WO2008065390A1

WO2008065390A1 - Exhaust gas treatment

Info

Publication number: WO2008065390A1
Application number: PCT/GB2007/004551
Authority: WO
Inventors: Clive Buckberry; Stuart Charles Davey
Original assignee: Imi Vision Limited
Priority date: 2006-12-01
Filing date: 2007-11-28
Publication date: 2008-06-05

Abstract

A combined engine gas recirculation (EGR) and selective catalytic reduction (SCR) apparatus has an EGR heat exchanger 9 for cooling exhaust gasses by heating exchange with a cooling fluid prior to return to the engine (1) for reducing NOx formation, and an SCR catalyst (7) for removing NOx from the exhaust gases prior to discharge of the exhaust gasses to atmosphere. A conduit (8) connects the outlet exhaust of the combustion chambers of the engine (1) to the hot exhaust gas inlet of the EGR heat exchanger (9) and a return conduit (4) connects the cooled exhaust gas outlet of the EGR heat exchanger (9) to the inlet of the combustion chambers of the engine (1). A hydrolysis reactor (12) for generating an ammonia containing gas for use in the SCR catalyst (7) is arranged within the EGR heat exchanger (9) for heat exchange with the exhaust gasses from the combustion chambers of the engine (1) and a dosing valve controls the addition of ammonia-containing gas to the exhaust gasses upstream of the SCR catalyst (7).

Description

Exhaust Gas Treatment

The present invention relates to an apparatus for reducing emissions of Nitrogen oxides (NOx ) in exhaust gasses of an internal combustion (IC) engine.

There are two methods of reducing NOx which are well known, exhaust gas recirculation (EGR) and selective catalytic reduction with an ammonia reagent (SCR).

EGR systems re-circulate exhaust into the intake stream to the engine combustion chamber replacing some of the normal intake air. The re-circulated exhaust gases have already combusted, so they do not burn when they are re-circulated back into the combustion chamber for a second time. This chemically slows and cools the combustion process by several hundred degrees, thus reducing NOx formation which occurs when temperatures in the combustion chamber get too hot.

hi the SCR process, reagents are introduced 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. The known systems for SCR principally fall into one of two categories, those which introduce gaseous ammonia into the exhaust conduit and those which introduce into the exhaust conduit a liquid reagent which decomposes into ammonia gas in the conduit.

Both of the above mentioned methods work successfully but have different benefits. SCR is most effective at high engine loads as it can enable better fuel efficiency. However its lower boundaries of effectiveness are dictated by its need for a minimum gas temperature of around 180 degrees to work (dependant on the particular catalyst material). Below this temperature, the SCR catalyst material is less effective in the process to remove all the NOx from the exhaust gas. EGR is effective at lower engine loads and associated exhaust gas temperatures but can not give the same fuel efficiencies as SCR at higher engine loads. Therefore to have a system that effectively reduces NOx across the full range of engine operating parameters it is beneficial to have both systems working together. This however generally leads to a complex solution and related high cost of having two separate systems that essentially perform the same function, that of reducing NOx emissions in the exhaust gas.

It is the purpose of the present invention to provide a simplified system of NOx reduction that reduces NOx at both low and high engine loads.

According to a first aspect of the present invention there is provided an engine gas recirculation (EGR) heat exchanger comprising: an inlet for hot exhaust gas, an outlet for cooled exhaust gas, an inlet for cooling fluid, an outlet for cooling fluid, and a hydrolysis reactor therein, said hydrolysis reactor and said cooling fluid being arranged for heat transfer with the exhaust gas.

In one preferred embodiment the inlet is adapted to accept a liquid cooling fluid. In a second preferred embodiment the inlet is adapted to accept air as the cooling fluid.

Preferably the hydrolysis reactor is arranged for containing a reagent, preferably an aqueous solution of urea, such that, in use, by means of heat exchange with the exhaust gas the reagent becomes heated and hydrolyses thereby releasing an ammonia- containing gas.

Preferably the hydrolysis reactor further comprises a valve means to control the release of ammonia-containing gas from the reactor such that, in use, the pressure within the hydrolysis reactor becomes elevated increasing the rate of the hydrolysis reaction.

According to a second aspect of the present invention there is provided a combined engine gas recirculation (EGR) and selective catalytic reduction (SCR) apparatus comprising: an exhaust conduit downstream of an internal combustion engine having an SCR catalyst therein; an EGR heat exchanger according to the first aspect of the invention; an inlet conduit connecting the outlet exhaust of the combustion chambers of the engine to the hot exhaust gas inlet of the heat exchanger; a return conduit connecting the cooled exhaust gas outlet of the heat exchanger to the inlet of the combustion chambers of the engine; a dosing conduit leading from said hydrolysis reactor into the exhaust conduit, upstream of the SCR catalyst, and a dosing valve to control the addition of ammonia-containing gas to the exhaust gas via the dosing conduit.

Preferably interposed between the hydrolysis reactor and the dosing valve is an ammonia-containing gas reservoir for temporarily storing at least a portion of the ammonia-containing gas.

Preferably the reservoir is heated by heat exchange with cooled exhaust gasses flowing through the return conduit. The return gasses have been cooled and as such provide a relatively stable temperature environment in which to keep the reservoir at an elevated temperature which is desirable to prevent the formation of solid deposits or condensation therein. In one arrangement the reservoir is located within the exhaust conduit. In an alternative arrangement the reservoir is located adjacent the exhaust conduit, hi another arrangement, the reservoir is located external to the exhaust system and the reservoir is provided with a heater, said heater operable to maintain the reservoir at a substantially constant temperature.

In use at low engine temperatures the EGR system works as is known in the art. Exhaust gas exits the combustion chambers of the engine and enters the heat exchanger wherein it is cooled by the cooling fluid which could be liquid or air. The cooling fluid circulates through the heat exchanger, where it absorbs heat from the exhaust gas and a cooling heat exchanger where it looses heat. This cooling heat exchanger may exchange heat between the cooling fluid and the cooling circuit of the engine or may have its own means of heat loss, for example an air system may have an intercooler as known in the art. The flow of the cooling fluid can be regulated to ensure the exhaust gas is cooled to a substantially constant temperature. The cooled exhaust gas is re-circulated back into the intake stream of the engine combustion chambers slowing the combustion and reducing its temperature resulting in reduced NOx formation.

In the heat exchanger of the invention, prior to or simultaneous with being cooled, the hot exhaust gas heats the hydrolysis reactor, heating the aqueous reagent therein causing it to hydrolyse to form an ammonia containing gas. The gas pressure builds within the reactor and is controlled by a control valve which allows some of the gas to exit the reactor via the ammonia conduit and enter the reservoir vessel. Where the reservoir vessel is in the cooled exhaust conduit, the ammonia-containing gas therein, is therefore maintained at a substantially constant temperature. Once the exhaust gas flowing through the exhaust conduit has heated the SCR catalyst to a temperature at which SCR can effectively take place, ammonia-containing gas is dosed into the exhaust conduit upstream of the SCR catalyst where it mixes with the exhaust gas and flows therewith through the catalyst thereby reducing the NOx. As the hydrolysis reactor absorbs some of the energy from the exhaust gas, the cooling capacity of the cooling fluid and cooling heat exchanger is reduced. This is greatly advantageous, for example in passenger vehicles, where there is a limited space envelope for the heat exchanger (the radiator) and furthermore where radiator size has been shown to increase the mortality rate in the event of a vehicle colliding with a pedestrian.

Preferably the cooling fluid within the heat exchanger can be selectively directed to cool the hydrolysis reactor, cooling the reagent therein and slowing the production of the ammonia-containing gas if there is a decreased demand in ammonia-containing gas output of the reactor.

Preferably the engine coolant fluid is used to preheat the urea feed lines to the reactor and also the valves and gas delivery lines to minimise cold spots during start up which may cause crystalline deposits to form therein. Furthermore, during normal operation of the system, i.e. once at normal operational temperature, the engine cooling fluid is preferably used to cool the valves to protect the components thereof from exposure to the full heat of the ammonia containing gas produced by the reactor. Embodiments of the invention will now be described, by means of example, with reference to the drawing in which:

Figure 1 is a system diagram of a combined EGR/SCR system according to the invention;

Figure 2 is a diagram of a hydrolysis reactor suitable for use with the invention; and

Figure 3 is a diagram of a heat exchanger according to the first aspect of the invention.

Referring to Figure 1 an engine system with an exhaust gas purification system according to the invention is shown. The system comprises an engine 1 which has an inlet manifold 2 which supplies air to the combustion chambers of the engine. The inlet manifold 2 is fed from two sources, with air from the turbocharger 3 and with recirculated exhaust gas via conduit 4. Gas from the inlet manifold 2 is drawn into the engine cylinders where it mixes with fuel and is combusted. The temperature of combustion can be controlled by controlling the percentage of air and re-circulated exhaust gas in the gas mixture and/or the temperature of the re-circulated exhaust gas. Waste exhaust gasses from the combustion chamber of the engine 1 enters the outlet manifold 16 from which a portion is diverted via valve 17 and conduit 8 to the EGR heat exchanger 9 wherein it looses heat to a coolant fluid passing therethrough. The remainder of the exhaust gas passes through the exhaust conduit 5, through an oxidation catalyst 6 and an SCR catalyst 7 and exits the exhaust for discharge to atmosphere. The cooling fluid is part of the main engine liquid cooling system such that the fluid from the EGR heat exchanger flows through the main engine liquid cooling system where it looses heat to the atmosphere. On board a vehicle this is achieved by passing the fluid through a radiator 10 where it cools. The cooled exhaust gas exits the EGR heat exchanger and flows back to the inlet manifold 2 via return conduit 4. The cooling circuit is provided with a controller to control the flow of coolant through the EGR cooler 9 and therefore the temperature of the recirculated gas. A temperature sensor (not shown) is placed in the return circuit, the sensed temperature can be used the control the flow of cooling fluid and therefore the temperature of the returning gas. Also placed in the EGR heat exchanger is a hydrolysis reactor 12 which is supplied with a pressurised supply of aqueous urea. In use the exhaust gas passes over the reactor which absorbs heat therefrom, causing the reactor, and therefore the aqueous urea it contains to increase in temperature. As the temperature increases the urea starts to decompose into an ammonia-containing gas and the pressure within the reactor 12 rises. As the reactor 12 is adsorbing heat from the exhaust gas, the exhaust gas is already partially cooled by the time that it interacts with the cooling fluid in the EGR heat exchanger 9. This reduces the load on the cooling circuit and enables a reduced size radiator 10 to be used. The reactor 12 has an ammonia conduit 13 leading to a reservoir 14 which is heated by heat exchange with the cooled exhaust gas flowing through the return conduit 4 which, although substantially cooled, still have a temperature above ambient, typically in the region of 150 to 250 degrees centigrade. As the temperature of the exhaust gas is maintained substantially constant, the temperature of the reservoir 14 is also maintained substantially constant. The ammonia conduit 13 has a pressure relief valve (not shown) which vents pressure above a certain value, which in this case is 18 barg, from the reactor 12 into the reservoir 14. Leading from the reservoir 14 into the exhaust conduit 5 is a dosing conduit 15 which is provided with a control valve (not shown) to control the flow therethrough to add a stoichiometric volume of the ammonia- containing gas from the reservoir to the exhaust gas flowing through the exhaust conduit 5. The ammonia containing gas is added to the exhaust gas upstream of the SCR catalyst 7 such that it mixes with the exhaust gas and passes therewith through the SCR catalyst where NOx is converted by catalytic reduction to less harmful substances, primarily carbon dioxide, and nitrogen.

In use, the SCR catalyst is only efficient above a certain temperature and the EGR loses effectiveness as the exhaust temperature rises, therefore with the system of the invention at lower temperatures primarily EGR is used to minimise the NOx production and as soon as the SCR catalyst reaches effective operating temperature (sensed by sensors therein) a combination of SCR and EGR can be used to minimise NOx production and then to reduce those which are produced to less harmful compounds. Within the EGR heat exchanger 9 the coolant circuit has a switchable section (not shown) which can optionally circulate the cooling fluid in thermal contact with the reactor 12 such that the reactor is cooled. This can be used to cool the reactor 12 when the engine is shut down or if it is over producing ammonia-containing gas.

Referring to Figure 2 a reactor 602 for use in a gas treatment apparatus is shown comprising an elongate body 624 with a bulbous head section 633 and a conical lower section 634. During use the reactor 602 is heated by heat transfer from the hot exhaust gasses of an engine (not shown) to hydrolyse the urea as described above). The reactor has a level sensor 627 entering at its top and extending downwards therefrom into the liquid reagent within the reactor. The liquid level sensor 627 is situated on the central axis of the reactor. By placing the liquid level sensor 627 on the central axis as the liquid moves slightly from side to side the level at the central axis should not change significantly. Preferably the liquid level sensor 627 measures the liquid level 628 on a continuous scale. The reactor 602 has an inlet 625 for the supply of pressurised liquid reagent and an outlet 626 which leads to a pressure control valve (not shown). The reactor 602 has a baffle 635 situated in its head section 633 above the liquid level and below the outlet 626. In the event of any splashing of the reagent within the reactor 602, for example due to motion of the vehicle, the baffle 635 prevents splashes of liquid from exiting from the outlet 626. The liquid level 628 may be controlled by controlling the volume of liquid reagent pumped into the reactor via inlet 625 dependant on the sensed liquid level. The heat transfer from the hot exhaust gas is dependent on the wetted surface area of the reactor 602. The geometry of the conical section 634 allows for a specific non linear relationship of heat transfer to liquid level to be achieved. To assist heat transfer from the exhaust gas to the reactor a number of heat exchange fins 636 are shown on the external surface of the reactor. The surface area of the fins 636 changes in relation to the height of the reactor 602 and thus the heat input to the liquid reagent can be controlled by varying the liquid level 628. For additional heat transfer to the liquid, heat exchange fins 637 fins are shown inside the reactor 602 to increase the contact surface area between the reactor body 624 and the liquid reagent within the reactor 602. The reactor 602 is also provided with temperature 631 and pressure 632 sensors to monitor the temperature and pressure of the gas within the reactor.

Referring to Figure 3 an EGR heat exchanger 300 according to the invention is shown. The heat exchanger 300 may be used in the system described previously. The heat exchanger 300 has an inlet 302 for hot exhaust gasses and an outlet 304 for cooled exhaust gasses. The term cooled is indicative of the exhaust gas temperature exiting the heat exchanger relative to the temperature of the exhaust gas entering the heat exchanger. Situated in the heat exchanger 300 is a heat exchange device 306 comprising heat exchange fins 308 through which passes a heat exchange fluid which enters the heat exchanger 300 via inlet 310 and exits via outlet 312. Also situated within the heat exchanger 300 is a hydrolysis reactor 314 having a reagent inlet 316, a pressure control valve 318 and an outlet conduit 320. In use the hydrolysis reactor 314 is supplied with a reagent, for example urea, via the reagent inlet 316 such that, by means of heat exchange with the exhaust gas, the reagent becomes heated and hydrolyses thereby releasing an ammonia-containing gas. The control valve 318 causes the pressure within the hydrolysis reactor 314 to become elevated thereby increasing the rate of the hydrolysis reaction. The control valve 318 also allows the ammonia- containing gas to leave the reaction vessel 314 to be used in an SCR process for NOx reduction.

Claims

1 An engine gas recirculation (EGR) heat exchanger comprising: an inlet for hot exhaust gas, an outlet for cooled exhaust gas, an inlet for cooling fluid, an outlet for cooling fluid, and a hydrolysis reactor therein, said hydrolysis reactor and said cooling fluid being arranged for heat transfer with the exhaust gas.

2 An engine gas recirculation (EGR) heat exchanger according to claim 1 wherein the hydrolysis reactor is arranged for containing a reagent, preferably an aqueous solution of urea, such that, in use, the reagent becomes heated by means of heat exchange with the exhaust gas and hydrolyses thereby releasing an ammonia- containing gas.

3 An engine gas recirculation (EGR) heat exchanger according to claim 1 or claim 2 wherein the hydrolysis reactor further comprises a valve means to control the release of ammonia-containing gas from the reactor such that, in use, the pressure within the hydrolysis reactor becomes elevated.

4 An engine gas recirculation (EGR) heat exchanger according to any one of claims 1 to 3 wherein the hydrolysis reactor has a plurality of fins thereon to promote heat exchange with the exhaust gas.

5 An engine gas recirculation (EGR) heat exchanger according to any one of the previous claims wherein the hydrolysis reactor has a cooling means associated therewith.

6 An engine gas recirculation (EGR) heat exchanger according to claim 5 wherein the cooling means comprises heat exchange between the contents of the hydrolysis reactor and the cooling fluid.

7 A combined engine gas recirculation (EGR) and selective catalytic reduction (SCR) apparatus comprising: an exhaust conduit downstream of an internal combustion engine having an SCR catalyst therein; an EGR heat exchanger according to any one of claims 1 to 6; an inlet conduit connecting the outlet exhaust of the combustion chambers of the engine to the hot exhaust gas inlet of the heat exchanger; a return conduit connecting the cooled exhaust gas outlet of the heat exchanger to the inlet of the combustion chambers of the engine; a dosing conduit leading from said hydrolysis reactor into the exhaust conduit upstream of the SCR catalyst, and a dosing valve to control the addition of ammonia-containing gas to the exhaust gas via the dosing conduit.

8 A combined engine gas recirculation (EGR) and selective catalytic reduction (SCR) apparatus according to claim 7 wherein interposed between the hydrolysis reactor and the dosing valve is an ammonia-containing gas reservoir for temporarily storing at least a portion of the ammonia-containing gas.

9 A combined engine gas recirculation (EGR) and selective catalytic reduction (SCR) apparatus according to claim 8 wherein the reservoir is heated by heat exchange with cooled exhaust gasses flowing through the return conduit.

10 A combined engine gas recirculation (EGR) and selective catalytic reduction (SCR) apparatus according to claim 9 wherein the reservoir is located within the exhaust conduit.

11 A combined engine gas recirculation (EGR) and selective catalytic reduction (SCR) apparatus according to claim 8 wherein the reservoir is located external to the exhaust system and the reservoir is provided with a heater, said heater operable to maintain the reservoir at a substantially constant temperature.

12. A combined engine gas recirculation (EGR) and selective catalytic reduction (SCR) apparatus according to any one of claims 7 to 10 wherein the hydrolysis reactor is heated by the hot exhaust gasses thereby cooling the exhaust gasses prior to, or simultaneous, heat exchange between the exhaust gasses and the cooling fluid.