Exhaust gas treatment

Abstract

A device ( 1 ) is disclosed for generating gaseous ammonia formed by the hydrolysis of an aqueous solution of urea at elevated temperature and pressure and for feeding the gaseous ammonia into the exhaust gas of an IC engine as it flows through the exhaust system of the engine. The device ( 1 ) is placed in the exhaust system and has an inlet ( 2 ) for the exhaust gas and an outlet ( 3 ) for the exhaust gas so that the exhaust gas will flow through the device ( 1 ) during use. The aqueous solution of urea is contained in a reaction vessel located between the inlet ( 2 ) and the outlet ( 3 ) such that, in use, the vessel and therefore the urea solution are heated by heat exchange with the exhaust gas as it flows from the inlet ( 2 ) to the outlet ( 3 ). A pump ( 32 ) is provided for pumping urea solution into the reaction vessel and a controller ( 33 ) controls the pump ( 32 ) in response to changing NOx output from the IC engine such that, in response to an increase in NOx output, the controller ( 33 ) controls the pump ( 32 ) to increase the level of urea solution in the reaction vessel, thereby increasing the surface area of urea solution available for heat exchange with the exhaust gas so as to increase the rate of production of gaseous ammonia in the reactor vessel.

1 . A device for generating gaseous hydrolysis product comprising ammonia, formed by the hydrolysis of an aqueous solution of urea (as hereinbefore defined) at elevated temperature and pressure, for feeding into the exhaust gas of an IC engine as it flows through the exhaust system of the engine, the device being adapted to be placed in the exhaust system so that the exhaust gas will flow through it during use, and comprising
 a) a housing having an inlet for the exhaust gas and an outlet for the exhaust gas;   b) a reaction vessel located within the housing between the inlet and the outlet for containing an aqueous solution of urea and arranged such that, in use, the vessel and therefore the urea solution become heated by means of heat exchange with the exhaust gas as it flows from the inlet to the outlet;   c) a urea solution inlet to the reaction vessel and a gaseous hydrolysis product outlet from the reaction vessel;   d) a pump for pumping urea solution into the reaction vessel via the urea solution inlet;   e) control means for controlling the pump in response to changing NOx output from the IC engine;   wherein, in response to an increase in said NOx output, the control means controls the pump to increase the level of urea solution in the reaction vessel, thereby increasing the surface area of urea solution available for heat exchange with the exhaust gas so as to increase the rate of production of gaseous hydrolysis product in the reactor vessel.   
   
   
       2 . The device of  claim 1  including an SCR catalyst downstream of the outlet. 
   
   
       3 . The device of  claim 1  wherein the device includes a NOx sensor arranged in the exhaust gas flow in proximity to the inlet for measuring the quantity of NOx in the untreated gas flow. 
   
   
       4 . The device of  claim 2  wherein the device includes a NOx sensor arranged downstream of the SCR catalyst, the sensor for measuring the NOx output from the exhaust system. 
   
   
       5 . The device of  claim 1 , wherein the control means interrogates engine management data in order to deduce the NOx output from the engine. 
   
   
       6 . The device of  claim 1 , wherein the device includes a reservoir for receiving gaseous hydrolysis product from the reaction vessel and storing said product. 
   
   
       7 . The device of  claim 6  wherein the device includes a conduit for interconnecting the reservoir and the exhaust system. 
   
   
       8 . The device of  claim 7  wherein the conduit includes valve means to selectively control the feed of hydrolysis product stored in the reservoir into the exhaust gas. 
   
   
       9 . The device of  claim 8  wherein the control means controller controls the valve means in order to dose a volume of gaseous hydrolysis product to match the NOx output from the IC engine. 
   
   
       10 . The device of  claim 1  wherein the reaction vessel includes a level sensor for monitoring the level of aqueous solution in the reaction vessel. 
   
   
       11 . The device of  claim 1  wherein the reaction vessel includes a quality sensor for monitoring the quality of the aqueous solution in the reaction vessel. 
   
   
       12 . The device of  claim 10  wherein the level sensor also acts as a quality sensor for monitoring the quality of the aqueous solution in the reaction vessel. 
   
   
       13 . The device of  claim 1  wherein the reaction vessel includes a pressure sensor for monitoring the pressure of the aqueous solution in the reaction vessel. 
   
   
       14 . The device of  claim 1  wherein the reaction vessel includes a temperature sensor for monitoring the temperature of the aqueous solution in the reaction vessel. 
   
   
       15 . The device of  claim 1 , wherein the reaction vessel includes a reaction vessel sensor, said reaction vessel sensor is provided in a single cluster. 
   
   
       16 . The device of  claim 1 , including an SCR catalyst downstream of the outlet wherein the device is provided with ammonia sensors downstream of the SCR catalyst for measuring quantity of ammonia in the exhaust system outlet. 
   
   
       17 . The device of  claim 1 , including an SCR catalyst downstream of the outlet wherein a temperature sensor is provided inside the SCR catalyst to measure the temperature of the catalyst. 
   
   
       18 . The device of  claim 1 , including an SCR catalyst downstream of the outlet wherein a temperature sensor is provided upstream and/or downstream of the SCR catalyst. 
   
   
       19 . The device of  claim 1 , wherein the device includes a valve at the outlet from the reaction vessel, the valve being adapted to cause the contents of the reaction vessel, in use, to attain an elevated pressure as it becomes heated, and periodically to discharge gaseous hydrolysis product into the reservoir. 
   
   
       20 . The device of  claim 19 , further comprising a pressure sensor for monitoring the pressure of the aqueous solution in the reaction vessel wherein the valve is controlled by the control means to actuate in response to the control means detecting a predetermined pressure within the reaction vessel. 
   
   
       21 . The device of  claim 19  wherein the valve is self actuating, discharging gaseous hydrolysis product into the reservoir when a preset pressure is achieved within the reaction vessel. 
   
   
       22 . The device of  claim 19  wherein the reaction vessel includes a temperature sensor for monitoring the temperature of the aqueous solution in the reaction vessel and wherein the control means controller actuates the valve in response to the temperature of the aqueous solution in the reactor. 
   
   
       23 . The device of  claim 1 , wherein the device includes a reservoir for receiving gaseous hydrolysis product from the reaction vessel and storing said product and wherein the device includes an auxiliary heating means for heating the reservoir. 
   
   
       24 . The device of  claim 23  wherein the auxiliary heating means is an electrically powered heater, or a diesel burning heater. 
   
   
       25 . The device of  claim 1  wherein the device includes a bypass valve which can selectively control the proportion of the exhaust gas which is in thermal contact with the reaction vessel in order to control the heat input into the reaction vessel. 
   
   
       26 . The device of  claim 1 , wherein the device includes a reservoir for receiving gaseous hydrolysis product from the reaction vessel and storing said product and wherein the reservoir acts as a secondary reaction vessel to, when the IC engine is re-started, heat the contents therein to hydrolyse an aqueous solution condensate. 
   
   
       27 . The device of  claim 1  wherein in response to a decrease in said NOx output, the control means controls the pump to decrease the level of urea solution in the reaction vessel, thereby decreasing the surface area of urea solution available for heat exchange with the exhaust gas so as to decrease the rate of production of gaseous hydrolysis product in the reactor vessel. 
   
   
       28 . The device of  claim 1  wherein the device includes a holding vessel into which, in response to a decrease in said NOx output, the control means controls the pump to remove a volume of the aqueous urea from the reaction vessel for temporary holding in the holding vessel. 
   
   
       29 . The device of  claim 28  wherein, when, in response to a subsequent increase in said NOx output, the controller controls the pump to pump any liquid in the holding vessel into the reaction vessel. 
   
   
       30 . The device of  claim 28  wherein the holding vessel is maintained at a temperature above which solids form within the aqueous solution. 
   
   
       31 . The device of  claim 1 , wherein the device includes a reservoir for receiving gaseous hydrolysis product from the reaction vessel and storing said product and wherein both the reaction vessel and the reservoir are heated by heat exchange with the exhaust gas. 
   
   
       32 . The device of  claim 1  wherein the device includes a catalyst arranged within the reaction vessel to advance the rate of hydrolysis of the aqueous solution. 
   
   
       33 . The device of  claim 32  wherein the catalyst is arranged on a substrate. 
   
   
       34 . The device of  claim 33  wherein the substrate is conical or frustoconical. 
   
   
       35 . A method of controlling the generation of a gaseous hydrolysis product comprising ammonia, and the feeding of that product into the exhaust gas of an IC engine, the method comprising the steps of:
 a) providing a housing having an inlet for the exhaust gas and an outlet for the exhaust gas;   b) providing a reaction vessel located within the housing between the inlet and the outlet for containing an aqueous solution of urea and arranged such that, in use, the vessel and therefore the urea solution become heated by means of heat exchange with the exhaust gas as it flows from the inlet to the outlet;   c) providing a urea solution inlet to the reaction vessel and a gaseous hydrolysis product outlet from the reaction vessel;   d) providing a pump for pumping urea solution from in to the reaction vessel via the urea solution inlet;   the method further comprising the steps of:   e) hydrolysing an aqueous solution of urea (as hereinbefore defined) at elevated temperature and pressure within the reactor vessel;   f) determining the level of NOx in the exhaust gas;   g) controlling the pump to increase the level of urea solution in the reactor vessel in response to an increase in NOx levels in the exhaust system, thereby increasing the surface area of urea solution available for heat exchange with the exhaust gas so as to increase the rate of hydrolysis in the reactor vessel.   
   
   
       36 . The method of  claim 35  including the steps of:
 providing reservoir for receiving gaseous hydrolysis product from the reaction vessel and storing said product;   providing an auxiliary heating means for heating the reservoir;   
   
   
       37 . The method of  claim 36  further including the step of:
 controlling the auxiliary heating means to heat the reservoir at cold start-up of the IC engine in order to deliver gaseous hydrolysis product from the reservoir into the exhaust gas before the reactor temperature and pressure is elevated sufficiently by the exhaust gas to hydrolyse the aqueous solution of urea contained therein.   
   
   
       38 . The method of  claim 36  further including the step of:
 controlling the auxiliary heating means to ensure that the temperature and pressure in the reservoir is maintained at a temperature above which solids form within the aqueous solution during normal operation of the IC engine.   
   
   
       39 . The method of  claim 35  further including the step of:
 providing a holding vessel for temporarily receiving aqueous solution from the reaction vessel;   controlling the pump to pump aqueous solution from the reaction vessel into the holding vessel in response to a decrease in NOx output.   
   
   
       40 . The method of  claims 39  further including the step of:
 providing an aqueous solution tank for containing a volume of aqueous solution;   controlling the pump to pump aqueous solution from the holding vessel into the reaction vessel in response to an increase in NOx output so as to empty the holding vessel before controlling the pump to further increase the level of urea solution in the reactor vessel from the aqueous solution tank as necessary.