gas to facilitate the catalytic partial oxidation reaction. The amount of hot engine exhaust gas is effective to light off the catalytic partial oxidation reaction in the fuel converter. Water present in the exhaust stream can reduce coke formation on the catalyst.
In one embodiment, a controller (not shown) monitors the temperature of the exhaust gas stream via a sensor (not shown). When the exhaust gas stream temperature drops below a determined set point the controller actuates valves so that the hydrogen-rich syngas co-reductant from the fuel converter is sent to the exhaust conduit for mixing with the exhaust stream and entering the SCR catalyst reactor At low exhaust temperatures (e.g. less than about 375 degrees Celsius), hydrogen-rich syngas can decrease the NOx concentration in the exhaust gas stream relative to the amount of NOx that would otherwise be present. The hydrogen-rich syngas co-reductant, therefore, can be used on an as-needed basis depending upon the temperature of the exhaust stream. Monitoring systems, control valves, thermocouples, and the like can be used to observe the exhaust stream temperature and determine when hydrogen-rich syngas is required to be fed to the SCR catalyst reactor. The fuel converter, therefore, can be operated on an as-needed basis to produce the hydrogen-rich syngas and deliver it to the exhaust stream. In one embodiment, the fuel converter can operate continuously along with the rest of the system, and the hydrogen-rich syngas co-reductant formed can be stored in a syngas storage tank (not shown). The syngas storage tank can include a control valve in operative communication with a thermocouple for monitoring the exhaust gas temperature.
When the temperature decreases to a predetermined threshold temperature or set point, the control valve can open to permit the hydrogen-rich syngas co-reductant to enter the exhaust stream conduit and contact the SCR catalyst. The predetermined threshold temperature may be greater than 250 degrees Celsius. The predetermined threshold temperature may be less than 650 degrees Celsius. In one embodiment, the predetermined threshold temperature is in a range of from about 250 degrees Celsius to about 300 degrees Celsius, from about 300 degrees Celsius to about 350 degrees Celsius, from about 350 degrees Celsius to about 400 degrees Celsius, from about 400 degrees Celsius to about 450 degrees Celsius, from about 450 degrees Celsius to about 500 degrees Celsius, from about 500 degrees Celsius to about 550 degrees Celsius, from about 550 degrees Celsius to about 600 degrees Celsius, or greater than about 600 degrees Celsius. If desired, the fuel converter can employ more than one fixed bed reactor to affect productivity. For example, the catalytic converter can employ from about 2 to about 6 fixed bed reactors if desired.
When needed, the hydrogen-rich syngas co-reductant produced during the conversion of primary fuel in the fuel converter, can be combined in the exhaust stream together with the secondary fuel upstream of the SCR catalyst reactor in the exhaust conduit. The reduction of NOx in the reactor with hydrogen-rich syngas can take place at temperatures of as low as about 150 degrees Celsius, according to the following reaction (2).
NO_x+H₂+CO+organic reductant→N₂+H₂O+CO₂ (2)
In one embodiment, reaction (2) occurs at a temperature that is greater than about 100 degrees Celsius.
As mentioned previously, the hydrogen-rich syngas co-reductant, produced from the primary fuel, can optionally be used to supplement the secondary fuel reductant when exhaust temperatures are in this lower range. The hydrogen-rich syngas co-reductant is disposed in fluid communication with the SCR catalyst such that, during operation, the co-reductant is introduced upstream along with the first reducing agent (i.e., the secondary fuel). The use of the hydrogen-rich syngas in this manner allows the SCR catalyst reactor to operate over a relatively wider temperature range when compared to systems where hydrogen is not employed as a co-reductant. Moreover, the use of hydrogen-rich syngas as a co-reductant permits the use of lower amounts of the secondary fuel.
In various embodiments, depending on whether the system is for mobile or stationary applications, the secondary fuel is maintained on-board in a separate fuel tank from the primary fuel source, and the hydrogen-rich syngas co-reductant is produced on-board from the readily available primary fuel.
With reference to FIG. 2, an exemplary flow chart, generally designated by reference numeral 200, illustrates a method according to an embodiment of the invention. A temperature of an exhaust gas stream is measured 210, and is determined to be greater than a determined threshold value 220. If the threshold is not met, the temperature of the exhaust continues to be measured until the threshold is met 230. In response to the threshold value determination, (that is, whether the exhaust gas stream temperature is at or above the threshold temperature) a co-reductant flows into the exhaust gas stream prior to the exhaust gas stream contacting a selective catalytic reduction catalyst 240. The co-reductant, the exhaust gas stream and a reductant contact the selective catalytic reduction catalyst 250, and control a concentration of one or more components of the exhaust gas stream 260.
In an exemplary embodiment, a diesel engine operates at a power level sufficient to heat an exhaust gas stream to a trigger point (threshold temperature) of 350 degrees Celsius. A temperature sensor signals a controller that the threshold temperature has been achieved, and the controller signals a valve actuator to cause a flow of a co-reductant into the exhaust gas stream. Alternatively, rather than a temperature trigger, the controller responds to another input. Such an input may be the engine operating speed, environmental conditions, or a closed loop feedback from an emissions sensor (e.g., a NOx sensor) that monitors the exhaust gas stream.
While the disclosure has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the disclosure without departing from the essential scope thereof. Therefore, it is intended that the disclosure not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this disclosure, but that the disclosure will include all embodiments falling within the scope of the appended claims.

Claims

1. A system, comprising:

an exhaust conduit configured to conduct a stream of exhaust gas, wherein the exhaust conduit comprises a selective catalytic reduction catalyst reactor comprising a first catalyst composition;

an fuel source configured to introduce a fuel into the exhaust gas stream within the exhaust conduit upstream of the selective catalytic reduction catalyst reactor;

a catalytic partial oxidation reformer in fluid communication with the exhaust gas stream and upstream from the selective catalytic reduction catalyst reactor, wherein the catalytic partial oxidation reformer can introduce a hydrogen-rich syngas co-reductant into the exhaust gas stream, when a temperature of the exhaust fluid is less than a determined threshold temperature.

2. The system of claim 1, wherein the fuel comprises a selected one of a primary fuel and a secondary fuel.

3. The system of claim 1, wherein the catalytic partial oxidation reformer comprises a second catalyst composition that can produce the hydrogen-rich syngas co-reductant from a primary fuel.

4. The system of claim 1, wherein the fuel source is a secondary fuel source and the fuel is a secondary fuel.

5. The system of claim 1, wherein the fuel source is a primary fuel source and the fuel is the primary fuel.

6. The system of claim 1, further comprising a controller operable to control the flow of the primary fuel to the second catalyst and control the production of hydrogen-rich syngas co-reductant.

7. The system of claim 1, wherein the determined temperature set point is about 375 degrees Celsius.

8. The system of claim 2, wherein the secondary fuel comprises one or both of gasoline and a short chain alcohol.

9. The system of claim 1, wherein the first catalyst composition comprises a catalyst material selected from the group consisting of gallium, indium, tungsten, molybdenum, bismuth, vanadium, and cobalt.

10. The system of claim 1, wherein the first catalyst composition comprises a catalyst material selected from the group consisting of gallium oxide, indium oxide, molybdenum oxide, bismuth oxide, and cobalt oxide.

11. The system of claim 1, wherein the first catalyst composition comprises one or both of tungsten oxide or vanadium oxide.

12. The system of claim 1, wherein the first catalyst composition comprises silver.

13. The system of claim 1, wherein the first catalyst composition consists essentially of silver, silver oxide, or both silver and silver oxide.

14. The system of claim 1, wherein the first catalyst composition is a zeolyte.

15. The system of claim 14, wherein the first catalyst composition comprises a selected one or all of a combination of silver with the zeolyte, tungsten oxide, and vanadium oxide.

16. The system of claim 3, wherein the second catalyst composition is capable of performing a catalytic partial oxidation function of the primary fuel.

17. The system of claim 1, wherein the second catalyst composition comprises a platinum group metal.

18. The system of claim 17, wherein the platinum group metal comprises a metal selected from the group consisting of palladium, iridium, osmium, and ruthenium.

19. The system of claim 17, wherein the platinum group metal comprises platinum or rhodium.

20. The system of claim 18, wherein the second catalyst composition further comprises one or more promoter metals selected from Group VIII, Group IB, Group VB, or Group VIB of the Periodic Table of Elements.

21. A method, comprising:

determining a condition of an exhaust gas stream to be less than a determined threshold value;

responding to the exhaust gas stream condition being at or above the threshold value by flowing a co-reductant into the exhaust gas stream prior to the exhaust gas stream contacting a selective catalytic reduction catalyst; and

contacting the co-reductant, the exhaust gas stream and a fuel with the selective catalytic reduction catalyst to control a concentration of one or more components of the exhaust gas stream.

22. The method of claim 16, wherein the condition is a selected one or both of an exhaust temperature and a concentration of an exhaust emission species.

23. The method of claim 16, wherein determining the value includes sensing the condition directly or sensing an engine operating parameter.

24. The method of claim 23, further comprising forming the co-reductant.

25. The method of claim 24, wherein flowing the co-reductant comprises forming a hydrogen-rich syngas via partial oxidation of a primary fuel for an engine that is producing the exhaust gas stream.

26. The method of claim 24, wherein forming the co-reductant is controlled to occur only on demand.

27. The method of claim 24, wherein forming the co-reductant further comprises storing the co-reductant until needed.

28. The method of claim 24, wherein the co-reductant is flowed continuously into the exhaust gas stream when the exhaust gas condition is at, or less than, the threshold value.

29. The method of claim 23, further comprising controlling a flow rate or concentration of the co-reductant into the exhaust gas stream.