WO2008043387A1

WO2008043387A1 - A method for the removal of nox components in flue gases

Info

Publication number: WO2008043387A1
Application number: PCT/EP2006/009920
Authority: WO
Original assignee: Mennen-Geenen G.C.M; Geurts-Honings S.A.M
Priority date: 2006-10-13
Filing date: 2006-10-13
Publication date: 2008-04-17

Abstract

The invention refers to a method to remove NOx components from flue gases originated from industrial combustion processes. The invention refers to a method in which a urea solution is used as a reduction medium. The urea solution is concentrated where after the urea is converted in the gas phase into ammonia and carbon dioxide. The in between product is isocyanic acid which converts to ammonia and carbon dioxide accelerated by a catalyst. The formed ammonia is distributed in the flue gas channel and reacts with the NOx to components which are environmental acceptable.

Description

A method for the removal of NO_x components in flue gases .

The invention refers to a method to remove NO_x components from flue gases originated from industrial combustion processes. The invention refers to a method in which a urea solution is used as a reduction medium.

The NO_x components in flue gases are normally removed by the application of ammonia or ammonia derivates such as for example urea. NO_x components refer to nitrogen and oxygen compounds such as nitrogen dioxide NO₂, nitrogen monoxide NO and laugh gas N₂O₅.

Normally the reduction reaction of ammonia and NO_x components takes place in the flue gas channel of burners, power plants, kettles or other installations in which combustion processes take place. The ammonia reacts with the NO_x components according the underneath equation under the formation of water and nitrogen that are both components friendly for the environment .

n NH₃ + m NO_x -> p N₂ + q H₂O

If pure ammonia is used, the reaction with NO_x components will be complete. The kinetics of the reaction depends on the applied temperature and together with the amount of introduced ammonia, the NO_x components will be converted completely. The advantage of the application of ammonia as a reduction medium is that the reaction kinetics is already sufficient high at relative low temperature to complete the reduction reaction. Relative low temperatures are temperatures lower than 500 ⁰C. Ammonia however is an environmental unfriendly component and poisonous and that makes that special requirements with respect to safety and environment are needed to store and handle this component. Especially in power plants and in combustion installations at locations which are non-chemical, it makes special attention and investments needed when ammonia is stored for the application as reduction medium. To overcome this safety and environmental issue, the application of urea as a reduction medium is an alternative. Urea is a solid and normally used as a fertilizer. Urea and urea solution are easy to store and special requirements with respect to safety and environmental issues are not needed. There are processes in which a urea solution is injected directly in the flue gas channel. The direct injection of a urea solution in a flue gas channel needs to be done at a location where sufficient high temperatures exists in order to complete the reduction reaction with the NO_x components. High temperatures in this respect refer to temperatures higher than 700 ⁰C.

Combustion processes at such high temperatures however can cause the formation of the for human life dangerous components such as dioxins. Therefore it is preferred to do the combustion process at moderate temperatures. The removal of NO_x components from flue gases at low temperature is for that reason generally done by the application of ammonia as a reduction medium in combination with an expensive catalyst containing platinum, palladium and/ or rhodium metals. The purpose of this catalyst is to accelerate the reduction reaction between ammonia and the NO_x components in the flue gas at moderate temperatures .

A well known process at which the NO_x reduction takes place at temperatures higher than 700 ⁰C with no catalyst used, is the SNCR process (Selective Non Catalytic Reduction) . It is described that for this process ammonia as well as urea solution can be applied for the reduction of NO_x components into nitrogen and water. It is also described that the urea solution on top of the reduction of the NO_x components forms dioxins and isocyanic acid (HNCO) . At these high temperatures flue gas contains hydroxyl radicals (OH^") . These hydroxyl radicals react with the isocyanic acid to form the active CNO molecule and this molecule forms together with nitrogen monoxide, carbon monoxide (CO) and laugh gas which are both environmental unfriendly components. Basically the formed reaction products can be demonstrated by the following simplified reaction equations.

NH₂-CO-NH₂ + H₂O - HNCO + NH₃

HNCO + OH^" → H₂O + CNO

CNO + NO → CO + N₂O

For this reason the application of urea as a reduction medium in SNCR processes causes a secondary emission in which both formed components CO and NO_x are environmental unfriendly and this makes urea as reduction medium far less suitable compared to pure ammonia .

NO_x removal processes where a catalyst is used are known as SCR technology (Selective Catalytic Reduction) . Generally this process is operated at temperatures lower than 500 ⁰C and a catalyst is being used. In general, ammonia is used as a reduction medium. Urea in the application of reduction medium in these processes is not suitable because of the applied low temperatures and thus low reactivity in these processes.

It is known that urea in water hydrolyses into ammonia and carbon dioxide according a chemical equilibrium reaction. Relative long retention and temperatures are required for this equilibrium reaction. This way of hydrolyzing urea in the liquid phase takes place without the formation of any in between products. The invention complies the advantage that it is a combination of the advantages that ammonia with respect to the reaction with NO_x products and urea solution with respect to storage and handling. A method is found in which a urea solution is used as a feedstock for the combustion of NO_x components in flue gases and in which in this method the formed ammonia takes care for the sufficient high reaction kinetic at temperatures higher than 100 ⁰C and especially at temperatures above 200 ⁰C.

It is found that a urea solution is injected to an apparatus in which urea solution is concentrated to a urea melt by adding heat. This formed urea melt is evaporated by adding heat and by this evaporation process formed gaseous isocyanic acid will completely convert into ammonia and carbon dioxide. This conversion reaction is accelerated if a catalyst is used. The reaction takes place in contrary to the well known liquid phase hydrolysis reaction, completely in the gas phase.

The conversion of urea in ammonia and carbon dioxide takes place via the in between product gaseous isocyanic acid. The formed isocyanic acid is dissociated spontaneously completely at sufficient high reaction rate at temperatures higher than 100 ⁰C in the presence of a catalyst. In this respect are all surface active components such as silicium oxide, silica, aluminum oxide and compounds which contained these components such as zeolites are meant to be used as catalyst. Also other metals are suitable to be used as catalyst in this respect. The reactions which take place when urea hydrolyses in the gas phase are as follows:

NH₂-CO-NH₂ + Heat -» HNCO + NH₃

HNCO + H₂O → NH₃ + CO₂

These reactions can take place in an apparatus placed in or outside the flue gas channel. If the apparatus is placed in the flue gas channel, the heat from the flue gas is completely or partly used for the concentration of the urea solution and the evaporation of the urea melt. The catalyst is a fixed bed or fluid or spouted bed. After the reaction has taken place the formed ammonia containing vapor is distributed across the flue gas channel.

If the apparatus is placed outside the flue gas channel, the required heat addition for the urea concentration and urea melt evaporation is done by steam or another heat source. The catalyst can be a fixed bed catalyst as well as a fluid bed catalyst. As a preference a fixed bed catalyst is being used.

As feedstock urea is used. The solid urea is solved in water to a concentration between 10 and 90 % of weight and more specific between 20 and 75 % by weight. It is also possible that a urea solution from a urea plant is used after being prepared in the before mentioned concentrations. The prepared urea solution is added to the apparatus in which this urea solution is concentrated where after the urea melt is evaporated and converted into ammonia and carbon dioxide. The pressure in the apparatus is in between 0.3 and 20 bar but preferable between atmospheric and 10 bar. The temperature in the apparatus depends on the chosen pressure but varies between 100 and 800 ⁰C but preferable between 200 and 600 ⁰C. The supplied heat to the apparatus for the concentration and urea melt evaporation can be done by the application of steam. This can be done by injection of life steam as well as the application of a steam heat exchanger. It is also possible that an electrical heat exchanger or thermal oil is used as heating source.

The invention is further explained by the following example shown in the accompanying figure 1 In the apparatus the prepared urea solution (US) is supplied and equally distributed across the device surface via a liquid distributor (LDIS) . The urea solution (US) enters the urea concentration zone (CONC) and in this zone the water in the solution is evaporated by the applied heat and thus the water fraction is separated from the urea fraction. Further heating takes place in the urea evaporation zone (EVAP) where the urea is converted in isocyanic acid and ammonia. After the urea evaporation zone, the vapor mixture containing the isocyanic acid, ammonia and steam enters the catalyst zone (CAT) in which the isocyanic acid is converted into ammonia and carbon dioxide. This vapor mixture is distributed across the exhaust flue gas channel (EFC) of the combustion process via a vapor distributor (VDIS) . The urea concentration zone (CONC) and urea evaporation zone (EVAP) can be installed inside the exhaust channel. It is also possible that the apparatus is installed outside this exhaust channel (EFC) . The heat input (HI) for this concentration zone (CONC) can be supplied by applying steam. It is also possible that the concentration zone (CONC) and evaporation zone (EVAP) are installed outside the exhaust channel (EFC) . In this case the heat input (HI) to both zones can be done i.e. by the application of steam. It is possible that the shell side for the concentration zone (CONC) and the evaporation zone (EVAP) is separated by a separation plate (SEP) in order to make it possible that the applied steam pressure for the heat input

(HI) in the concentration zone part (CONC) is different from the pressure of the steam for the heat input (HI) in the evaporation zone (EVAP) .

Furthermore the invention can be explained by a next example illustrated by figure 2. Solid urea is dissolved in water in a dissolving vessel (A) . The prepared urea solution is supplied to a reactor (B) . To this reactor heat is supplied needed for concentrating the solution to a melt where after furthermore heat is supplied for evaporating the urea melt to isocyanic acid and ammonia in the vapor phase. In this reactor is a catalyst which can be of a fixed type, a fluid bed or a spouted bed type. When the isocyanic acid is contacting the catalyst together with the formed water vapor released by the urea concentration, spontaneous the isocyanic acid is converted in ammonia and carbon dioxide. The mixture containing ammonia, carbon dioxide and water vapor is distributed in the exhaust flue gas channel (C) in a ratio such that the NO_x is converted to almost complete into nitrogen and water vapor. The exhaust flue gas channel can be executed with a catalyst as supplied in the SCR processes as well as without a catalyst as applied in the SNCR process.

Claims

1. A process for producing ammonia (NH₃) and introducing the produced ammonia into a flue gas stream as a reduction means for reducing nitrogen oxides contained in the flue gas stream which is an exhaust stream generated by the combustion processes of power plants, kettles, burners and other industrial processes in which combustion takes place. The method comprises:

Feeding a liquid urea solution to a device that concentrates the urea solution by adding heat to a melt where after this formed urea melt is evaporated by adding heat and converted into isocyanic acid vapor that is subjecting to a catalyst to generate in the presence of the formed steam by the concentration step, ammonia and carbon dioxide.

2. A process according to claim 1 wherein the urea solution applied, has a concentration between the 10 and the 90 % by weight and in particular a concentration between 25 and 75 % by weight.

3. A process according to claim 1 and 2 wherein the process is operated at a pressure between 0.3 and 20 bar and in particular between atmospheric pressure and 10 bar.

4. A process according to claim 1 to 3 wherein the process is operated between 100 "C and 800 °C and in particular between 200°C and 600°C.

5. A process according to claim 1 to 4 wherein the applied catalyst is a metal, a metal oxide or a combination of a metal and metal oxide. Particular zeolites, aluminum oxide and silicium oxide and combinations of these are suitable for this purpose .

6. A process according to claim 1 to 5 wherein the heat input for urea concentration zone and the urea evaporation zone is done by steam and/ or hot gases in the exhaust flue gas channel of the combustion installation.

7. An apparatus that comprises a urea concentration zone followed by a urea evaporation zone followed by a catalyst and followed by a vapour distribution system.

8. An apparatus according to claim 7 wherein the urea solution applied, has a concentration between the 10 and the 90 % by weight and in particular a concentration between 25 and 75 % by weight.

9. An apparatus according to claim 7 and 8 wherein the process is operated at a pressure between 0.3 and 20 bar and in particular between atmospheric pressure and 10 bar.

10. An apparatus according to claim 7 to 9 wherein the process is operated between 100 °C and 800 °C and in particular between 200°C and 600°C.

11. An apparatus according to claim 7 to 10 wherein the applied catalyst is a metal, a metal oxide or a combination of a metal and metal oxide. Particular zeolites, aluminum oxide and silicium oxide and combinations of these are suitable for this purpose.

12. An apparatus according to claim 7 to 11 wherein the heat input for urea concentration zone and the urea evaporation zone is done by steam.

13. An apparatus according to claim 7 to 11 wherein the heat input for the urea concentration zone is done by steam and the urea evaporation zone is done by the hot gases in the exhaust flue gas channel of the combustion installation.

14. An apparatus according to claim 7 to 11 wherein the heat input for the urea concentration zone is done by steam operated at a lower pressure than the applied steam for the steam input on the urea evaporation zone.

15. An apparatus according to claim 7 to 11 wherein the heat input for the urea concentration zone is done by electric and/ or thermal oil.