GB2576511A

GB2576511A - Producing ammonium carbamate and reducing nitrogen oxides

Info

Publication number: GB2576511A
Application number: GB1813541.8A
Authority: GB
Inventors: Wilson Jonathan; Hargrave Graham
Original assignee: Loughborough University
Current assignee: Loughborough University
Priority date: 2018-08-20
Filing date: 2018-08-20
Publication date: 2020-02-26
Also published as: GB201813541D0

Abstract

A process for reducing nitrogen oxides in an exhaust stream in large-scale applications, such as marine vessels and powerplants, and for use at a filling station or a depot.The process comprises providing a first reservoir ( 12) having a first composition therein, the first composition comprising aqueous urea; transferring a portion of the first composition to a second reservoir ( 14): heating the portion of the first composition in the second reservoir to generate a second composition, the second composition comprising aqueous ammonium carbamate: and introducing a portion of the second composition into an exhaust scream (10) comprising nitrogen oxides. The second reservoir comprises at least two tanks ( 14A, 14B, 14C), an anti-slosh device; and the second composition is heated to avoid precipitation of ammonium carbamate. A portion of the second composition is introduced into the exhaust stream by means of pressure-driven injection and a control system may be employed that responds to on-board diagnostics.

Description

PRODUCING AMMONIUM CARBAMATE FOR REDUCING NITROGEN OXIDES

This invention relates to processes for producing ammonium carbamate for reducing nitrogen oxides (NOx) in an exhaust stream, and to apparatus for carrying out the processes.

Vehicle engines manufactured for sale in Europe must abide by a set of stringent emission targets laid out by the European Commission which covers a range of possible pollutants. Nitrogen oxides (NO and NO₂, referred to collectively as NOx) are one such emission. NOx emission limits are set in other fields as well. For example, MARPOL 73/78 (the International Convention for the Prevention of Pollution from Ships, 1973 as modified by the Protocol of 1978) sets limits for NOx emissions from ships.

A selective catalytic reduction (SCR) system can be employed to reduce NOx emissions. The SCR system has been used in power plants for some time and uses ammonia as a reductant which reacts with NOx in the presence of a catalyst and produces nitrogen and water.

The use of SCR in vehicles is a more recent development and employs an ammonia precursor in place of ammonia because ammonia is impractical to refuel and potentially unsafe to carry.

US2001/0053342 describes a device where gaseous ammonia is made available by heating a solid storage medium in a container. The storage medium binds ammonia by physical and/or chemical absorption. The container is large (e.g. 10 litres) in order to store enough ammonia to last for the period between vehicle services.

For vehicles, there has been an industry-wide adoption of an ammonia precursor consisting of 32.5% urea and 67.5% deionized water. This aqueous urea solution is standardised as AUS32 (aqueous urea solution) in ISO 22241. It is commonly known as AdBlue® or diesel exhaust fuel (DEF).

AdBlue® is injected directly into a hot exhaust stream where, due to the elevated temperatures, it decomposes producing ammonia. However, a temperature of 350°C within the exhaust stream is desirable for full decomposition. Therefore periods of operation can exist where temperatures are insufficient to fully support decomposition to ammonia. SCR operation must be ceased since the urea can produce alternative decomposition products, which form deposits. These deposits can build up within the exhaust system increasing back pressure and leading to engine failure. Additionally, due to its high water content AdBlue® is prone to freezing.

US2016/153335 Al (Toyota) describes an exhaust gas purification apparatus of an internal combustion engine that includes an SCR NOx catalyst, and a tank that stores aqueous urea, a hydrolysis catalyst and a heater. The hydrolysis catalyst is heated under high pressure to prevent gasification of the reducing agent.

W02018100187 describes a process for reducing nitrogen oxides in an exhaust stream. The process comprises providing a first reservoir having the first composition therein, the first composition comprising aqueous urea; and transferring a portion of the first composition along a flow path, the flow path being in communication with a second reservoir. A portion of the first composition is converted into a second composition, the second composition comprising aqueous ammonium. The second composition is introduced into an exhaust stream comprising nitrogen oxides.

According to a first aspect of the present invention there is provided a process for reducing nitrogen oxides in an exhaust stream, the process comprising providing a first reservoir having a first composition therein, the first composition comprising aqueous urea;

transferring a portion of the first composition to a second reservoir;

heating the portion of the first composition in the second reservoir to generate a second composition, the second composition comprising aqueous ammonium carbamate; and introducing a portion of the second composition into an exhaust stream comprising nitrogen oxides;

wherein the second reservoir comprises at least two tanks.

According to a second aspect of the present invention there is provided an apparatus for reducing nitrogen oxides in an exhaust stream, the apparatus comprising: a first reservoir for a first composition, the first composition comprising aqueous urea; a second reservoir for a second composition, the second composition comprising aqueous ammonium carbamate;

a passage for an exhaust stream;

flow control apparatus arranged to couple the first reservoir to the second reservoir and thereby transfer a portion of the first composition to the second reservoir; and flow control apparatus arranged to couple the second reservoir to the passage and thereby introduce the second composition into the exhaust stream;

wherein the second reservoir comprises at least two tanks.

The use of at least two tanks for the second composition (comprising ammonium carbamate) is beneficial. The second composition can be generated in one or more tanks while a portion of the second composition is being introduced into the exhaust stream from another tank. Hence aqueous ammonium carbamate can be simultaneously generated and employed for reduction of NOx.

The process allows aqueous ammonium carbamate to be employed for reduction of NOx in place of aqueous urea (e.g. AdBlue®) while benefitting from the ready availability of aqueous urea.

The process is especially useful for large-scale applications, such as marine vessels and powerplants. The process would also be useful at a filling station or a depot.

The exhaust stream may be an exhaust stream from a marine vessel, such as a ship (including container ships and ferries), a hovercraft, or a catamaran.

The exhaust steam may be an exhaust stream from a power plant i.e. an industrial facility for generating electricity, e.g. a fossil-fuel power station.

The portion of the first composition may be heated within a tank that is closed to the atmosphere. In this way, heating causes the pressure within the tank to increase. The second reservoir may comprise at least one (such as two or more) closed (sealed) tanks.

The second reservoir may comprise at least 3, at least 4, at least 5, at least 8, at least 10 or at least 15 tanks and/or the second reservoir may comprise 50 tanks or fewer, 40 tanks or fewer, 30 tanks or fewer or 20 tanks or fewer. For example, the second reservoir may comprise 3 to 10 tanks. It will be appreciated that each of the tanks is a separate container.

The second reservoir comprises at least two tanks and can be considered to comprise a first tank and a second tank, wherein the portion of the first composition is transferred to the first tank and the portion of the second composition is introduced into the exhaust stream from the second tank.

The volume of the second reservoir may be described by reference to the average (mean) volume of the tanks. The tanks may have an average volume of at least 10 litres (1 litre = 1000cm³), at least 20 litres, at least 30 litres, at least 40 litres, at least 60 litres, at least 100 litres or at least 150 litres and/or the tanks may have an average volume of 500 litres or less, 200 litres or less, 150 litres or less, 120 litres or less, 90 litres, 50 litres or less or 30 litres or less.

The volume of the second reservoir may be described with reference to a specific tank. At least one tank may have a volume of at least 10 litres (1 litre = 1000cm³), at least 20 litres, at least 30 litres, at least 40 litres, at least 60 litres, at least 100 litres or at least 150 litres and/or at least one tank may have a volume of 500 litres or less, 200 litres or less, 150 litres or less, 120 litres or less, 90 litres or less or 50 litres or less.

The total volume of the second reservoir (i.e. the sum of the volumes of each of the tanks) may be at least at least 20 litres, at least 40 litres, at least 60 litres, at least 100 litres, at least 200 litres, at least 500 litres or at least 1000 litres and/or the total volume of the second reservoir may be 1500 litres or less, 1000 litres or less, 500 litres or less, 200 litres or less, 100 litres or less or 50 litres or less.

The volume (total volume) of the first reservoir may be greater than the volume (total volume) of the second reservoir. The first reservoir holds a store of aqueous urea which is readily available and can be converted to aqueous ammonium carbamate when desired.

Alternatively the volume (total volume) of the first reservoir may be less than the volume (total volume) of the second reservoir. It is relatively easy to obtain aqueous urea so the first reservoir can be re-filled readily.

The volume (total volume) of the first reservoir may be at least at least 20 litres, at least 40 litres, at least 60 litres, at least 80 litres, at least 100 litres, at least 150 litres or at least 200 litres and/or the volume of the first reservoir may be 1500 litres or less, 1000 litres or less, 500 litres or less, 300 litres or less, 200 litres or less, 100 litres or less or 50 litres or less.

Heating the portion of the first composition in the second reservoir to generate the second composition may comprise heating for at least 4, at least 6, at least 8 or at least 10 hours and/or heating for 48 hours or less, 24 hours or less, 16 hours or less or 12 hours or less. For example, heating for 8 to 16 hours. Heating for a longer period allows a lower temperature to be employed.

Heating the portion of the first composition in the second reservoir to generate the second composition may comprise heating at an average (mean) temperature of 300°C or less, 200°C or less, or 150°C or less and/or heating at an average (mean) temperature of at least 50°C, at least 75°C, at least 100°C or at least 150°C. The average temperature may be determined by placing a temperature probe in the aqueous solution

Heating the portion of the first composition in the second reservoir to generate the second composition may comprise heating the portion of the first composition in a tank to a pressure of at least 200kPa, at least 500kPa, at least lOOOkPa or at least 10000 and/or a pressure of 30000kPa or less, lOOOOkPa or less, 2000kPa or less, lOOOkPa or less or 500kPa or less.

Heating the portion of the first composition in the second reservoir to generate the second composition may comprise heating for 6 to 24 hours at an average temperature of 100 to 200°C.

Heating the portion of the first composition in the second reservoir to generate a second composition may comprise heating in the absence of a catalyst or heating in the presence of a catalyst. The inventors have determined that a catalyst (e.g. a hydrolysis catalyst) is not necessary for conversion of aqueous urea to aqueous ammonium carbamate.

The second reservoir may comprise one or more heat sources. For example, the heat source may be a heat exchanger or an electrical heater.

The heat source(s) may be located within one or more of the tanks. Each of the tanks may comprise a heat source.

The heat source(s) may be located outside the tanks.

Heating may be stopped before the portion of the second composition is introduced into the exhaust stream. In this way a tank may be allowed to cool naturally or by means of a heat exchanger.

According to a third aspect of the present invention there is provided a process for reducing nitrogen oxides in an exhaust stream, the process comprising providing a first reservoir having a first composition therein, the first composition comprising aqueous urea;

transferring a portion of the first composition to a second reservoir;

heating the portion of the first composition to generate a second composition, the second composition comprising aqueous ammonium carbamate; and introducing a portion of the second composition into an exhaust stream comprising nitrogen oxides;

wherein the second reservoir comprises an anti-slosh device.

According to a fourth aspect of the present invention there is provided an apparatus for reducing nitrogen oxides in an exhaust stream comprising nitrogen oxides, the apparatus comprising a first reservoir for a first composition, the first composition comprising aqueous urea; a second reservoir for a second composition, the second composition comprising aqueous ammonium carbamate;

a passage for an exhaust stream;

flow control apparatus arranged to couple the first reservoir to the second reservoir and thereby transfer a portion of the first composition to the second reservoir; and flow control apparatus arranged to couple the second reservoir to the passage and thereby introduce a portion of the second composition into the exhaust stream; wherein the second reservoir comprises an anti-slosh device.

In fluid dynamics, “slosh” refers to the movement of liquid inside another object (which is, typically, also undergoing motion). The second composition is stored in the second reservoir before being introduced (e.g. injected) into the exhaust stream. As such, movement of the second reservoir will cause movement of the second composition.

A suitable anti-slosh device includes a baffle to compartmentalise the second reservoir. The second reservoir may comprise at least 2, at least 5 or at least 8 baffles and/or the second reservoir may comprise 20 baffles or fewer or 10 baffles or fewer.

The shape and orientation of the second reservoir will affect how the second composition moves.

The exhaust stream may flow in a generally horizontal direction i.e. generally parallel with the ground. Alternatively the exhaust stream may flow in a generally vertical direction i.e. generally perpendicular to the ground, e.g. upwards. This invention may be particularly useful when the exhaust stream flows upwards.

The second reservoir may be generally cylindrical, i.e. it may have a longitudinal axis and a circular cross-section in a plane perpendicular to the longitudinal axis. The longitudinal axis may be upright when the second composition is introduced into the exhaust stream.

According to a fifth aspect of the present invention there is provided a process for reducing nitrogen oxides in an exhaust stream, the process comprising providing a first reservoir having a first composition therein, the first composition comprising aqueous urea;

transferring a portion of the first composition to a second reservoir;

heating the portion of the first composition to generate a second composition, the second composition comprising aqueous ammonium carbamate; and introducing a portion of the second composition into an exhaust stream comprising nitrogen oxides, wherein the second composition is heated to avoid precipitation of ammonium carbamate.

According to a sixth aspect of the present invention there is provided an apparatus for reducing nitrogen oxides in an exhaust stream comprising nitrogen oxides, the apparatus comprising a first reservoir for a first composition, the first composition comprising aqueous urea; a second reservoir for a second composition, the second composition comprising aqueous ammonium carbamate;

a passage for an exhaust stream;

flow control apparatus arranged to couple the first reservoir to the second reservoir and thereby transfer a portion of the first composition to the second reservoir; and flow control apparatus arranged to couple the second reservoir to the passage and thereby introduce a portion of the second composition into the exhaust stream;

wherein the second reservoir comprises a heat source for heating the second composition to avoid precipitation of ammonium carbamate.

Ammonium carbamate precipitates out of solution at low temperature, which can cause problems for the reduction of NOx. For example, solid ammonium carbamate can block dosing apparatus. Moreover, precipitation affects the concentration of the aqueous ammonium carbamate so that it is difficult to determine if the right amount is being employed.

The second composition can be heated selectively to avoid precipitation. For example, heating can be carried out only in certain circumstances where precipitation is likely.

The second reservoir may be heated and/or any lines feeding and exiting the second reservoir may be heated. Other components, such as injectors can also be heated.

According to a seventh aspect of the present invention there is provided a process for reducing nitrogen oxides in an exhaust stream, the process comprising providing a first reservoir having a first composition therein, the first composition comprising aqueous urea;

transferring a portion of the first composition to a second reservoir;

heating the portion of the first composition to generate a second composition, the second composition comprising aqueous ammonium carbamate; and introducing a portion of the second composition into an exhaust stream comprising nitrogen oxides, wherein the portion of the second composition is introduced into the exhaust stream by means of pressure-driven injection.

According to an eighth aspect of the present invention there is provided an apparatus for reducing nitrogen oxides in an exhaust stream comprising nitrogen oxides, the apparatus comprising a first reservoir for a first composition, the first composition comprising aqueous urea; a second reservoir for a second composition, the second composition comprising aqueous ammonium carbamate;

a passage for an exhaust stream;

flow control apparatus arranged to couple the first reservoir to the second reservoir and thereby transfer a portion of the first composition to the second reservoir; and flow control apparatus arranged to couple the second reservoir to the passage and thereby introduce a portion of the second composition into the exhaust stream; wherein the second reservoir comprises a pressure-driven injector.

The invention provides benefits relative to non-pressurized injection, such as metering.

The second reservoir may be sealed and heated to increase the pressure therein, and thereby provide the pressure-driven injection.

A pump may be employed to drive the injection.

According to a ninth aspect of the present invention there is provided a process for reducing nitrogen oxides in an exhaust stream, the process comprising providing a first reservoir having a first composition therein, the first composition comprising aqueous urea;

transferring a portion of the first composition to a second reservoir;

heating the portion of the first composition to generate a second composition, the second composition comprising aqueous ammonium carbamate; and introducing a portion of the second composition into an exhaust stream comprising nitrogen oxides, wherein a control system is employed that responds to on board diagnostics.

The control system (e.g. ECU) may control the transfer of the portion of the first composition to the second reservoir; heating the portion of the first composition; and/or introduction of the portion of the second composition into the exhaust stream.

According to a tenth aspect of the present invention there is provided an apparatus for reducing nitrogen oxides in an exhaust stream comprising nitrogen oxides, the apparatus comprising a first reservoir for a first composition, the first composition comprising aqueous urea; a second reservoir for a second composition, the second composition comprising aqueous ammonium carbamate;

a passage for an exhaust stream;

wherein the apparatus additionally comprises a control unit that responds to on board diagnostics.

Detailed description

Ammonium carbamate decomposes on heating to form ammonia and the resulting ammonia reduces the NOx in the exhaust stream. Ammonium carbamate decomposes to form ammonia very quickly and at low temperatures.

As such, ammonium carbamate does not form deposits. Moreover the present invention also allows greater control of ammonia delivery. Rapid decomposition times shorten the delay between injection command and ammonia delivery. This allows a control system capable of dealing with rapid NOx transients, (i.e. below 1 second).

The process of the present invention requires the production of ammonium carbamate in situ. As discussed below, it is not practical or safe to employ ammonium carbamate solution as a direct replacement for AdBlue®. It would be necessary to transport large volumes of the solution, which is technically difficult and potentially unsafe. Instead generation of ammonium carbamate in situ allows ammonium carbamate to be prepared and stored, such that is available for use when needed.

The process of the present invention allows a high NOx strategy to be employed, e.g. to allow an engine to run at high temperature and thereby produce less CO₂. The ammonium carbamate that is generated in situ can be used alone or with support from the first composition (e.g. AdBlue®) to increase the total amount of ammonia it is possible to deliver to the exhaust stream. This will allow an SCR system to have a greater NOx conversion capacity and therefore allow a greater engine output of NOx.

An additional benefit of the present invention is that the second composition is volatile and rapidly generates ammonia. This effervescent process acts as a mild mechanical cleaning process to remove deposit growth on surfaces.

Moreover, ammonium carbamate has benefits even relative to ammonium carbonate and ammonium bicarbonate. Ammonium carbamate decomposes at lower temperature than both ammonium carbonate and ammonium bicarbonate.

Ammonium carbamate as the sole reducing agent

It will be appreciated that both the first composition and the second composition can be employed to generate ammonia and thereby reduce NOx. However, in some circumstances is may be beneficial to employ only the second composition.

In this way, there is no need to include hardware to inject and decompose urea to form ammonia, which simplifies the system architecture.

In embodiments the second composition (comprising ammonium carbamate) is the sole reducing agent for NOx. i.e. the first composition is employed only to generate the second position, and not to reduce NOx.

Threshold temperature

In embodiments the second composition is introduced into the exhaust stream when the exhaust stream has a temperature below a threshold temperature and the first composition is introduced into the exhaust stream when the exhaust stream has a temperature at or above the threshold temperature.

Both urea and ammonium carbamate decompose on heating to form ammonia and the resulting ammonia reduces NOx. Ammonium carbamate decomposes to form ammonia more readily than urea. Hence, we propose the use of ammonium carbamate to generate ammonia at lower temperatures and urea at higher temperatures. An advantage of the process is that NOx can be reduced even when present in a low temperature exhaust stream i.e. a cold-start process. The process has a significant environmental impact. Not only does the process reduce NOx, it may also have a positive impact on other engine emissions such as particulate matter (PM) and CO₂since the increased ability to treat NOx allows for more efficient engine operation.

The suitability of the first and second compositions for reducing NOx depends on the temperature of the exhaust stream. The first composition (comprising urea) may be more appropriate at higher temperatures and the second composition (comprising ammonium carbamate) is more appropriate at lower temperatures.

For example, an exhaust stream temperature of at least 350°C is desirable for full decomposition of AdBlue® to form ammonia. Deposits are likely to form when the temperature is lower. In particular, a practical implementation of the invention with direct AdBlue® dosing is challenging at temperatures below 190°C. In general, when using AdBlue®, deposits may need to be considered at temperatures below 250 °C. Therefore, if AdBlue® is employed as the first composition the skilled person may select a threshold temperature of at least 150°C, at least 200°C, at least 250 °C, at least 275 °C, or at least 300 °C.

In one series of embodiments the threshold temperature is at least 100, 150, 200, 230, 250, 270, 300, 350 or 400°C and / or no more than 1000, 800, 600, 400, 200 or 150°C. It may be that the threshold temperature is selected from the range of from 100 to 600°C, or from 100 to 500°C, or from 100 to 450°C, or from 100 to 400°C. It may be that the threshold temperature is selected from the range of from 150 to 500°C, or from 150 to 450°C, or from 150 to 400°C. It may be that the threshold temperature is selected from the range of from 200 to 500°C, or from 200 to 450°C. In a particular embodiment, the threshold temperature is selected from the range 200 to 400°C. In other embodiments, it may be that the threshold temperature is selected from the range of from 250 to 500°C, or from 250 to 450°C, or from 250 to 400°C.

It will be understood that it is necessary to know the temperature of the exhaust stream in order to determine whether the first composition and / or the second composition should be introduced. The temperature could be measured by means of a dedicated sensor. Alternatively an approximate temperature might be inferred from the conditions. In embodiments the process additionally comprises measuring the temperature of the exhaust stream. The temperature of the exhaust stream may be compared to the threshold temperature and the first composition and / or the second composition can be selected for introduction into the exhaust stream. In embodiments the process comprises measuring the temperature of the exhaust stream and reporting the value to a programmable electronic control unit (ECU). The ECU can be programmed to release the first composition and / or the second composition into the exhaust steam, e.g. by means of a dosing valve.

First composition

Urea has the formula (I):

O u h₂n^nh_{2 (i)}

The first composition comprises aqueous urea and is in liquid form at standard ambient temperature and pressure (SATP). The first composition comprises water and urea: CO(NH₂)2- In one such embodiment the first composition consists essentially of urea and water, e.g. AdBlue® (aqueous urea solution, standardised to AUS32, also known as a diesel engine fuel (DEF)). There is already an existing market acceptance for AdBlue® as an ammonia precursor and an extensive distribution network.

In one series of embodiments the first composition comprises at least 5, 10, 15, 20, 25, 30, 35, 40, 45 or 50wt% urea / or no more than 80, 70, 60, 50, 40, 30, 20 or 10wt% urea. The balance may be water.

In one embodiment the first composition comprises 30 to 35wt%, 3 1 to 34wt%, 32 to 33wt% or approximately 32.5% urea, with the balance being water. The commercially available product AdBlue® consists of 32.5wt% urea and 67.5wt% water. This is said to be correspond closely to a eutectic mixture. In a preferred embodiment the first composition is AdBlue®.

Second composition

The second composition comprises ammonium carbamate: H2NCO2NH4. Ammonium carbamate is the ammonium salt of carbamic acid (NH₂CO₂H) and has the formula (II):

O

JI h₂n^o- nh_{4 (ii)}

Ammonium carbamate decomposes to produce ammonia and carbon dioxide. Decomposition can occur as early as 25°C, meaning that the reduction of NOx can occur even at low temperatures (c.f. 350°C for AdBlue®). This is a benefit at low temperature but leads to difficulties at higher ambient temperatures, since sealed storage is required.

Ammonium carbamate dissolves in water to provide a solution of ammonium ions and carbamate ions. The use of ammonium carbamate solution for the selective catalytic reduction of NOx is known (WO96/06674). However, it would not be practical to transport aqueous ammonium carbamate solution as a direct replacement of AdBlue®, especially in vehicle engines.

A benefit of AdBlue® is that it is very stable at ambient temperatures; it is stored in a tank in a vehicle and refilled as necessary. A passenger car may have a tank that holds 15 to 20 litres of AdBlue®. A heavy goods vehicle would have a much larger tank. It would not be safe or practical to carry such high volumes of ammonium carbamate solution. The ammonium carbamate would decompose to form ammonia, which is toxic and also flammable in sufficient quantities. Therefore a specifically designed tank would be required to maintain the vapour pressure and prevent sublimation. WO96/06674 confirms that shipping of the aqueous solution is impractical and recommends shipping the ammonium carbamate in dry form and then dissolving it on the site of the NOx reduction process.

In embodiments the second composition comprises at least 5, 10, 15, 20, 25, 30, 35, 40, 45 or 50wt% ammonium carbamate and / or no more than 70, 60, 50, 40, 45, 30, 35, 20 or 10wt% ammonium carbamate. The balance may be water.

In embodiments the second composition comprises from 25 to 45wt%, from 30 to 44wt%, from 33 to 43wt% or approximately 42wt% ammonium carbamate. When the ammonium carbamate is produced in situ from AdBlue® (32.5wt% urea and 67.5wt% water), the resulting ammonium carbamate solution comprises approximately 42.25wt% ammonium carbamate (assuming complete conversion).

Reduction of NOx

The urea solution and the ammonium carbamate solution may each produce ammonia, which reduces NOx in the exhaust stream. In embodiments the exhaust stream comprises a reduction chamber having a selective catalytic reduction (SCR) catalyst therein.

Applications

The process comprises reducing NOx in an exhaust steam from a combustor, such as an internal combustion engine. The combustor may be stationary (e.g. a utility boiler, a natural gas generator, a gasoline generator or diesel generator) or mobile (e.g. a vehicle). The process is especially useful for on highway and off highway vehicles which currently employ SCR reduction of NOx.

The process is beneficial for an exhaust stream from heavy equipment including excavators, cranes, articulated trucks, asphalt pavers, backhoe loaders, cold planers, compactors, and bulldozers.

The invention can be useful at a depot or filling stations. Such locations already store large quantities of aqueous urea for conventional SCR in vehicles including lorries and buses.

Aqueous urea could be converted to aqueous ammonium carbamate on site and used to fill a “standard” reductant injection SCR system. This would allow the invention to be applied to vehicles without retro-fitting since the existing SCR system would be employed. This could completely eradicate deposits from the aqueous urea.

In one embodiment a miniature “dissolver” is fitted to an outlet of ammonium carbamate tank. Such a dissolver could be a small heater in the flow to dissolve any precipitates on cold days.

By generating aqueous ammonium carbamate on demand, the reducing agent is “fresh” thereby mitigating risks associated with storage. A vehicle could return to the filling station or depot regularly (e.g. daily) and thereby need not carry very large volumes of aqueous ammonium carbamate on board.

An anti-spill design can be used to ensure safety.

A replacement filler cap can be employed to provide a very low vapour pressure increase and thereby prevent sublimation.

The process is not limited to diesel engines and can also be used for gasoline. The process of the present invention enables an SCR system designed for gasoline since the exhaust temperatures are likely to be lower than that of a comparable diesel engine. The process may allow a decrease in NOx emission limits or an engine no longer running at conditions applicable to existing gasoline after-treatment.

The process is applicable to dual fuel vehicles or generators.

The process is especially useful for hybrid electric vehicles i.e. an electric vehicle that combined an internal combustion engine with an electric propulsion system. The internal combustion engine may be cold, even if the vehicle has been running using the electric system for some time. In embodiments the vehicle exhaust stream is a hybrid electric vehicle (e.g. diesel hybrid or gasoline hybrid) exhaust stream.

The first reservoir may correspond to a conventional AdBlue® tank on a vehicle. In embodiments the first reservoir has a volume of no more than 500, 200, 100, 50, 30 or 20 litres and / or at least 10, 20 or 50 litres. In embodiments the first reservoir has a volume of 10 to 100 litres.

The second reservoir stores the aqueous ammonium carbamate that is produced. Hence a portion of the aqueous solution of urea is transferred from the first reservoir to the second reservoir and may be converted to ammonium carbamate on the way or in the second reservoir. As discussed above, it may not be safe or practical to transport large volumes of aqueous ammonium carbamate. In embodiments the second reservoir has a volume of no more than 10, 8, 5, 3, 2, 1 or 0.5 litres and / or at least 0.05, 0.1, 0.2, 0.5, or 1 litres.

In embodiments the first reservoir has a greater volume than the second reservoir. In embodiments, the volume of the first reservoir is at least 5, 10, 20, 50 or 100 times greater than the volume of the second reservoir.

In embodiments the portion of the first composition (aqueous urea) is heated by an electrical heater.

In embodiments the portion of the first composition is heated by a heat-exchanger, e.g. a heat-exchanger in communication with an exhaust stream, engine coolant or an electrical heater. It is an advantage of the present process that the ammonium carbamate can be produced when an exhaust stream is hot, using energy that would otherwise be wasted. This means that ammonium carbamate solution is available in the second reservoir for subsequent use.

In embodiments the heating of the portion of the first composition comprises heating to a temperature of at least 100, 150, 200, 250, 300, 350 or 400°C and / or no more than 1000, 800, 600 or 500°C. Higher temperatures minimise the generation of alternative decomposition products which cause deposits.

In embodiments the heating of the portion of the first composition is carried out at or above atmospheric pressure. In one embodiment heating may be carried out in a sealed environment, such that pressure increases as ammonia and carbon dioxide is generated. In another embodiment a pressure release valve may be employed to maintain a constant pressure.

In embodiments the heating of the portion of the first composition is carried out at a pressure of at least 100, 130, 150, 200, 250, 300, 500, 1000, 2000, 3000, 4000, 5000, 10000 or 15000kPa and / or no more than 200000, 150000, 100000, 50000, 10000, 5000, 4000, 3000, 2000, 1500, 1000, 800, 600, 400, 200 or lOOkPa. In a particular embodiment the heating takes place at a pressure of 100 to 500kPa or 100 to 300kPa (abs).

In embodiments the heating is carried out below atmospheric pressure. In a series of such embodiments the heating is carried out at a pressure of no more than 99, 95, 90, 80 or 70kPa and/or at least 10, 30 or 50kPa.

In embodiments heating comprises heating the first portion of the first composition to produce gaseous ammonia, carbon dioxide and water, which subsequently forms aqueous ammonium carbamate.

In embodiments heating comprises heating the portion of the first composition to generate ammonia and carbon dioxide in liquid water. A benefit of heating at elevated pressure is that the water in the aqueous urea solution may be kept liquid, rather than becoming a gas (steam), which encourages full decomposition of the urea.

In embodiments the heating takes place in a flow path between the first and second reservoirs. In one such embodiment the flow path comprises or constitutes a decomposition chamber. In embodiments the decomposition chamber may have a volume of at least 0.05, 0.1, 0.15, 0.2, 0.3 or 0.5 litres and / or no more than 2, 1.5, 1 or 0.5 litres. Ammonia can be corrosive so the flow path should be made from a material that can withstand ammonia, especially if heating is to take place there. In one embodiment the flow path is formed from stainless steel.

Cooling the mixture comprises cooling the mixture to form an aqueous solution of ammonium carbamate i.e. a liquid. In embodiments the mixture is cooled by means of a heat exchanger.

In embodiments the cooling takes place in a flow path between the first and second reservoirs. In one such embodiment the flow path comprises a chiller, e.g. a condenser having a coolant fluid flowing therein. Additionally or alternatively, the flow path may comprise a pressure reducing valve e.g. a constant back pressure valve. Since enthalpy is constant over the valve, expansion of a gas forces a temperature drop. These are examples of forced cooling.

In embodiments the cooling comprises cooling the mixture of ammonia, carbon dioxide and water to a temperature of no more than 200, 150, 100, 50, 40 or 30°C and/or at least 5, 10, 15, 20 or 25°C.

In embodiments the cooling is carried out at or above atmospheric pressure. Ambient and higher pressures bias the reaction towards the formation of ammonium carbamate. In one embodiment cooling may be carried out in a sealed environment, such that pressure reduces as aqueous ammonium carbamate is generated. In another embodiment a pressure release valve may be employed to maintain a constant pressure.

In embodiments the heating and the cooling take place in the flow path.

In embodiments the heating takes place in the flow path and the cooling takes place in the second reservoir.

In embodiments each of the heating and the cooling takes place in the second reservoir. In one such embodiment the second reservoir comprises a heat exchanger which heats the aqueous urea solution and cools the resulting mixture comprising ammonia, carbon dioxide and water, thereby producing aqueous ammonium carbamate.

In embodiments of the invention, a hydrolysis catalyst is employed to assist with the conversion of aqueous urea to aqueous ammonium carbamate. It will be understood that the ammonium carbamate is stored in the second reservoir before being introduced into the exhaust stream.

In embodiments of the invention, no hydrolysis catalyst is employed to convert urea to ammonium carbamate. The use of a hydrolysis catalyst complicates the apparatus and increases cost.

Ammonia can be corrosive so the second reservoir should be made from a material that can withstand ammonia, especially if heating or cooling is to take place there. In one embodiment the second reservoir is formed from stainless steel.

In embodiments the process is carried out to produce no more than 500, 400, 300, 200, 100, 80, 60, 40, 30, 20 or lOg of ammonium carbamate solution per minute. Such quantities may not be useful in a commercial process, but may provide sufficient ammonium carbamate for a cold-start reduction of NOx.

Control unit

In embodiments the apparatus additionally comprises a programmable control unit (ECU). The ECU may be programmed to deliver the first composition and / or the second composition to the exhaust stream in accordance with specific criteria. In one such embodiment the ECU controls one or more valves and / or pumps which limit the release of the first and / or second compositions into the exhaust stream. In one such embodiment the ECU controls one or more valves and /or pump which meter the transfer of the first composition into the flow path.

Ammonia Oxidation Catalyst (AMOX)

In embodiments the apparatus additionally comprises an ammonia oxidation catalyst (AMOX) located downstream from the reduction chamber, to oxidize any excess ammonia.

Sensors

In embodiments the apparatus comprises a temperature sensor located in the exhaust stream to determine the exhaust stream temperature. This information can be relayed to the ECU.

In embodiments the apparatus comprises at least one NOx sensor. In one embodiment a NOx sensor is located upstream of the first reservoir (and reduction chamber). The NOx sensor can then provide information to the ECU to determine whether NOx reduction is required. In one embodiment a further NOx sensor is located downstream of the reduction chamber. The NOx sensor can provide information to determine the efficiency of the reduction.

In embodiments the apparatus comprises at least one ammonia sensor. In one such embodiment the ammonia sensor is located downstream from the reduction chamber (and optional AMOX) to determine whether excess ammonia is present.

Fitting and retro-fitting

The invention also resides in fitting the apparatus to a combustor, e.g. a vehicle or generator. The apparatus may be fitted when the combustor (e.g. vehicle or generator) is manufactured or at a later date (retro-fitting). The system requires very little engine design alteration whilst dramatically reducing the total NOx output. Increased NOx reduction capability can also benefit other regulated emissions allowing an engine to operate at a more efficient condition reducing both CO₂ and PM.

The invention also resides in a combustor configured for the process of the invention. The invention also resides in a system comprising such a combustor e.g. a vehicle or generator.

In embodiments, the flow control apparatus is arranged so as to couple the second source to the passage when the exhaust stream has a temperature below a threshold temperature, and to couple the first source to the passage when the exhaust stream has a temperature above the threshold temperature.

The apparatus may comprise a temperature sensor coupled to the flow control apparatus. Alternatively, the flow control apparatus may be arranged to infer an approximate temperature from the conditions.

The first reservoir may correspond to a conventional AdBlue® tank on a vehicle. In embodiments the first reservoir has a volume of no more than 500, 200, 100, 50, 30 or 20 litres and / or at least 10, 20 or 50 litres. In embodiments the first reservoir has a volume of 10 to 100 litres. The first reservoir may contain the first composition.

In embodiments the apparatus is arranged to prepare the ammonium carbamate from a precursor. The precursor may be the first composition. As such, the first reservoir may be coupled to the second reservoir through a flow path, the flow path and the second reservoir together forming a reaction path being configured to convert the first composition into the second composition.

The reaction path may be provided with heating means arranged to heat fluid flowing through a heating part of the reaction path. Typically, this will heat the first composition as it passes through the heating part of the reaction path, in order to encourage the conversion of the first composition to the second composition. In embodiments the heating means may be an electrical heater. Alternatively or additionally, the heating means may comprise a heat-exchanger, e.g. a heat-exchanger in communication with an exhaust stream, engine coolant or an electrical heater. It is an advantage of the present process that the ammonium carbamate can be produced when an exhaust stream is hot, using energy that would otherwise be wasted.

The heating means may be arranged to heat the contents of the heating part of the reaction path to a temperature of at least 100, 150, 200, 250, 300, 350 or 400°C and / or no more than 1000, 800, 600 or 500°C. Higher temperatures increase the rate of ammonium carbamate production.

The heating part of the reaction path may be arranged to withstand and/or achieve heating at or above atmospheric pressure. The heating part of the reaction path may provide a sealed environment, such that pressure increases as ammonia and carbon dioxide is generated. In another embodiment the heating part of the reaction path may comprise a pressure release valve arranged so as to maintain a constant pressure in the heating part of the reaction path.

The heating part of the reaction path may be arranged to withstand and/or achieve a pressure of at least 100, 130, 150, 200, 250, 300, 500, 1000, 2000, 3000, 4000, 5000, 10000 or 15000kPa and / or no more than 200000, 150000, 100000, 50000, 10000, 5000, 4000, 3000, 2000, 1500, 1000, 800, 600, 400, 200 or lOOkPa. In a particular embodiment the heating part of the reaction path may be arranged to withstand and/or achieve a pressure of 100 to 300kPa.

The heating part of the reaction path may be arranged to withstand and/or achieve heating at below atmospheric pressure. In a series of such embodiments the heating part of the reaction path may be arranged to withstand and/or achieve pressure of no more than 99, 95, 90, 80 or 70kPa and/or at least 10, 30 or 50kPa.

In embodiments, the heating means is arranged so as to heat the aqueous solution of urea to generate ammonia and carbon dioxide in liquid water. A benefit of heating at elevated pressure is that the water in the aqueous urea solution may be kept liquid, rather than becoming a gas (steam), which encourages full decomposition of the urea.

In one embodiment, the heating part of the reaction path comprises a decomposition chamber, to which the heating means may be thermally coupled. In embodiments the decomposition chamber may have a volume of at least 0.05, 0.1, 0.15, 0.2, 0.3 or 0.5 litres and / or no more than 2, 1.5, 1 or 0.5 litres. Ammonia can be corrosive so the heating part of the reaction path should be made from a material that can withstand ammonia, especially if heating is to take place there. In one embodiment the heating part of the reaction path is formed from stainless steel.

A second, cooling part, of the reaction path may be provided with cooling means arranged to cool material passing through the cooling part of the reaction path; the material will typically be a mixture of ammonia, carbon dioxide and water output the heating part of the reaction path. In one such embodiment, the cooling means comprises a chiller, e.g. a condenser having a coolant fluid flowing therein. Additionally or alternatively, the cooling means may comprise a pressure reducing valve e.g. a constant back pressure valve. Since enthalpy is constant over the valve, expansion of a gas forces a temperature drop.

In embodiments the cooling means is arranged to cool the material passing through the cooling part of the reaction path to a temperature of no more than 200, 150, 100, 50, 40 or 30°C and / or at least 5, 10, 15, 20 or 25°C.

The cooling means may be arranged such that the cooling is carried out at or above atmospheric pressure. Ambient and higher pressures bias the reaction towards the formation of ammonium carbamate. The cooling part of the reaction path may provide a sealed environment, such that pressure reduces as aqueous ammonium carbamate is generated. In another embodiment the cooling part of the reaction path may comprise a pressure release valve in the cooling part to maintain a constant pressure.

Typically, the heating part of the reaction path comprises the flow path, and the cooling part of the reaction path comprises the second reservoir. As such, the heating means may be thermally coupled to the flow path, and the cooling means may be thermally coupled to the second reservoir.

In other embodiments, both the heating and cooling parts of the reaction path comprise the second reservoir. In one such embodiment the second reservoir comprises a heat exchanger which heats the aqueous urea solution and cools the resulting mixture comprising ammonia, carbon dioxide and water.

The flow control apparatus may be arranged so as to deliver the first composition and I or the second composition to the exhaust stream in accordance with specific criteria. The flow control apparatus may comprise at least one valve and/or pump which limit the release of the first and/or second compositions into the exhaust stream. Each valve and /or pump may be arranged to meter the transfer of the first composition into the flow path.

The urea solution and the ammonium carbamate solution each produce ammonia, which reduces NOx in the exhaust stream. In embodiments the passage comprises a reduction chamber having a selective catalytic reduction (SCR) catalyst therein.

In accordance with a sixth aspect of the invention, there is provided a vehicle having the combustor of the fifth aspect of the invention, wherein the combustor is an internal combustion engine of the vehicle. Typically, the vehicle will be a wheeled vehicle, such as a road vehicle, and the internal combustion engine will be arranged to power at least one wheel of the vehicle.

Embodiments of the invention will now be described with reference to the following figures in which:

Figures 1 and 2 are schematic diagrams of systems for carrying out embodiments of the invention;

Figure 3 is a side view of a reactor for use in embodiments of the invention;

Figure 4 is a cross-section of a reactor for use in embodiments of the invention;

Figures 5A and 5B are a side view and a cross-section respectively of a reactor for use in embodiments of the invention;

Figures 6 to 10 are schematic diagrams of systems for carrying out embodiments of the invention;

Figure 11 is a plot showing the modelled relationship between reaction temperature and final pressure after conversion of aqueous urea to aqueous ammonium carbamate; and

Figure 12 is a plot showing the change in vapour pressure and temperature during the conversion of aqueous urea to aqueous ammonium carbamate.

Referring to figure 1 there is shown an exhaust stream 10 containing NOx that flows from left (upstream) to right (downstream), as indicated. The system comprises a first reservoir 12 containing aqueous urea (e.g. an AdBlue® tank).

A portion of the aqueous urea is released from the first reservoir 12 and transferred to a second reservoir 14, which comprises three closed tanks: first tank 14A, a second tank 14B and a third tank 14C. Each of the tanks contains a heat exchanger in communication with the exhaust stream 10 and uses heat from the exhaust stream 10 to decompose the aqueous urea solution and form aqueous ammonium carbamate.

The aqueous ammonium carbamate is injected into the exhaust stream 10 from one of the tanks 14A, 14B or 14C when required. The aqueous ammonium carbamate decomposes in the exhaust stream to form ammonia and carbon dioxide which pass downstream to a reduction chamber where the ammonia reduces NOx.

The aqueous ammonium carbamate is introduced into the exhaust stream as a liquid, rather than as a gas. This liquid forms droplets which penetrate the flow of the exhaust stream and ensure good mixing of the reducing agent, i.e. a homogenous mixture of ammonia. This is beneficial as compared to injecting a gas since there is a risk that a gas might “flash” in the exhaust stream. Moreover it allows the concentration of reducing agent to be maintained at a level which is sufficient for the amount of NOx in the exhaust stream, rather than using an excess of reducing agent as a precaution.

Aqueous ammonium carbamate can be injected into the exhaust stream from the first tank 14A, while another batch is being generated in the second tank 14B. The aqueous ammonium carbamate may be generated when convenient to build up a stock that is ready to be injected into the exhaust stream.

As a back-up, the aqueous urea can be metered from the first reservoir 12 into the exhaust stream 10 via flow control apparatus comprising a conventional delivery system (dashed line). This can be useful if no aqueous ammonium carbamate is available for any reason.

Referring to figure 2 there is shown an exhaust stream 10 containing NOx that flows from left (upstream) to right (downstream), as indicated. The system comprises a first reservoir 12 containing aqueous urea (e.g. an AdBlue® tank).

A portion of the aqueous urea is released from the first reservoir 12 and transferred to a second reservoir 16, which comprises three closed tanks: first tank 16A, a second tank 16B and a third tank 16C. The first tank 16A contains a heat exchanger in communication with the exhaust stream 10 and uses heat from the exhaust stream 10 to decompose the aqueous urea solution and form aqueous ammonium carbamate.

The second and third tanks 16B, 16C do not contain a heat exchanger but are in communication with the first tank 16A. As such, the second and third tanks serve only to store and dispense the aqueous ammonium carbamate that is generated in the first tank 16A.

The aqueous ammonium carbamate is injected into the exhaust stream 10 from one of the tanks 16A, 16B or 16C when required. The aqueous ammonium carbamate decomposes in the exhaust stream to form ammonia and carbon dioxide which pass downstream to a reduction chamber where the ammonia reduces NOx.

Aqueous ammonium carbamate can be injected into the exhaust stream from the first second tank 16B or the third tank 16C, while another batch is being generated in the first tank 16A. The aqueous ammonium carbamate may be generated when convenient to build up a stock that is ready to be injected into the exhaust stream.

The systems shown in figures 1 and 2 are particularly useful for a marine vessel (e.g. ship) that has capacity to carry large volumes of reducing agent. An ECU may be employed to control the system and to react is response to on board diagnostics (OBD).

The system shown in figures 1 and 2 are particularly useful for power generation where heat from a cooling tower can be harnessed to generate ammonium carbamate.

Figure 3 shows a side view of a second reservoir in the form of a reactor 20 for the conversion of aqueous urea to aqueous ammonium carbamate, for use in embodiments of the invention. This reactor is in the form of a coil which surrounds an exhaust pipe 22. As such, the heat of the exhaust pipe can be used to generate the ammonium carbamate reducing agent.

Figure 4 shows a cross-section of another reactor 24. The reactor 24 also surrounds the exhaust 22, but instead of being in the form of a coil, it is annular in shape. Arrows are employed to indicate possible locations for the input (aqueous urea) and output (aqueous ammonium carbamate).

Figure 5 shows a double skin annular reactor 26 having an inner chamber 26a and an outer chamber 26b. The inner chamber 26a is closer to the exhaust 22 and therefore hotter than the outer chamber 26b. Aqueous urea can be passed first through the inner chamber 26a and then through the outer chamber 26b (or vice versa), and thereby increase the time spent in the reactor.

Figure 6 shows a sealed reactor 28 in a generally upright orientation. The reactor 28 has a longitudinal axis and a circular cross-section in a plane perpendicular to the longitudinal axis. The longitudinal axis is upright when the aqueous ammonium carbamate is introduced into the exhaust stream (not shown).

A heater 30 is located within the reactor 28 and heats aqueous urea to form aqueous ammonium carbamate. A temperature probe 32 is located within the reactor so that it is submerged in the fluid in use and a pressure sensor 34 monitors the vapour pressure. A valve 36 meters fluid into and out of the reactor 28. The sealed reactor constitutes a second reservoir or a tank in the context of certain embodiments of the invention.

Figure 7 shows a series of reactors 28, as described in figure 6. The number of reactors can be varied as desired, i.e. 28a, 28b...28n. The series of reactors (tanks) together form a second reservoir.

Figure 8 shows various locations for the input (aqueous urea) and the output (aqueous ammonium carbamate) on the reactor 28. From the top: the input and output are at the upper end of the reactor; the input and output are at the lower end of the reactor; the input is at the lower end and the output is at the upper end of the reactor; and the input is at the upper end and the output is at the lower end of the reactor.

Figure 9 shows a sealed reactor 38 in a generally horizontal orientation. The reactor 38 has a longitudinal axis and a circular cross-section in a plane perpendicular to the longitudinal axis. The longitudinal axis is generally horizontal when the aqueous ammonium carbamate is introduced into the exhaust stream 22.

Figure 9 shows various locations for the input (aqueous urea) and the output (aqueous ammonium carbamate). From the top: the input and output on the upper side of the reactor; the input and output are on the lower side of the reactor; the input is on the lower side and the output is on the upper side of the reactor; and the input is on the upper side and the output is on the lower side of the reactor.

Figures 10A and 10B shows the upright reactor 28 and the horizontal reactor 38 respectively having two inputs and two outputs.

Figure 11 shows a plot of final vapour pressure against temperature. This relationship was modelled using an extended UNIQUAC (UNIversal QUAsiChemica) method. Pressure is shown in bar with 1 bar = 100000 Pa = lOOkPa. This graph represents the pressure of the fluid at a specified temperature after complete conversion in a sealed container. For example, heating at 110°C corresponds to a final vapour pressure of around 12.5 bar.

Example

Aqueous urea (32.5% urea and 67.5% deionized water) was heated in a sealed vessel to convert it to aqueous ammonium carbamate. The temperature was measured by a probe located within the liquid and a pressure monitor was secured to the reactor. The change in temperature (upper line) and pressure (lower line) is illustrated in figure 12.

Referring to figure 12 the aqueous urea reached a temperature of 100 to 110°C quite quickly and was then maintained at this temperature for the rest of the experiment. This is demonstrated by the steep rise at the beginning of the plot followed by a plateau.

Pressure rises slowly and reaches a maximum at around 13 to 14 bar. This is consistent with that predicted by the plot shown in figure 11.

Claims

1. A process for reducing nitrogen oxides in an exhaust stream, the process comprising providing a first reservoir having a first composition therein, the first composition comprising aqueous urea;

transferring a portion of the first composition to a second reservoir;

wherein (a) the second reservoir comprises at least two tanks;

(b) the second reservoir comprises an anti-slosh device;

(c) the second composition is heated to avoid precipitation of ammonium carbamate;

(d) the portion of the second composition is introduced into the exhaust stream by means of pressure-driven injection; and/or (e) a control system is employed that responds to on board diagnostics.

2. The process of claim 1, wherein the first composition is not introduced into the exhaust stream.

3. The process of claim 1 or claim 2, wherein the exhaust stream is an exhaust stream from (a) a marine vessel; (b) a power station; or (c) heavy equipment.

4. An apparatus for reducing nitrogen oxides in an exhaust stream, the apparatus comprising:

a first reservoir for a first composition, the first composition comprising aqueous urea; a second reservoir for a second composition, the second composition comprising aqueous ammonium carbamate;

a passage for an exhaust stream;

flow control apparatus arranged to couple the first reservoir to the second reservoir and thereby transfer a portion of the first composition to the second reservoir; and flow control apparatus arranged to couple the second reservoir to the passage and thereby introduce the second composition into the exhaust stream; wherein (a) the second reservoir comprises at least two tanks;

(b) the second reservoir comprises an anti-slosh device.

(c) the second reservoir comprises a heat source for heating the second composition and thereby avoiding precipitation of ammonium carbamate;

(d) the second reservoir comprises a pressure-driven injector; and/or (e) the apparatus comprises a control system that responds to on board diagnostics.

5. The process of any one of claims 1 to 3, or the apparatus of claim 4, wherein the second reservoir comprises at least two tanks.

6. The process or apparatus of claim 4 or claim 5, wherein the second reservoir comprises from 3 to 10 tanks.

7. The process or apparatus of any one of claims 4 to 6, wherein the tanks have an average volume of at least 10 litres and/or the tanks have an average volume of 500 litres or less.

8. The process of any one of claims 1 to 3, or the apparatus of claim 4, wherein the second reservoir comprises an anti-slosh device.

9. The process or apparatus of claim 5, wherein the anti-slosh device comprises a baffle.

10. The process or apparatus of any one of the preceding claims, wherein the exhaust stream flows in a generally horizontal direction.

11. The process or apparatus of any one of the preceding claims, wherein the exhaust stream flow in a generally vertical direction.

12. The process or apparatus of any one of the preceding claims, wherein the second reservoir has a longitudinal axis that is upright in use.

13. The process or apparatus of any one of the preceding claims, wherein the exhaust stream flows upwards.