Patent Application
20070289291
The proposed system includes an internal combustion engine or a turbine as a source of emissions, a multi-mode plasma fuel processor, selective catalytic reduction (SCR) catalyst and means to implement a predetermined control algorithm.
The SCR catalyst suitable for this system is usually a silver/alumina catalyst. The fuel processor is able to process liquid hydrocarbon fuel such as diesel or petrol to deliver the reducing agent in required quantity and of required composition while operating in a certain sequence of the following modes in accordance with a predetermined control algorithm: partial oxidation (POX), incomplete pyrolysis, evaporation, combustion, atomization.
The fuel processor could be based on one of the plasmatrons described in a U.S. patent application No. 11/330,515 entitled “Plasma Reformer With Extended Volume Discharge” and U.S. patent application No. 11/352,138 entitled “High Enthalpy Low Power Plasma Reformer”.
The system can also be equipped with a diesel particulate filter (DPF) installed before or after the SCR catalyst. The system can further be equipped with temperature sensors. The system can also be equipped with a source of hydrogen such as an electrolysis apparatus and/or an on-board hydrogen storage device.
The fuel processor should also be able to:
Possible soot formation could be controlled by changing O/C ratio to a higher value. The O/C ratio could also control the concentration of hydrocarbons in the reducing agent. For example, by changing O/C ratio from 1 to 1.3 the concentration of (C2H2+C2H4+C2H6) decreased from 3.78 vol. % to 1.82 vol. %. The diesel flow rate to the fuel reformer decreased by 25% (from 0.8 g/s to 0.6 g/s). The hydrogen concentration remained the same.
An alternative embodiment is depicted in
Yet another embodiment is shown in
(a) High temperature of exhaust gas (up to 300-500° C.) improves conditions for fuel atomization, vaporization and mixing; it also improves ignition stability.
(b) Since exhaust gas contains much less oxygen compared to air (˜6 vol. % instead of 21 vol. %), a much higher amount (over ×3) of exhaust gas is needed to achieve the same O/C ratio. Using higher volume of an oxidizer improves fuel atomization and plasma discharge stability. More importantly, under such conditions a significant part of NOx emissions would be reduced to N2 within the plasma fuel processor.
(c) High concentration of water vapor (up to 12%) in exhaust gas could prevent soot formation during partial oxidation or incomplete pyrolysis modes of operation.
Yet another embodiment is shown in
It should be noted that in all described embodiments the use of additional oxidation catalyst (not shown) might be required. This catalyst could be installed after the SCR catalyst for oxidation of unreacted H2, CO and HC.
The present invention must not be regarded as being limited to the examplary embodiments described above as several further modifications are feasible without departing from the scope of the claims contained herewith.
The present invention also relates to a method of controlling the system, especially its NOx reduction efficiency. As opposed to a simple single-mode SCR operating scheme offered by the prior art whereby the system is controlled primarily by varying the amount of the reducing agent, the present invention in several of its aspects contemplates control over various operating modes and parameters. More specifically, the proposed control strategy is based on choosing the most appropriate operating mode or a sequence thereof for any given condition to provide an optimum combination of the quantity, composition, thermal properties and feed rate in respect of the reducing agent.
Implementing this control strategy generally involves estimation of the catalytic activity by measuring the catalyst temperature using temperature sensors or other suitable means. Also, the control parameters can be based on an engine map and, therefore, can be modified or adjusted via the EMS or any other suitable controller.
One aspect of the invention improves over conventional methods by first operating the fuel processor in a combustion mode to enable fast “light off” of the SCR catalyst. According to this aspect of the present invention, the system operates in this mode until the temperature of the SCR catalyst reaches its activation point (i.e. NOx reduction efficiency over 60% and/or temperature over 150° C.). Activation of the catalyst can be determined by temperature sensors.
As an additional step, with a view of improving overall NOx reduction efficiency of the system, when the catalyst is nearing its activation temperature (i.e. is from about 10 to about 50% or more preferably from about 20 to about 30% of the target temperature) the fuel processor may intermittently alternate between the combustion and POX modes in a predetermined sequence, for example 5 sec of combustion followed by 5 sec of POX, or 10 sec of combustion followed by 5 sec of POX, or 5 sec of combustion followed by 10 sec of POX. Such a change of modes could be accomplished by changing fuel flowrate at the constant air flowrates. In addition to the simplicity of such an approach, the liquid fuel is not compressible and mode change could be done very fast,
According to yet another aspect of the present invention, once the catalyst reaches its activation point, the fuel processor is automatically switched into POX mode. It would operate in that mode until the temperature of the catalyst reaches the range of 300° C. -500° C. after which the fuel processor would be automatically switched into the evaporation mode or fuel atomization mode or an intermittent alternating sequence thereof.
As an option, in any of these modes an additional fuel may be injected either into the hot stream of the reducing agent generated by the fuel processor or into the exhaust stream in close proximity to the SCR catalyst in order to increase the overall volume of the reducing agent and improve NOx reduction efficiency of the system.
The fuel that is injected may be in liquid or gaseous form. It may be the same fuel that is used to propel the vehicle, for example diesel or petrol, or any other suitable hydrocarbon fuel.
Another method of the present invention relates to the use of an on-board source of hydrogen such as an electrolysis apparatus or a hydrogen storage device. Such a source can be used in addition to the fuel processor or instead of it in conjunction with an additional injection of diesel fuel into the exhaust stream. Alternatively, for additional flexibility a hydrogen storage could also be used to store the hydrogen rich gas produced by the fuel processor.
According to yet another aspect of the present invention, the fuel processor can be operated in several modes intermittently, with the ratio of on/off cycles with the mode being 1:1, 2:1, 1:2, 3:1, 1:3, 4:1, 1:4, 5:1 or 1:5. A predetermined sequence of alternating modes allows to reduce the fuel penalty and improve NOx reduction efficiency.
According to yet another aspect of the present invention, should the temperature of the catalyst drop below or start dropping towards its activation point, the fuel processor should operate in a combustion mode as described above.
According to yet another aspect of the present invention, should the temperature of the catalyst rise above or start approaching a certain predetermined point (typically 650° C.-700° C.), means to cool the exhaust stream and/or the reducing agent will be automatically activated.
The invention has been shown and described with respect to certain aspects, examples, and embodiments. While a particular feature of the invention may have been disclosed with respect to only one of several aspects, examples, or embodiments, the feature may be combined with one or more other features of the other aspects, examples, or embodiments as may be advantageous for any given or particular application.
A well-controlled temperature in the exhaust system improves NOx reduction efficiency as well as minimizes the risk of an incomplete reaction of the reducing agent or the risk of the reducing agent being destroyed by excessively high temperatures. Also, an appropriate temperature and thermal mass management provides for an increased duty cycle of a catalyst.
