WO1997033678A1

WO1997033678A1 - A method for removing nitrogen oxides, sulfur oxides, and other acid gases from a gas stream

Info

Publication number: WO1997033678A1
Application number: PCT/DK1997/000105
Authority: WO
Inventors: Steen Brummerstedt Iversen; Niels Thulstrup; Vinay K. Bhatia; Bjarne Henning Jensen
Original assignee: Fls Miljø A/S
Priority date: 1996-03-11
Filing date: 1997-03-11
Publication date: 1997-09-18
Also published as: EP0888165A1; CA2248679A1; AU2022297A

Abstract

In a method for removing nitrogen oxides, sulfur oxides, and other acid gases from a gas stream in which an adsorption agent is suspended in the gas stream in a reaction zone wherein these oxides and acid gases are adsorbed by the adsorption agent; whereafter the adsorbent agent is at least partially withdrawn from the reaction zone as an entrained suspension and substantially removed from the gas stream in a separation zone; whereafter the exit gas is discharged to the atmosphere; while separated adsorbent agent is introduced into a regeneration zone where the adsorbed oxides and acid gases are substantially removed and withdrawn in a concentrated form; while the regenerated adsorption agent is recycled to the reaction zone, whereby the separation zone comprises a first electrostatic filter and optionally an additional filter arranged upstream the first electrostatic filter and/or a low grade fuel is used as a reducing agent in the regeneration zone.

Description

A METHOD FOR REMOVING NITROGEN OXIDES, SULFUR OXIDES, AND OTHER ACID GASES FROM A GAS STREAM

The present invention relates to a method for removing nitrogen oxides, sulfur oxides, and other acid gases from a gas stream in which an adsorption agent suitable for adsorbing nitrogen oxides, sulfur oxides, and other acidic gases is introduced and contacted with the gas stream in a reaction zone wherein the nitrogen oxides, sulfur oxides, and other acidic gases are adsorbed by the adsorption agent,

- whereafter the adsorption agent is at least partially withdrawn from the reaction zone with the gas stream as an entrained suspension and substantially removed from the gas stream in a separation zone,

- whereafter the exit gas from the separation zone is discharged into the atmosphere, - while separated adsorption agent is introduced into a regeneration zone where the adsorbed nitrogen oxides, sulfur oxides, and other acidic gases are substantially removed from the adsorption agent and withdrawn from the regeneration zone in a concentrated form, - while the regenerated pulverous adsorption agent is recycled to the reaction zone.

Various processes for removing nitrogen oxides and sulfur oxides from a gas stream, e.g. from flue gas from power plants and incinerators are known.

Most of these fall within one of the following main groups:

(1) Wet methods, where the gas is scrubbed with aqueous suspensions or solutions of hydroxides or carbonates of alkali or alkaline-earth metals wherein the reaction products are withdrawn as a sludge.

(2) Semi-dry methods, where the gas is brought into contact with aqueous suspensions or solutions of hydroxides or carbonates of alkali or alkaline-earth metals under such conditions that the water is evaporated and the reaction products are withdrawn as a dry powder.

(3J Dry methods, where the gas is brought into contact and reacted with dry adsorption agents in a reaction zone, e.g. in a fixed or a fluidized bed or in an entrained suspension in the gas to be treated, whereafter the reaction products are withdrawn as a dry solid material which can be deposited as an end product or subjected to further processing in a regeneration zone and recycled to the reaction zone.

The methods according to the present invention may be classified as dry methods with gas-solid contact and regeneration of the adsorption agent.

Dry methods with regeneration of the adsorption agent have been described in a number of articles, patents and patent applications:

A number of patents claiming priority from US patent application No. 659996 (priority date 12 Oct. 1984) such as UK patent No. GB 2176771 B, US patent No. 4,755,499, US patent No. 4,798,711, US patent No. 4,940,569, and CA patent No. 1 261 314 describe the so-called NOXSO Process in which a flue gas stream containing both nitrogen oxides (NO,.) and sulfur oxides (SO.) is passed through a fluid bed adsorber containing suitable sorbent particles or beads, such as for example those disclosed in the above-mentioned US patent No. 4,755,499. The various stages in the development of the NOXSO process have been reported in a number of publications, vide e.g. Yeh, J.T., et al., "The NOXSO Process: Simultaneous Removal of SO and NO. from Flue Gas", 1987 AIChE Spring National Meeting, Houston, Texas, March 29- April 2, 1987/ Yeh, J.T. et al, Integrated testing of the NOXSO process: Simultaneous removal of SO; and NO., from flue gas, Chem. Eng. Comm. 114, (1992), 65-88; Neal, L.G. et al, Pilot Plant Test Data For The NOXSO Flue Gas Treatment System, presented at the 4th International Conference on Processing and Utilization of High Sulfur Coals, Idaho Falls, Idaho, August 26-30, 1991; Ma, W.T. et al, NOXSO S0 /NO_., flue gas treatment process, Adsorption chemistry and kinetics, presented at the 1994 AIChE annual meeting, San Fransisco, November 1994, paper 214c; and Ma, W.T. et al, NOXSO S0₂/NO_;. flue gas treatment process, Regeneration chemistry and kinetics, presented at the 1995 AIChE Summer National meeting, Boston, July 1995, paper 16e.

According to the NOXSO process nitrogen oxides and sulfur oxides are adsorbed on the sorbent beads and removed from the gas stream. The beads consist of a 840-2000 μm alumina carrier activated with alkali or alkaline-earth metal oxides.

The adsorption can take place in a single or multiple stage fluid bed reactor.

The adsorption of SO,- and NO, on the sorbent proceeds in several steps through a complex surface chemistry mechanism which is not fully understood. A simple overall adsorption reaction which may be used as a partial basis for process design is: 4 Na₂0 + 3 SO; + 2 NO + 3 O- = 3 Na SO, + 2 NaN0

The sorbent particles having adsorbed the SO and NO_. from the gas is transported to a fluid bed heater wherein the N0_> is liberated by raising the temperature of the sorbent particles above 532°C using air as heating media. Since both NO and N0₂ are seen to evolve in this first regeneration step, the overall reactions may be shown as:

2 NaN0₃ = Na₂0 + 2 N0₂ + HO;

2 NaN0₃ = Na₂0 + NO; + NO + 0^

The NO- is thus stripped from the sorbent particles and carried away in the heating gas stream.

The hot sorbent particles with the N0_X removed therefrom are transferred into a two stage moving bed regenerator where they are contacted with a suitable regenerant gas stream in the first stage and steam in the second stage. As regeneration gases are mentioned H₂, CO, H₂-CO mixtures, H_?S, CH.,, CjH_θ, Natural gas, and the before mentioned gases mixed with CO; and/or H;0. Other gases mentioned are all gases that will result in the formation of H₂S at regenerator process conditions, among these are COS and CS;.

Both SO;, H_tS, CO; and H₂0 have been observed to evolve in this second regeneration step. Therefore it may be assumed that the following overall reaction takes place in the regenerator:

a Na S0₄+ b CH) = c Na_;0 ^•+ d H_..S + e SO. ι I CO;+ g H_?0

SO; and H;S evolved from the second regeneration step are converted into elemental sulfur in the Claus process: SO- + 2 H;S = 3 S , + 2 H-O

The regenerated sorbent particles are cooled in a fluid bed cooler and recirculated to the moving bed adsorber.

A new variant of the NOXSO Process, in the following referred to as the "ES-NOXSO Process" (entrained suspension NOXSO Process), is disclosed in EP 549 891 Al and EP 657 203 A2 :

The sorbent bead particles (840-2000 μm) are replaced with sorbent powder particles of the size 30-500 μm. In this process the adsorption is performed in an entrained suspension adsorber.

The advantage of the ES-NOXSO process is found in the fact that the adsorption rate for the comparatively large NOXSO sorbent beads (1.23 mm) is diffusion controlled while the adsorption rate in the ES-NOXSO process is chemically controlled.

According to the ES-NOXSO Process as disclosed in EP 549 891 Al NO and SO are removed from the gas in a process comprising the following steps: - introducing and suspending sorbent particles suitable for adsorbing NO and SO. and having a size in the range from about 140 mesh (105 μm) to about 70 mesh (210 μm) in the gas stream containing NO_x and SO. to be removed;

- transporting said sorbent particles through a transport line adsorber to cause said sorbent particles to substantially adsorb said NO,, and SO, from said gas;

- separating said sorbent particles from said gas to provide a stream of substantially NO and SO free gas; discharging the sorbent free gas stream to the atmosphere; - heating said separated sorbent particles in entrained suspension in a multi-cyclone heater to remove NO from said sorbent particle and to produce an off gas stream of hot gas carrying away said NO_* ; - contacting said heated sorbent with a regenerant gas to substantially remove said SO., from said sorbent particles and to produce heated regenerated sorbent and an off gas stream of regenerant gas carrying away said S0_κ removed from said heated regenerated sorbent; - cooling said heated regenerated sorbent in entrained suspension in a multi-cyclone cooler to produce cooled regenerated sorbent; and

- returning said cooled regenerated sorbent to the first step.

According to EP 657 203 A2 this process may be modified in the following way: sorbent particles of an average size in the range of about 30 μm to about 500 μm are used; and the stream of separated sorbent particles is fed to a sorbent particle splitter where it is divided into a first stream which is recirculated directly to the first NO_.. and SO. adsorption step and a second stream which is directed to the multi-cyclone heating NO. removal step.

It is mentioned in the patents that the ES-NOXSO process, compared to the NOXSO process, has the disadvantage that the relatively small sorbent particles are not easily distinguished from fly ash or other pulverous solids suspended in the gas stream, hence requiring an efficient particle removal both upstream and downstream of the adsorber to prevent accumulation of fly ash or other pulverous solids in the sorbent, and to prevent discharge of sorbent particles into the atmosphere. The SNAP Process.

A modification of the ES-NOXSO process, the so-called SNAP Process is disclosed by Leif Mortensen, SK Power Company, Copenhagen, Denmark, Stig Bue Lading, FLS iljø a/s, Copenhagen, Denmark, and Mark C. Woods, NOXSO Corporation, Bethel Park, Pennsylvania in "Experiences from a 10 MWe Demonstration Project with an innovative SO and NO. Adsorption Process (SNAP)", EPRI DOE EPA 1995 SO, Control Symposium, Miami Florida, March 28-31 1995.

The major components of the SNAP process accomplish the same basic operations as occur in the bead or powder based process according to the NOXSO/ES-NOXSO concept. The major components of the SNAP process are shown schematically in Figure 1. The major components of the SNAP process as shown in Figure 1 are described below.

Gas Suspension Adsorber (GSA) . The flue gases from a coal-fired power plant are introduced into the GSA reactor downstream of the plant's particulate control unit.

The GSA reactor comprises a riser with suitably designed conical inlet, cyclones for primary gas-solid separation, and means for solid recirculation to the bottom of the riser. Flue gas enters the reactor through the conical inlet at the bottom, and contacts suspended sorbent which adsorbs SO^ and NO..,. The GSA reactor design allows for high flue gas velocity (3-6 m/s) and gas-solid contact time of 2-3 seconds, at a relatively low pressure drop

(200-300 mm WG) . The sorbent entrained in the flue gas is separated in the cyclones and is partly recycled to the bottom of the reactor, partly fed to the regeneration unit. Fine sorbent particles entrained in the gas exiting the GSA cyclones are removed in a particulate control unit and returned to the process. Clean flue gas proceeds through a booster fan directly to the stack from where it is released into the atmosphere.

This type of set-up necessitates, like the ES-NOXSO process, two particulate control devices:

- one up-stream of the adsorber for removing the fly-ash, and one down-stream of the adsorber for removing the uncollected fine sorbent entrained in the flue gas.

These two particulate control devices must be suitably designed in order to avoid accumulation of fly ash in the system and in order to comply with the particulate emission requirements.

Sorbent heater section The sorbent heater section represents the first stage of the sorbent regeneration system. A slip stream of loaded sorbent from the GSA cyclone recycle loop and sorbent collected in the particulate control unit are introduced into the heater.

The first sorbent heater is a multi-stage fluidized bed where the sorbent is heated by indirect heat transfer from heat transfer coils immersed in the fluidized bed. The heat transfer medium inside the heating coils is Hi Tec heat transfer medium. In this heater the sorbent reaches a temperature around 300^'C.

The first heater is followed by a second heating step where the sorbent is heated by indirect heat transfer from heat transfer coils immersed in the fluidized bed.

The heat transfer medium inside these heating coils is combustion off-gases from the NOx destruction unit and the auxiliary burner. In this heater the sorbent reaches a temperature around 620"C.

During the second heating step all NO., adsorbed on the sorbent is released. Since the fluidized bed sorbent heater is operated at low air velocity, the total volume of this NO_., bearing stream is relatively small. The NO in this air stream is reduced to N- by staged combustion in the NO. destruction burner.

Regenerator.

The sorbent regenerator is a two-stage fluidized bed. In the first stage the heated sorbent is contacted with a regeneration gas, which also serves as the fluidization medium. The sulfated sorbent reacts with regeneration gas, and sulfur is released as S0₂ and H₂S . In the second stage H₂S remaining on the surface is stripped off with steam. Sodium sulfide, which may be formed during the regeneration, is also converted into H;S by steam hydrolysis. The paper describes that the regeneration can be adopted to use several types of regeneration gases, e.g. natural gas, hydrogen, fuel gas etc.

Sorbent cooler section.

The sorbent cooler section represents the last stage of the sorbent regeneration system.

The first sorbent cooler is a multi-stage fluidized bed where the sorbent is cooled by indirect heat transfer from heat transfer coils immersed in the fluidized bed.

The heat transfer medium inside the coils is Hi Tec heat transfer medium. In this cooler the sorbent reaches a temperature around 300°C. The heated Hi Tec heat transfer medium is from the cooler circulated to the heater, which ensures that the energy consumption in the heating process is minimized.

The first cooler is followed by a second cooling step where the sorbent is cooled by indirect heat transfer from heat transfer coils immersed in the fluidized bed. The heat transfer medium inside these cooling coils is a water/glycol solution. The temperature of the sorbent after this final cooling step is m the range 100-300°C.

Claus unit.

The sulfur compounds from the regeneration step are fed to a Claus unit, where they are converted into elemental sulfur.

The tail gas from the Claus process is passed through a burner to convert all remaining sulfur compounds into SO . The gas is then cooled and recycled to the flue gas stream entering the GSA.

The problem to be solved according to the present invention is to provide an improved version of dry adsorption processes for removal of e.g. nitrogen oxides, sulfur oxides and other acid gases from a gas stream with regeneration and recirculation of the adsorption agent, such as e.g. the SNAP process.

A manor drawback of these processes is the requirement for an efficient particulate control device upstream of the reaction zone m order to avoid contamination of the adsorption agent with fly ash or other particulates contained in the inlet gas.

Thus, according to a first aspect of the present invention this problem is solved by a metnod for removing nitrogen oxides, sulfur oxides, and other acid gases from a gas stream containing fly ash and/or other solid materials in which

- an adsorption agent suitable for adsorbing nitrogen oxides, sulfur oxides, and other acidic gases is introduced and contacted with the gas stream in a reaction zone wherein the nitrogen oxides, sulfur oxides, and other acidic gases are adsorbed by the adsorption agent, - whereafter the adsorption agent is at least partially withdrawn from the reaction zone with the gas stream as an entrained suspension and substantially removed from the gas stream in a separation zone,

- whereafter the exit gas from the separation zone is discharged into the atmosphere,

- while separated adsorption agent is introduced into a regeneration zone where the adsorbed nitrogen oxides, sulfur oxides, and other acidic gases are substantially removed from the adsorption agent and withdrawn from the regeneration zone in a concentrated form,

- while the regenerated adsorption agent is recycled to the reaction zone, which is characterized in that the separation zone comprises a first electrostatic filter and optionally an additional filter arranged upstream of the first electrostatic filter.

It has been discovered that the resistivity of the adsorption agent decreases considerably during the adsorption in the reaction zone. This phenomenon makes it possible to obtain an extremely efficient removal of the adsorption agent relatively to fly ash and/or other solid materials present in an electrostatic filter downstream of the reaction zone. Hence, this discovery allows operation under high dust conditions (i.e. high concentrations of fly ash and/or other solid materials in the inlet gas) without excessive loss or contamination of the adsorption agent with fly ash and/or other solid materials.

It is believed that the above mentioned change of resistivity during the adsorption process is primarily due to adsorption of SO; on the adsorption agent surface, which is particular efficient when using the NOXSO sorbent particles, disclosed e.g. in the above-mentioned NOXSO patents, included by reference.

According to a preferred embodiment the adsorption agent separated in the separation zone is partly recycled to the reaction zone, and partly introduced into the regeneration zone.

When the separation zone comprises a first electrostatic filter as well as an additional filter, e.g. a system of one or more cyclones, the solid material precipitated in the additional filter is preferably divided into two streams, where the first stream is recycled to the reaction zone, whereas the second stream is directed to the regeneration zone. The solid material precipitated in the first electrostatic filter is divided and recycled in the same way.

The reaction zone may be accommodated in all types of adsorption reactors, including fluid bed reactors (e.g. dense bed, spouted bed etc.), moving bed reactors and entrained suspension adsorbers. In the last mentioned case pulverous adsorption agents are used.

According to preferred embodiments the reaction zone comprises gas-solid contact in an entrained suspension adsorber, but in some applications the reaction zone comprises a fluid bed or a moving bed. In such applications fines of the adsorption agent and fly ash are elutriated from the bed and carried with the gas as an entrained suspension. In these cases an additional particulate stream is withdrawn directly from the reaction zone, introduced into a regeneration zone, and returned to the reaction zone.

According to another preferred embodiment fly ash and/or other solid materials contained in the gas stream introduced into the reaction zone are partially removed in a second filter arranged upstream of the reaction zone.

In the present context the term "partial removal of fly ash and/or other solid materials" is used to designate removal to particulate concentrations in the outlet gas from said second filter of at least 100

such as at least 1 g/Nπr and in particular in the range 1-10 g/Nm^J.

The second filter may be a member selected from the group consisting of cyclones, ceramic filters, bag filters, gravimetric settling filters, and electrostatic filters.

According to another preferred embodiment fly ash and/or other solid materials contained in the exit gas stream from the first electrostatic filter for removal of the adsorption agent are removed in a third filter arranged downstream of the first electrostatic filter.

The third filter may be a member selected from the group consisting of bag filters, ceramic filters, and electrostatic filters. The third filter is preferably an electrostatic filter arranged adjacent to the first electrostatic filter in a common housing.

In still another preferred embodiment the first electrostatic filter is used for separation of the sorbent with respect to the sulfur content. Hence, the operation of the electrostatic filter is adjusted so that the particulate stream withdrawn from said first electrostatic filter has a substantially higher sulfur content than the particulate part of the incoming entrained suspension.

According to another preferred embodiment the adsorption agent consists of particles having an average particle size within the range 20-200 μm, preferably within the range 40-80 μm consisting of:

(a) a gamma alumina substrate having a surface area in the range 100-500 πr/g and a pore volume in the range 0.3-0.8 ml/g,

(b) an alkali metal component, the substrate being impregnated with the alkali metal component, and

(c) an alumina stabilizer selected from the group consisting of silica, lanthana, other rare earths, titania, zirconia, clay, alkaline earths and mixtures thereof in an amount from an effective amount up to about 30 mole-?;.

The gamma alumina substrate has preferably a biraodal pore size distribution comprising micropores and macropores, the micropores having an average pore diameter d> in the range 30 - 400 Angstroms and the macropores having an average pore diameter d. in the range 80 - 3000 Angstroms. According to a preferred embodiment the adsorbed nitrogen oxides, sulfur oxides, and other acidic gases are substantially removed from the adsorption agent in the regeneration zone comprising the following steps: - a heating step;

- a reducing gas treatment step;

- a stripping step; and

- a cooling step, and optionally - an adsorption agent buffer tank.

According to a preferred embodiment

- the reaction zone comprises an entrained suspension adsorber; - the heating step comprises heating of the adsorption agent in a fluidized bed heater to remove NO, and the main part of the other acidic gases from the adsorption agent, withdrawing the liberated gases and the adsorption agent from the fluidized bed heater; - the reducing gas treatment step comprises contacting the exit adsorption agent from the heating step with a reducing gas in a fluidized bed reactor to remove and release SO, as a mixture of SO> and H₂S, withdrawing the mixture of SO; and H_?S and the adsorption agent from the fluidized bed reactor; the stripping step comprises contacting the exit adsorption agent from the reducing gas treatment step with water vapour in a fluidized bed to remove adsorbed H-S from the adsorption agent; - the cooling step comprises cooling the exit adsorption agent from the stripping step in a fluidized bed cooler; and

- the exit adsorption agent from the cooler is recycled to the reaction zone either directly or via an adsorption agent buffer tank. The heating step/cooling step is preferably carried out by indirect heat transfer from heating/cooling coils immersed in a fluidized bed.

According to a preferred embodiment the reducing gas is used as fluidization medium in the fluidized bed reactor in the reducing gas treatment step.

According to another embodiment the reaction temperature in the fluidized bed reactor in the reducing gas treatment step is within the range 200-700 °C, preferably 300-600 °C, and in particular 300-500 °C.

The regeneration process is a crucial part of any regenerable adsorption process, that constitutes a major part of the operational costs in the heating of the adsorption agent to the regeneration temperature, and in the addition of reducing agent.

The regeneration temperature is known to depend on the reducing agent. As a rule of thumb the regeneration temperature required for pure substances may be correlated to the spontaneous ignition temperature of the substance. For example, the regeneration temperature required for methane is in the range 620-640 °C, being close to the spontaneous ignition temperature of 632 °C.

It would therefore be an obvious choice to carry out the regeneration by using a reducing agent having a low spontaneous ignition temperature (SIT) . Examples of such reducing agents are: acetylene (SIT = 305 °C) and ethanol

(SIT = 392 °C) . It has, however, been observed that such reducing agents exhibit a strong tendency to soot and coke formation on the adsorption agent, resulting in a physical blockage of the pores impeding the regeneration process.

Tests with acetylene as well as ethanol showed a very pronounced darkening of the adsorption agent due to the above mentioned formation of soot and coke. For both tests a considerable, unacceptable increase in residual sulfur was found.

It can thus be concluded that the above mentioned reducing agents and other coke forming compounds are unsuitable for the regeneration process.

It is has also been discovered that mixtures of gases in general decrease the temperature of regeneration compared to the regeneration temperature for the pure substances. For example, the regeneration temperature for methane is in the range 620-640 °C, while the regeneration temperature for natural gas (containing approximately 90 % Methane) is in the range 560-580 °C. This may be due to a synergistic effect of the mixture. Natural gas may contain up to 10 ?^; non-combustibles. Such non-combustible compounds, viz. CO , H_:0, etc., may be responsible for synergetic effects, thus enhancing the regeneration significantly. Further, it has been discovered that the conditions in the reducing agent treatment step in the regeneration zone are able to convert sulfur- and nitrogen-containing compounds into H₂S, SO. and N... The produced sulfur compounds may thus participate directly in a regular downstream Claus process, while nitrogen is a non- pollutant .

It- is known from prior art, US Patent No. 4,323,544, that water vapour and/or carbon dioxide may be present in the reducing gas, that the presence of those gases will improve the removal of sulfide from adsorption agent, and that regeneration without these components will necessitate a second regeneration (stripping) step where the adsorption agent is treated with one of these gases, as described and realized in the SNAP second regeneration stage.

This means that regeneration with a mixture containing substantial amounts of H₂0 and/or CO; may exhibit the advantage of eliminating at least one downstream processing (stripping) step and thus reduce the capital and operational costs.

Another problem to be solved by the present invention is to provide another improved version of the above mentioned dry adsorption processes with regeneration and recirculation of the adsorption agent.

According to a second aspect of the present invention it has been found that regeneration temperature as well as operational cost may be reduced by use of low grade fuel in the reducing agent treatment step.

Therefore the present invention does also relate to a method for removing nitrogen oxides, sulfur oxides, and other acid gases from a gas stream, in particular according to any of claims 1-14, in which

- an adsorption agent suitable for adsorbing nitrogen oxides, sulfur oxides, and other acidic gases is introduced and contacted with the gas stream in a reaction zone wherein the nitrogen oxides, sulfur oxides and other acidic gases are adsorbed by the adsorption agent,

- while separated adsorption agent is introduced into a regeneration zone where the adsorbed nitrogen oxides, sulfur oxides, and other acidic gases are substantially removed from the adsorption agent and withdrawn from the regeneration zone in a concentrated form, said regeneration zone containing a reducing agent treatment step,

- while the regenerated adsorption agent is recycled to the reaction zone,

which is characterized in that the reducing agent used in the reducing gas treatment step is a low grade f el. In the present context the term "low grade fuel" is intended to designate fuels containing impurities in an amount necessitating further processing in order to provide a generally acceptable fuel, and/or having a low heating value.

Low grade fuel is usually processed before it is used in other processes, due to the general necessity of using pollutant free fuels. These pollutants are usually sulfur containing compounds, nitrogen containing compounds or non-combustibles, such as nitrogen, water vapour, sulfur oxides and carbon oxides. By means using a low grade fuel it is thus obtained that the synergistic effect of the gas mixture is utilized for performing the regeneration at a relatively lower temperature and that the low grade fuel is used without any prior processing, both improvements resulting in lower operational costs.

As low grade fuel, the following gases may i.a. be used according to the present invention:

unprocessed, sulfur containing effluents from the following processes: coal gasification, coal liquefaction, oil shale processing, tar sands processing, petroleum processing, mineral processing, and geothermal energy utilization;

- various unprocessed gas streams from refineries, e.g. flare gas;

- various gas streams in the petroleum processing;

- sour water stripper off-gas from refineries; and - bi oga s

According to a preferred embodiment the low grade fuel contain at least one non-combustible component, e.g. CO , H„0, SO , NO, NO , N, , 0_., etc.

The low grade fuel may contain non-combustible compounds in an amount up to 95, preferably up to 50, in particular up to 25 % by volume.

According to a preferred embodiment the low grade fuel contains non-combustible compounds in an amount of at least 10, preferably at least 15, in particular at least 20 by volume.

According to another preferred embodiment the low grade fuel is a waste fuel.

The low grade fuel may in particular contain sulfur containing compounds, such as sulfides, mercaptans, thioethers, thioaldehydes, and thioketones.

The low grade fuel may in particular also contain nitrogen containing compounds, such as ammonia, urea, urethanes, amines, amides, jmides, and CN-containing compounds as well as mixed sulfur and nitrogen containing compounds . According to a preferred embodiment of the present invention the adsorption agent consists of particles having an average particle size within the range 20-200 μm, preferably within the range 40-80 μm consisting of: (a) a gamma alumina substrate having a surface area in the range 100-500 m /g and a pore volume in the range 0.3-0.8 ml/g,

(c) an alumina stabilizer selected from the group consisting of silica, lanthana, other rare earths, titania, zirconia, clay, alkaline earths and mixtures thereof in an amount from an effective amount up to about 30 mole-%.

According to another preferred embodiment of the present invention the gamma alumina substrate has a bimodal pore size distribution comprising micropores and macropores, the micropores having an average pore diameter d_; in the range 30 - 400 Angstroms, and the macropores having an average pore diameter d; in the range 80 - 3000 Angstroms .

According to yet another preferred embodiment of the present invention the adsorbed nitrogen oxides, sulfur oxides, and other acidic gases are substantially removed from the adsorption agent in the regeneration zone comprising the following steps: - a heating step;

- a reducing gas treatment step;

- a stripping step; and

- a cooling step, and optionally - an adsorption agent buffer tank. In a further embodiment of the present invention

- the reaction zone comprises an entrained suspension adsorber;

- the heating step comprises heating of the adsorption agent in a fluidized bed heater to remove NO and the main part of the other acidic gases from the adsorption agent, withdrawing the liberated gases and the adsorption agent from the fluidized bed heater;

- the reducing gas treatment step comprises contacting the exit adsorption agent from the heating step with a reducing gas in a fluidized bed reactor to remove and release SOv as a mixture of SO and H_:S, withdrawing the mixture of SO and H;S and the adsorption agent from the fluidized bed reactor; - the stripping step comprises contacting the exit adsorption agent from the reducing gas treatment step with water vapour in a fluidized bed to remove adsorbed H₂S from the adsorption agent; and

- the cooling step comprises cooling the exit adsorption agent from the stripping step in a fluidized bed cooler.

In a further embodiment of the present invention the adsorbed nitrogen oxides, sulfur oxides, and other acidic gases are substantially removed from the adsorption agent in the regeneration zone comprising the following steps:

- a heating step;

- a reducing gas treatment step; and

- a cooling step, and optionally - an adsorption agent buffer tank.

In the following the invention will be further described with reference to the drawings, in which fig. 1 is a diagrammatic illustration of the SNAP process described in the introductory part of the description,

fig. 2 schematically illustrates a process according to the present invention for removal of nitrogen oxides, sulfur oxides and other acid gases from a gas stream,

fig. 3 shows a preferred embodiment of the above process having a particle stream withdrawn from the reaction zone and sent to the regeneration zone,

fig. 4 shows further details of the regeneration process, according to a preferred embodiment, and

fig. 5 shows an example of an preferred embodiment that serves as a process diagram for example 4.

As shown in fig. 2, the gas stream (10) is passed through a reaction zone (1) where nitrogen oxides, sulfur oxides and other acid gases are adsorbed on an adsorption agent. The adsorption agent is withdrawn from the reaction zone (1) with the gas stream as an entrained suspension (11) and substantially removed from the gas m a separation zone (2) consisting of a first electrostatic filter (4) and an optional filter (5) upstream of the first electrostatic filter (4) . Separated adsorption agent (13) is introduced into a regeneration zone (3) or may be partly recycled (14) to the reaction zone (1) . In the regeneration zone (3) nitrogen oxides, sulfur oxides and other acid gases adsorbed on the adsorption agent are substantially removed from the adsorption agent and withdrawn from the regeneration zone (3) in a concentrated form (17) . Regenerated adsorption agent (15) is recycled to the reaction zone (1) . An optional second filter (6) may be placed upstream of the reaction zone (1) for partial removal of fly ash and/or other solid materials from the inlet gas stream (16) . The gas stream

(12) from the separation zone (2) may be further processed for the removal of fly ash and/or other solid materials in an optional third filter (7) before the exit gas stream (18) is discharged into the atmosphere. In the case of choosing an electrostatic filter as the third filter (7), it will be possible to arrange the first electrostatic filter (4) and the third electrostatic filter (7) in the same housing creating a filter where adsorption agent containing particulate stream can be withdrawn from the one end of the filter, and an adsorption agent free particulate stream can be withdrawn from the other end of the filter.

Figure 3 illustrates a process similar to the process illustrated in figure 2 with a stream of particles (19) withdrawn from the reaction zone (1) and sent to the regeneration zone (3) .

Figure 4 illustrates a process similar to the process illustrated in figure 2 with a regeneration zone (3) consisting of

-a heating step (20) ;

-a reducing gas treatment step (21); -a stripping step (22);

-a cooling step (23) ; and

- an optional adsorption agent buffer tank (31) .

Figure 5 show a process diagram for a process according to the present invention, m which a flue gas from a power generation station containing SO , NOx and fly ash

(10) is passed through a reaction zone (1) where nitrogen oxides, sulfur oxides, and other acid gases are adsorbed on an adsorption agent. The pulverous adsorption agent is withdrawn from the reaction zone (1) with the gas stream as an entrained suspension (11) and 26

substantially removed from the gas in a separation zone (2) consisting of a first electrostatic filter (4) and a cyclone (5) upstream of the electrostatic filter. Separated adsorption agent (13) is partly introduced into a regeneration zone (3), and partly recycled (14) to the reaction zone (1) . The regeneration zone consists of

-a fluidized bed heater (20) using air (24) as the fluidizing media; -a_ reducing gas treatment step (21) using natural gas

(26) as the fluidizing media;

-a stripping step (22) using steam (27) as the fluidizing media;

-a cooling step (23) using air (29) as the fluidizing media; and

-an adsorption agent buffer tank (31) .

In this example the regeneration zone produces three outlet streams, -a stream (25) from the fluidized bed heater (20) containing NOx in a concentrated form,

-a stream (28) from the reducing gas treatment step (21) and the stripping step (22) containing H,_:S and SO; in a concentrated form, and - a stream (30) from the fluidized bed cooler (23) that can be discharged to the atmosphere.

Regenerated adsorption agent (15) is recycled to the reaction zone (1) . The gas stream (12) from the separation zone (2) is further processed for the removal of fly ash and/or other solid materials in an electrostatic filter (7) before the exit gas stream (18) is discharged into the atmosphere. It will be possible to arrange the first electrostatic filter (4) and the third electrostatic filter (7) in the same housing creating a filter where adsorption agent containing particulate stream is withdrawn from the one end of the filter, and an adsorption agent free particulate stream (32) is withdrawn from the other end of the filter.

The invention is further illustrated by way of the following non-limiting examples:

Example 1 : Sour water stripper off-gas from a refinery as the reducing agent in the SNAP.

A slip stream from the process of petroleum refining is the sour water stripper off-gas. This stream has a composition approximately as follows: H₂S 45% H₂0 30%

NH₃ 25%

This gas with the above composition is a typical example of a low grade fuel that is not generally acceptable as a fuel. The mixture contains two combustible components (H S and NH₃) and a non combustible component (H O) . The sour water stripper off-gas is today burned in the flare.

The sour water stripper off-gas can be used as the reducing agent used in the regeneration zone for the SNAP, thus lowering the operational cost for the reducing agent treatment step considerably, and simultaneously reducing the emission of sulfur-oxides and nitrogen oxides to the atmosphere from the refinery flare.

Example 2: Combustible gas mixture from a refinery as the reducing agent in the SNAP.

A by-product stream from the process of petroleum refining is a gas stream containing a mixture of hydrocarbon, hydrogen sulphide and hydrogen. This stream has a composition approximately as follows:

H.S 0.2

H 13.1 %

C,H» and CH, 17.1 "_n d-compounds (3 different) 5.5 '-

This gas is a typical example of a low grade fuel due to the complex composition of the mixture and the high content of hydrogen sulfide. The gas mixture is today used in the refinery for heat generation.

The above mentioned gas mixture can be used as the reducing agent used in the regeneration zone for the SNAP, thus lowering the operational cost for the reducing agent treatment step considerably.

Example 3: Mixture of sour water stripper off-gas and a combustible gas mixture from a refinery as a reducing agent in the SNAP.

A mixture of the sour water stripper off-gas mentioned in example 1 and the gas mixture mentioned in example 2 will contain a variety of hydrocarbon, hydrogen, hydrogen sulfide, ammonia and non-combustible water vapour and must thus be classified as a low grade fuel.

This gas mixture can be used as the reducing agent used in the regeneration zone for the SNAP, thus lowering the operational cost for the reducing agent treatment step considerably.

Example 4 : Using an electrostatic filter in the separation of SNAP adsorption agent and fly ash. The process disclosed in this example corresponds to the diagram shown in figure 5.

100000 NmVh at 120 °C flue gas (10) from a fossil fired power station is to be cleaned.

The flue gas contains

600 ppmv SO; 200 ppmv NO_;,

10 g/Nm^; fly ash

The gas is led to the reaction zone (1) comprising a gas suspension adsorber (GSA) with a diameter of 2.8 m. In the GSA the flue gas is mixed with adsorption agent consisting of a γ-alumina carrier impregnated with sodium oxide with a particle size of d,. = ^"0 μm.

S0₂ and NOx are adsorbed on the adsorption agent in the reaction zone, and the clean gas leaves the reaction zone with loaded adsorption agent (11) .

The gas stream (11) here contains

S0₂ 30 ppm NOx 25 ppm

Fly ash 60 g/Nm'

Adsorption agent 500 g/Nm'

The flue gas enters the separation zone (2) that comprises a cyclone (5) and an electrostatic filter (4) .

Part of the solid material is removed from the gas stream in the separation zone (2) whereafter the outgoing stream (12) contains: 10 g/Nm^: Fly ash

<5 mg/Nm adsorption agent The remaining part of the solid material is efficiently removed down to a total concentration of 50 mg/Nm in the second part of the electrostatic filter (7) , whereafter the gas stream is led to the atmosphere (18) . The particles collected in this second electrostatic filter are discharged as the stream (32) .

In this example the two electrostatic filters (4) and (7) are placed in the same housing, and actually constitute one electrostatic filter containing two outlets for collected particles.

The solid material collected (13) in the separation zone (2) corresponds to a total amount of 54995 kg/h, of which

49495 kg/h are recirculated via (14) to the reaction zone

(1), and 5500 kg/h are sent to the regeneration zone (3) .

In the regeneration zone (3) the adsorption agent is led to a fluidized bed heater (20) that is using air as the fluidizing medium (24) .

The adsorption agent is heated indirectly to 600 'C, by which NOx is liberated into the fluidization gas in a concentration of 9 Ϋ and leaves the system through (25) .

The hot adsorption agent is led to the reducing agent treatment step where it is treated in a fluidized bed reactor (21) with a diameter of 1.2 m with natural gas (26) 43 Nπr/h in 45 minutes, and then led to the steam treatment step, where it is treated in a fluidized bed reactor (22) with a diameter of 0.6 m with steam (27) 7 NmVh for 10 minutes.

The gas stream leaving the steam treatment and reducing agent treatment step is mixed (28) and contains 74.6 kg/h sulfur as H_..S and SO; in concentrations of 30' and 2", respectively.

The adsorption agent is afterwards led to the fluid bed sorbent cooler (23) that is using air as a fluidizing medium (29) where it is indirectly cooled to 125"C. The gas leaving the sorbent cooler does not contain any pollutants and is discharged into the atmosphere.

The cooled adsorption agent particles are recycled (15) to the reaction zone (1) via an adsorption agent buffer tank (31) .

Claims

Cl aims

1. A method for removing nitrogen oxides, sulfur oxides, and other acid gases from a gas stream containing fly ash and/or other solid materials in which

- while the regenerated adsorption agent is recycled to the reaction zone, characterized in that the separation zone comprises a first electrostatic filter, and optionally an additional filter arranged upstream of the first electrostatic filter .

2. A method according to claim 1, characterized in that the adsorption agent separated in the separation zone is partly recycled to the reaction zone and partly introduced into the regeneration zone.

3. A method according to any of the claims 1-2, characterized in that an additional particulate adsorption agent stream is withdrawn directly from the reaction zone, and is introduced in to a regeneration zone, and returned to the reaction zone.

4. A method according to claim 1-3 , characterized in that fly ash and/or other solid materials contained in the gas stream introduced into the reaction zone are partially removed jn a second filter arranged upstream of the reaction zone.

5. A method according to any of the preceding claims, characterized in that fly ash and/or other solid materials contained in the exit gas stream from the first electrostatic filter for removal of the adsorption agent are removed in a third filter arranged downstream of the first electrostatic filter.

6. A method according to claim 5, characterized m that the third filter is an electrostatic filter ad acent to the first electrostatic filter in a common housing.

7. A method according to any of the preceding claims, characterized in that the particulate stream withdrawn from said first electrostatic filter has a higher sulfur content than the particulate part of the incoming entrained suspension.

8. A method according to any of the preceding claims, characterized in that the adsorption agent consists of particles having an average particle size within the range 20-200 μm, preferably within the range 40-80 μm, consisting of:

(a) a gamma alumina substrate having a surface area in the range 100-500 m /g and a pore volume m the range 0.3-0.8 ml/g,

9. A method according to claim 8, characterized in that the gamma alumina substrate has a bimodal pore size distribution comprising micropores and macropores, the micropores having an average pore diameter d, in the range 30 - 400 Angstroms, and the macropores having an average pore diameter d₂ in the range 80 - 3000 Angstroms.

10. A method according to any of the preceding claims, characterized in that the adsorbed nitrogen oxides, sulfur oxides and other acidic gases are substantially removed from the adsorption agent in the regeneration zone comprising the following steps:

- a heating step;

- a reducing gas treatment step;

- a stripping step; and - a cooling step, and optionally

- an adsorption agent buffer tank.

11. A method according to claim 10, characterized in - that the reaction zone comprises an entrained suspension adsorber; that the heating step comprises heating of the adsorption agent in a fluidized bed heater to remove NO, and the main part of the other acidic gases from the adsorption agent, withdrawing the liberated gases and the adsorption agent from the fluidized bed heater; that the reducing gas treatment step comprises contacting the exit adsorption agent from the heating step with a reducing gas in a fluidized bed reactor to remove and release SO as a mixture of SO; and H-S, withdrawing the mixture of SO- and H₂S and the adsorption agent from the fluidized bed reactor;

- that the stripping step comprises contacting the exit adsorption agent from the reducing gas treatment step with water vapour in a fluidized bed to remove adsorbed H-S from the adsorption agent; and that the cooling step comprises cooling the exit adsorption agent from the stripping step in a fluidized bed cooler.

12. A method according to any of claims 10-11, characterized in that the heating step/cooling step is carried out by indirect heat transfer from heating/cooling coils immersed in a fluidized bed.

13. A method according to any of claims 10-12, characterized in that the reducing gas is used as fluidization medium in the fluidized bed reactor in the reducing gas treatment step.

14. A method according to any of claims 10-13, characterized in that the reaction temperature in the fluidized bed reactor in the reducing gas treatment step is within the range 200-700 °C, preferably 300-600 °C and in particular 300-500 °C.

15. A method for removing nitrogen oxides, sulfur oxides, and other acid gases from a gas stream, in particular according to any of claims 1-14, in which

- an adsorption agent suitable for adsorbing nitrogen oxides, sulfur oxides, and other acidic gases is introduced and contacted with the gas stream in a reaction zone wherein the nitrogen oxides, sulfur oxides, and other acidic gases are adsorbed by the adsorption agent,

- whereafter the exit gas from the separation zone is discharged into the atmosphere, - while separated adsorption agent is introduced into a regeneration zone where the adsorbed nitrogen oxides, sulfur oxides and other acidic gases are substantially removed from the adsorption agent and withdrawn from the regeneration zone in a concentrated form, said regeneration zone containing a reducing agent treatment step,

- while the regenerated adsorption agent is recycled to the reaction zone, characterized m that the reducing agent used in the regeneration zone is a low grade fuel.

16. A method according to claim 15, characterized m that the low grade fuel contains non-combustible compounds m an amount up to 95, preferably up to 50, in particular up to 25 % by volume.

17. A method according to claim 16, characterized in that the low grade fuel contains non-combustible compounds in an amount of at least 10, preferably at least 15, in particular at least 20 ^l by volume.

18. A method according to any of claims 15-17, characterized in that the low grade fuel is a waste fuel.

19. A method according to any of claims 15-18, characterized m that the low grade fuel is a biogas .

20. A method according to any of claims 15-19, characterized in that the low grade fuel contains sulfur containing compounds.

21. A method according to any of claims 15-20, characterized in that the low grade fuel contains nitrogen containing compounds.

22. A method according to any of the claims 15-21, characterized in that the adsorption agent consists of particles having an average particle size within the range 20-200 μm, preferably within the range 40-80 μm, consisting of: (a) a gamma alumina substrate having a surface area in the range 100-500 m²/g and a pore volume in the range 0.3-0.8 ml/g,

23. A method according to claim 22, characterized in that the gamma alumina substrate has a bimodal pore size distribution comprising micropores and macropores, the micropores having an average pore diameter di in the range 30 - 400 Angstroms, and the macropores having an average pore diameter d; in the range 80 - 3000 Angstroms .

24. A method according to any of the claims 15-23, characterized in that the adsorbed nitrogen oxides, sulfur oxides, and other acidic gases are substantially removed from the adsorption agent in the regeneration zone comprising the following steps:

- a heating step;

- a reducing gas treatment step;

- a stripping step; and

- a cooling step, and optionally

- an adsorption agent buffer tank.

25. A method according to claim 24, characterized in that the reaction zone comprises an entrained suspension adsorber; that the heating step comprises heating of the adsorption agent in a fluidized bed heater to remove NO_> and the main part of the other acidic gases from the adsorption agent, withdrawing the liberated gases and the adsorption agent from the fluidized bed heater; that the reducing gas treatment step comprises contacting the exit adsorption agent from the heating step with a reducing gas in a fluidized bed reactor to remove and release SO_., as a mixture of S0₂ and H₂S, withdrawing the mixture of S0₂ and H₂S and the adsorption agent from the fluidized bed reactor; - that the stripping step comprises contacting the exit adsorption agent from the reducing gas treatment step with water vapour in a fluidized bed to remove adsorbed H.-S from the adsorption agent; and that the cooling step comprises cooling the exit adsorption agent from the stripping step in a fluidized bed cooler.

26. A method according to claim 24, characterized in that the adsorbed nitrogen oxides, sulfur oxides, and other acidic gases are substantially removed from the adsorption agent in the regeneration zone comprising the following steps:

- a heating step;

- a reducing gas treatment step; and

- a cooling step, and optionally

- an adsorption agent buffer tank.