Patent Application
20080279732
The present invention relates to an apparatus for reducing emissions of Nitrogen oxides (NOx) in exhaust gasses of an internal combustion (IC) engine.
The introduction of reagents into the flow of an exhaust gas of an IC engine prior to the gas passing through a catalyst in order to effect selective catalytic reduction (SCR) of NOx is well known.
The current industry solution is to inject a liquid reagent into the hot exhaust gas where it decomposes into ammonia which then reacts with NOx in an SCR catalyst converting it to harmless substances (wet spray systems). The liquid reagent is, at ambient temperatures, a stable medium, but it decomposes at elevated temperatures to form at least ammonia gas. It is preferably an aqueous solution of urea or related substance such as biuret or ammonium carbamate, collectively referred to, and defined, herein as “urea”. While this solution to the problem provides a satisfactory result, changing legislation is requiring ever better NOx reduction and, while wet spray systems operate adequately, due to the their need for a minimum temperature exhaust gas sufficient to fully decompose the urea in the distance traveled between point of introduction and SCR catalyst, they are not well suited to start-up or idle engine conditions where the exhaust temperatures can be much cooler. Specifically, wet spray systems are operating at their limit at about 200 degrees Centigrade and during start-up and idle situations the temperature of the exhaust gas can be well below this. This means that either current systems have to be switched off during these engine conditions, and are therefore not removing the NOx, or another means has to be provided for coping with these conditions. Currently exhaust gas recirculation (“EGR”) is proposed in combination with wet spray systems to meet NOx output limits, however this results in a vehicle having to be provided with two independent systems to solve a common problem, adding expense and weight to the vehicle. In addition EGR reduces the fuel efficiency of the engine.
Several alternative solutions to wet spray systems have been proposed which produce an ammonia containing gas on board and incorporate reservoirs to temporarily store the ammonia before dosing it into the exhaust gas system. Such a systems is shown in U.S. Pat. No. 6,361,754 and while this systems may overcome the problem of dosing during idle conditions when the system has been operating for a while they do not tackle the problem of meeting start-up conditions so would still need another technology, for example exhaust gas recirculation in parallel with them.
A system is disclosed in U.S. patent U.S. Pat. No. 6,301,879 which has a cold start ammonia generator for trapping ammonium carbamate during the normal operation of the system for use in cold start up. However, the system is overly complex, requires lots of heating to keep all parts warm and is designed for a situation where only a small amount of gas is needed on start-up. This is sufficient with older type SCR catalysts which are largely inefficient at lower temperatures, but the newer catalysts perform fully at lower temperatures requiring larger volumes of ammonia to be available during the cold start cycle. In addition, as it draws off the main volume of gas produced during operation to charge the cold start, an increase in the overall production rate of the device is needed. In addition a means would be needed to evacuate air out of the ammonium carbamate trap before any gas could pass into it as it is a closed-ended vessel.
It is the purpose of the to providing an improved apparatus for the production of ammonia gas for use in normal running in cold start cycles of SCR systems for IC engines, especially but not exclusively diesel engines.
According to the present invention there is provided an apparatus for generating an ammonia-containing gas for use in the selective catalytic reduction of NOx contained in the exhaust gases of an IC engine, the apparatus comprising:
a) a hydrolysis reactor for containing an aqueous solution of urea (as hereinbefore defined)
b) means for heating the solution to an elevated temperature by way of heat exchange with said exhaust gases, whereby the urea is hydrolysed and the ammonia containing gases are liberated;
c) valve means operable between a substantially closed position for enabling the pressure of the ammonia-containing gas to attain a predetermined elevated pressure within the reactor, and an open position when the gas is above said predetermined pressure;
d) a reservoir having an inlet for receiving all of the ammonia-containing gas discharged from the reactor when said valve is in its open position, and an outlet for feeding ammonia-containing gas to the exhaust gases, the reservoir serving to store ammonia-containing gas during operation of the IC engine and, following the IC engine being switched off, ammonia-containing gas condensate; and
e) means for heating the reservoir,
the arrangement being such that on cold start-up of the IC engine, the means for heating the reservoir is operable to decompose the condensate into ammonia-containing gas.
By decomposing the condensate into ammonia-containing gas a source thereof is thereby provided at cold start-up of the IC engine for use in NOx reduction before the hydrolysis reactor reaches its normal working elevated temperature and pressure.
Preferably, during normal operation the reservoir is maintained at a pressure above the pressure within the exhaust conduit. The reservoir, by providing a store of ammonia containing gas during normal operation of the system enables a fast response to transient changes in the demand for ammonia to be dosed as the load on the engine changes. While it is relatively easy to use a system without a reservoir and where the ammonia is effectively produced “on demand” in a situation where there is little or only gradual changes in the demand on the system, in a highly dynamic operating situation such as that found onboard a commercial or a passenger vehicle there will normally be a time lag between a change in engine operating conditions and the ammonia-containing gas supply being matched to those conditions due to the finite time taken to hydrolyse the reagent “on demand”.
By placing the reservoir between the point of generation of ammonia-containing gas product and point of introduction to the exhaust, the requirement for ammonia containing gas to be dosed into the exhaust can be substantially met in real time. In addition the separation of the reservoir from the reactor ensures that the operating conditions within the reactor are kept constant, i.e. the pressure within the reactor does not fluctuate as a result of dosing the ammonia-containing gas into the exhaust, therefore resulting in a substantially consistent gas product mixture exiting the reactor.
Preferably the reactor is solely heated by thermal heat transfer with the exhaust gas effecting a simple heating system utilising the “free” energy available in the exhaust. To that end, the reactor is preferably placed within the exhaust conduit such that there is direct contact between the hot exhaust gas and at least a part of the exterior surface of the reactor.
Alternatively the reactor may be heated at least in part by electric means. Preferably the reactor is initially heated by both heat exchange with the exhaust conduit and electric means and, once the exhaust reactor is at operating temperature and pressure, the electric heating means is turned off and the reactor is maintained at operating temperature and pressure by heat exchange with the exhaust gas only.
In another alternative preferred arrangement the reactor is preheated by electric heating means prior to the IC engine being started such that the reactor can produce ammonia-containing gas substantially immediately from the time the IC engine is started.
When the IC engine is turned off there is a residual volume of ammonia-containing gas within the reservoir and the ammonia-containing gas will continue to be produced for a short time. As the reservoir cools the pressure of the ammonia-containing gas in the reservoir drops and the H2O condenses on the surface of the reservoir. As the temperature and pressure further drop some of the ammonia and carbon dioxide will combine to produce ammonium carbamate which then dissolves in the condensed water forming a solution of ammonium carbamate. The pressure and temperature within the reactor will also drop and the gas product contained within the reactor will undergo a similar process, the aqueous ammonium carbamate mixing with the aqueous urea solution within the reactor. As the ammonia-containing gas product within the reservoir cools and condenses, the pressure within the reactor will drop to substantially atmospheric pressure, preferably slightly below atmospheric pressure, as will the pressure within the reactor, thereby substantially eliminating the danger of ammonia escaping form the system while the engine is not running. This is particularly important for mobile IC engines, for example commercial or passenger vehicles where the vehicle may be stored within an enclosed space, for example a garage where any ammonia escaping from a pressurised system would be in a contained environment creating a build up of contained ammonia.
During cold start, i.e. when the IC engine is started from ambient temperature there is a time period before the reactor will produce ammonia-containing gas product for use in the NOx reduction process resulting in a time lag before ammonia is available for use in the NOx reduction process. This time lag is the result of a combination of several factors including: the time taken for the exhaust gas to reach its normal operating temperature (compounded by the fact that IC engines are normally started under no load or very light load conditions therefore taking longer to reach normal operating temperatures), the coefficient of thermal transfer between the exhaust gas and the liquid reagent within the reactor and the ratio of volume of liquid within the reactor to head space above the liquid. The result is that meeting requirements of emission standards is difficult during initial start up. As emissions standards are becoming ever increasingly stringent, this period during which the NOx is untreated will become unacceptable.
On cold start, heat is applied to the reservoir which then acts as a secondary reactor, evaporating the condensed water and thermally decomposing the ammonium carbamate dissolved therein to create ammonia and carbon dioxide gas thus reverting the contents of the reservoir back to their original state prior to the IC engine being shut down. When operational the reservoir is maintained at an elevated temperature to prevent the gasses therein condensing. Preferably the reservoir is maintained at a substantially constant temperature.
Preferably, heat is supplied to the reservoir by heat transfer from the hot exhaust gas both during normal operation and cold start-up. For that purpose the reservoir is preferably located such that a part of it protrudes through, or forms a part of, the exhaust conduit. As the heat and time required for the cold-start reaction in the reservoir is much less than that needed to drive the hydrolysis reaction in the reactor, the gas from the reservoir can be made available much sooner for introduction to the exhaust.
However, preferably, an electric heating element is provided for initially heating the contents of the reservoir which may be used in isolation or in combination with the heat supplied by the hot exhaust gasses.
In another preferred arrangement the electric heater is used on start up to supplement the heat transfer from the hot exhaust gas thus enabling a faster reaction of the aqueous ammonium carbamate. Preferably once the system is up to operational temperature the electric heating element is not used and the temperature of the reservoir is substantially maintained by the exhaust gas. In periods of low engine load when the exhaust gas is relatively cool the electric heater may be used to supplement the heating effect of the hot exhaust gas. When an electric heater is used during cold start the heater is preferably turned on before the IC engine is started such that the aqueous ammonium carbamate within the reservoir is substantially thermally decomposed into ammonia-containing gas such that it is immediately available on start up of the engine.
In one preferred arrangement the reservoir is isolated from direct contact with the hot exhaust gas by an air gap, optionally containing an insulating material, and is provided with an electric heating element. Heat transfer across the air gap is sufficient to produce the majority of the heat needed to maintain the reservoir at an elevated temperature under operating conditions and the electric heater is used in start up and, if needed, to supplement the heating effect of the heat transfer with the exhaust gas.
Preferably the reservoir has a means of losing heat to the environment such that a balance of heat input to heat output can be achieved approximately at its operating temperature such that continued input of heat does not cause the reservoir to continue to rise.
In an alternative arrangement where it is preferable to control the temperature of the reservoir independently of the exhaust gas temperature, the reservoir is placed completely externally of the exhaust conduit and preferably is heated by means of its proximity to the exhaust conduit. Preferably an electric heater is provided for use on start up to thermally decompose the aqueous ammonium carbamate as described above. The electric heater, or an additional heater, may optionally also heat the entire outer surface of the reservoir to ensure no re-condensation of the gas occurs during start up. Preferably, once the system is up to operational temperature the electric heating element(s) is not used and heat input to the reservoir is provided by radiated and conducted heat from the exhaust gas. Where more accurate control of the temperature of the reservoir is required a variable cooling circuit is provided operable to remove excess heat and maintain the reservoir at a substantially constant temperature less than the temperature of the exhaust gas. Preferably this cooling circuit is either a part of the engine cooling circuit or has a heat exchanger to transfer heat to the engine cooling or lubrication circuit. Preferably the reservoir is maintained at a temperature between 125 and 250 degrees centigrade, more preferably between 180 and 225 degrees centigrade.
In one preferred arrangement the reservoir is substantially positioned externally from the exhaust conduit but has a section that extends into the exhaust conduit for heat transfer arranged such that any liquid within the reservoir drains toward the section extending into the exhaust conduit. On start up any liquid within this section is directly acted on by the hot exhaust gas converting it to ammonia-containing gas. Preferably the part of the reservoir extending into the exhaust conduit comprises a heat pipe.
A system of the invention may include any one or more of the preferred features referred to above.
The invention will now be described, by way of example only, with reference to the following drawings in which:
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It will be appreciated that within the scope of the invention various components described herein with reference to one or other of the embodiments are interchangeable. For example systems falling within the scope of the invention may include a combination of features not explicitly described in respect to any particular embodiment.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0503181.0 | Feb 2005 | GB | national |
| 0505916.7 | Mar 2005 | GB | national |
| 0519322.2 | Sep 2005 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB2006/000543 | 2/16/2006 | WO | 00 | 3/18/2008 |
