US3921389A

US3921389A - Method and apparatus for combustion with the addition of water

Info

Publication number: US3921389A
Application number: US403057A
Authority: US
Inventors: Katsuyuki Kawaguchi
Original assignee: Mitsubishi Heavy Industries Ltd
Current assignee: Mitsubishi Heavy Industries Ltd
Priority date: 1972-10-09
Filing date: 1973-10-03
Publication date: 1975-11-25
Anticipated expiration: 1992-11-25
Also published as: FR2202230B1; DE2350658A1; GB1449150A; DE2350658C3; FR2202230A1; DE2350658B2; IT1006614B; CH567695A5

Abstract

The invention is a method of burning fuel in a combustion chamber in which fuel and air are admitted to the combustion chamber through concentric orifices and are burned in the presence of water in a high temperature combustion zone. This method of combustion reduces the formation of smoke and of oxides of nitrogen. A preferred method of providing water in the high temperature combustion zone is to add water to the fuel before the fuel is admitted to the combustion chamber. The invention further extends to apparatus for burning fuel in accordance with the preferred method.

Description

United States Patent Kawaguchi 1 1 Nov. 25, 1975 [5 METHOD AND APPARATUS FOR 2,763.987 9/1956 Kretschmer 60/3955 COMBUSTION WITH THE ADDITION 3,087,304 4/1963 Walter WATER 3,088,280 /1963 Lefehvre et ul 3,167,913 2/1965 Miihlberg et a1. [75] Inventor: Katsuyuki Kawaguchi, Akashi, 3,359,723 12/1967 Bohensky et a1. Japan 3,527,581 9/1970 Brownawell et a1. 44/51 3,608,309 9/1971 Hill et al. v. 60/3965 [73] Assignee: Mitsubishi Jukogyo Kabushiki 3 93,347 9 972 Kydd t a1. 60/3905 Kaisha, Tokyo, Japan 3,741,712 6/1973 Delatronchette 431/190 [22] Filed: 1973 Primary ExaminerC. J. Husar [21] Appl. No.: 403,057 Assistant Examiner-L. J. Casaregola Attorney, Agent, or Firm-Cushman, Darby and Foreign Application Priority Data cushman Oct. 9, 1972 Japan 47-100645 2 Oct 197 Japan 47 100646 The inventlon 1s a method of. burning fuel in a com- [52] S Cl 60/39 05. 60/39 59 bustion chamber in which fuel and air are admitted to [51] 6 {202C the combustion chamber through concentric orifices [58] Fieid "60/39 05 39 58 and are burned in the presence of water in a high temi Z d perature combustion zone. This method of combustion reduces the formation of smoke and of oxides of [56] References Cited nitrogen. A preferred method of providing water in the high temperature combustion zone is to add water UNITED STATES PATENTS to the fuel before the fuel is admitted to the combus- 324,828 8/1885 Gassett 60/3958 tion chamber The invention further extends to appag g--- ratus for burning fuel in accordance with the preferred e wic 2,648,950 8/1953 Miller 60/3955 method 2,722,100 11/1955 Goddard 60/3955 5 Claims, 5 Drawing Figures )b in [L J: if [#62 F a at 5 3 o US. Patent Nov. 25, 1975 Sheet2of2 3,921,389

METHOD AND APPARATUS FOR COMBUSTION WITH THE ADDITION OF WATER Field of the Invention This invention relates to the combustion of fuel in a manner such that production of smoke and of oxides of nitrogen is reduced.

Background of the Invention It has been considered difficult to suppress the formation of smoke and of oxides of nitrogen while burning fuel. Increasing the air-fuel ratio and promoting mixing of air and fuel to raise the combustion temperature to lower the amount of smoke produced and extending the residence time of the fuel particles in the combustion zone in an attempt to re-burn carbon formed both tend to promote formation of oxides of nitrogen.

In Japan, the following three methods among others, have been employed for suppressing the formation of oxides of nitrogen when burning fuel in a combustion chamber of a gas turbine. However each method has disadvantages, as set forth below.

Method 1 Aqueous vapor is injected from a manifold tube connected to the outlet of an air compressor into the space surrounding the inner tube of the combustion chamber.

In this method, a large amount of vapor flows into the intermediate combustion zone and dilution zone from the mid portion to the rear portion of the combustion chamber, lowering the gas temperature in the combustion chamber, with the result that the amount of smoke generated increases. Furthermore, this method requires an extremely large amount of aqueous vapor (about twice as much aqueous vapor as fuel) for reducing the amount of oxides of nitrogen formed.

Method 2 Aqueous vapor is injected from around a fuel injection valve.

In this method the spray cone angle occasionally decreases depending upon the relation betweenthe position and the amount of aqueous vapor injection, with the result that the amount of smoke generated may be increased.

Method 3 This method is described in a report entitled Study on a method of promoting the combustion of heavy oil by the addition of water (Technical Bulletin of Mitsubishi Heavy Industrial Company, Vol. 18, No. 4, July 31, 1971, pages 625632), which is an improvement of the above-described methods.

In this method heavy oil mixed with 5% of water is burned, whereby. the amount of dust (carbon concentration) can be reduced to one-half of that when water is not added. However, an experiment conducted with a gas turbine combustion chamber has revealed that the Bacharach Smoke Number (hereinafter abbreviated to B.S.N.) of the smoke does not change substantially and in the case of boiler combustion for example the addition of the necessary amount of water is substantially ineffective in decreasing the amount of oxides of nitrogen produced. Furthermore the amount of oxides of nitrogen discharged increases drastically (to -200 PPM) even with a slight increase (up to 3%) in the air ratio.

It will be appreciated that in the case ofa gas turbine combustion chamber in which the air ratio is as high as 500%, the simple addition of water to the fuel is insufficient for decreasing the amount of oxides of nitrogen produced. This is the case whether the water is added by injecting aqueous vapor into the combustion zone or by previously mixing water with the fuel.

Summary of the Invention According to one aspect of the present invention there is provided a method of burning fuel. to which water has been added, in a combustion chamber in which water-containing fuel and air are admitted to the combustion chamber via substantially concentric orifices and combustion takes place in the presence of the water in a high temperature combustion zone.

The water can be added to the fuel as liquid or as aqueous vapor or steam.

The preferred high temperature for the combustion is in the range of from I500 to l600C.

According to a further aspect of the invention there is provided apparatus for burning fuel in accordance with the above-defined method which apparatus comprises a combustion chamber provided with substantially concentric orifices for the admission of fuel and air to the combustion chamber, and means for supplying fuel to the combustion chamber including a mixing chamber and a water-injection tube extending into the mixing chamber which water-injection tube has a plurality of apertures in its wall whose cross-sectional areas decrease progressively towards the end of the water-injection tube, for uniformly mixing water with the fuel.

The present invention will be described hereinafter by way of example as applied to the combustion chamber of a gas turbine. It should be understood, however, that the invention is also applicable to combustion chambers in which combustion takes place constantly. such as combustion chambers of boilers and supercharging boilers, and that the invention is not restricted to the use of a particular fuel injection valve.

A combustion chamber for a gas turbine can be divided over its length into three sections, or zones, according to what happens in that zone. In the forward section of the combustion chamber, the primary combustion zone, air entering through ports in the liner tends to recirculate and move up-stream against the fuel spray. During combustion this action permits rapid combustion and prevents flame blowout by forming a low-velocity stabilization zone which acts as a continuous pilot for the rest of the combustion chamber. In the next section, or intermediate combustion zone, air en tering through ports in the liner mixes and burns any remaining combustible matter which was not burned in the primary combustion zone. In the third section, the dilution zone, air enters through ports in the liner and cools and dilutes the hot gases.

In one embodiment of the present invention. orifices for admission of combustion air to a combustion chamber are distributed concentrically around a fuel valve to increase the air ratio in the primary combustion zone and thereby tend to raise the combustion temperature at the combustion zone and provide conditions under which the combustion of fuel particles is accomplished in a very short period of time. The fuel is burned in the presence of water. Under these conditions the combustion temperature is lowered due to the fuel particle dividing action of the water evaporating in the fuel, so that the flames become short in length and the residence time of the fuel becomes short. Consequently. the fuel particles burn out in a very short period of time in the primary combustion zone and the amount of smoke generated is reduced. The ratio of air to oxides of nitrogen is also high but the major amount of oxygen is consumed in the complete combustion of fuel. The residence time of air retained by the short flames is short, which provides less opportunity for formation of oxides of nitrogen.

The formation of oxides of nitrogen in an intermediate combustion zone can also be suppressed by closing the combustion air inlet ports in the intermediate combustion zone.

Volatile oils can be atomized relatively easily. Therefore. when volatile oils are burned. the generation of smoke and oxides of nitrogen can be controlled by concentrating the combustion air to the primary combustion zone to elevate the combustion temperature and to shorten the flames at the primary combustion zone and thus to form a local zone in which complete combustion occurs and by injecting a slight amount of water controllably into the zone, The injection of water into the high temperature combustion zone is an important factor for encouraging the generation of active hydroxyl groups by the decomposition of water at high temperature. but substantially the same condition can also be realized by previously mixing water in the fuel.

According to the present invention there is provided an apparatus of simple construction in which fuel and water can be uniformly mixed, for use in the combustion method of the invention. In the apparatus of the invention, a water injection tube extends into a flow of fuel passing in a fuel supply tube leading to a fuel injection device. The water injection tube is provided with water injection apertures the opening area ratio of which progressively reduces from the upper portion to the lower portion of the tube. With such construction of the water injection tube, since the opening area ratio of the injection apertures progressively decreases towards the lower portion of the tube where the flow rate of water is low and the static pressure thereof is large, water can be injected uniformly into the fuel from all apertures, and the water and fuel can be mixed uniformly although they differ in specific gravity. Further. according to the apparatus of the invention the water can be mixed in the fuel in the optimum particle size.

Particularly important features of the combustion method of the invention are the following:

I. Combustion air is supplied concentrically to a primary combustion zone to elevate the temperature of said zone locally and reduce the length of the combustion flames, and thereby to achieve complete combustion of fuel particles in a short period of time.

2. As means for dividing the fuel particles finely, aqueous vapor is injected into the spray cone of fuel particles concurrently with the supply of the combustion air as described above or water is previously mixed in the fuel, and the carbon eliminating effect and the oxides of nitrogen reducing effect of the active hydroxyl groups formed in the high temperature combustion zone are utilized.

3. In order to restrict the high temperature combustion zone to the primary combustionzone, all of the 4 combustion air inlet ports at an intermediate combustion zone are closed.

The effects of the features will be described hereunder: The generation of smoke is reduced or prevented by increasing the combustion rate by atomizing the fuel into as small a particle size as possible, and by burning out the fuel particles before they are cooled by the cool air from a dilution zone. Further, the oxygen partial pressure decreases and the rate of the reaction forming oxides of nitrogen is slowed down as the length of the hydrocarbon flames becomes shorter and the time for the oxidation reaction of nitrogen in air to oxides of nitrogen becomes shorter. I

The water added to the fuel or the aqueous vapor injected into the high temperature combustion zone as fuel atomizing fluid lowers the combustion temperature at the zone but also further divides the fuel particles in the vicinity of the zone by the energy which is released upon evaporation. Furthermore, the water decomposes at high temperature as shown below and generates active hydroxyl groups OH.

2H,0 -H, 20H

These hydroxyl groups OH are highly reactive and react with the carbon particles formed in an enriched state of the mixture as shown below, thereby reducing the carbon concentration, as is well known:

C 20H 'CO2 H2 Thus, owing to the presence of water in the high temperature combustion zone, the fuel particles are further finely divided. which brings about the flame shortening effect, and consequently the amount of smoke is decreased. Further, the addition of water is effective for reducing the production of oxides of nitrogen and smoke because the time in which oxides of nitrogen are allowed to form is short, the lowered combustion tern perature suppresses the formation of oxides of nitrogen and the carbon formed is burned out by the aforesaid effect of the hydroxyl groups. However, it will be readily understood from the foregoing description that the presence of water in the intermediate combustion zone or dilution zone lowers the gas temperature and rather increases the smoke level.

BriefSummary of the Drawings The invention will be further illustrated with reference to the accompanying drawings showing, by way of example, embodiments of the invention, in which:

FIG. 1 is a graph showing the effect of the combustion method according to the present invention;

FIG. 2 is a view showing briefly one form of combustion chamber employing the combustion method of the invention;

FIG. 3 is a sectional view showing one form of wateradding device used in the combustion apparatus according'to the invention;

FIG. 4 is a sectional view taken along the line IV-IV of FIG. 3; and

FIG. 5 is a diagram showing an example of fuel supply system comprising the water-adding device of FIG. 3.

Detailed Description of the Presently Preferred Embodiment The effect of the invention in reducing the smoke level and the production of oxides of nitrogen is illustrated more practically in FIG. 1. FIG. 1 shows the measured B.S.N. and oxides of nitrogen content of the exhaust gas from a combustion chamber in' accordance with the invention, which was used with a 11 MW gas turbine and operated at full load with heavy oil fuels A and B with water mixed therein. It will be seen that, while the effect of the combustion chamber is variable depending upon the type of fuel used, the effect appears from a mixing ratio of water to fuel of about 2% and is highest at a mixing ratio of about 8l0%. In the experiment, the B.S.N. value of the smoke decreased about 1 unit and the smoke became substantially invisible when water was added to the fuel at a ratio of about 6%. In this case, the concentration of oxides of nitrogen also decreased materially to the order of 30 PPM.

An important feature of the invention is that, by mixing a very small amount (68%) of water with a fuel by a simple method, it is possible to reduce the level of smoke and oxides of nitrogen and the toxic components in the exhaust gases can be controlled by adjusting the amount of water added. In comparison, in the conventional Japanese method of reducing production of oxides of nitrogen by the injection of aqueous vapor, the injection of aqueous vapor at an inadequate position rather results in an increase in the smoke level and an amount of aqueous vapor equal to or more than the amount of fuel is required to be injected. In adding water, it is more effective to mix previously the necessary amount of water in the fuel rather than to inject water separately, because the water reaches the high temperature combustion zone without disturbing the particle size distribution of the fuel particles in the spray cone.

A combustion chamber which can be used in the combustion method of the invention is illustrated in FIG. 2. In FIG. 2, reference numeral 1 designates a fuel injection valve, 2 a swirler valve, 3 combustion air inlet ports and 4 cooling air inlet ports. Reference character a designates a primary combustion zone, b an intermediate combustion zone and c a dilution zone. The swirler valve 2 has rotary vanes (suitably inclined to the axis of the combustor at an angle of 3035) arranged around the fuel injection valve 1 and air, preferably from to 14% of the total, is admitted to the combustion chamber via the swirler valve. The combustion air passage ports 3 provided in the wall of the combustion chamber at the primary combustion zone are respectively provided with combustion air guide tubes, the total opening area of the combustion air passage ports in the primary combustion zone being 25-40% of the total opening area of all air passage ports of the combustion chamber. About 25 to 30% of the total air supplied enters the primary combustion zone a via inlet ports 3. The primary combustion zone a constitutes a local high temperature zone, into which fuel with water mixed therein is injected. About 25 to 30% of the total air supplied enters the combustion chamber via inlet ports 4 in the dilution zone 0. The remaining 30 to 35% of the total air supplied flows past and cools the outer periphery of the combustion chamber.

One form of water-adding device according to the invention, by which water is mixed in the fuel, will be described with reference to FIGS. 3, 4 and 5. As shown in FIGS. 3 and 4, a portion of a fuel supply tube 11 leading to the fuel injection valve is enlarged to form a fuelwater mixing chamber or barrel and a flange I3 is formed at the top end of a branch tube extending upwards from the fuel-water mixing chamber as shown. A water injection tube 14 provided with water-injection apertures 5 extends into the fuel-water mixing chamber substantially perpendicularly to the flow of fuel. The tube 14 is removably secured to the flange 13 by mean's of bolts. The water-injection tube 14 is removed from the mixing chamber when the adjustment of the water injection apertures becomes necessary or when the addition of water is not desired, and in the latter case the top opening of the flanged tube is closed with a blind cover. The apertures 5 are formed through the wall of that portion of the water injection tube 14 which is lo cated within the fuel-water mixing barrel in three vertical rows in three directions as shown in FIG. 4 and each of said apertures 5 has a diameter of l-2mm. One of these three vertical rows of apertures is directed to inject water in countercurrent with the fuel flow. Where the flow rate of fuel is low, some of these apertures are selectively closed to obtain the desired fuel-water mixing ratio. The totalopening area of the water injection apertures is made equal to or less than the transverse cross-sectional area of the water injection tube and the amount of water injected is adjusted by adjusting the number of apertures used. In the event that the diamc ter of the water injection tube 14 is larger than the di ameter ofthe water injection apertures 5 and the pitch of the apertures, the frictional loss within the water injection tube is small and the velocity of water in the tube progressively decreases downwards since the water flows out of the tube through the apertures at each pitch and becomes zero at the lower end of the tube. Since the water restores the static pressure towards the lower end of the water injection tube, an increasing amount of water is discharged from the tube from the lower apertures. Due to the specific gravity difference between fuel and water, the water particles tend to settle on the bottom of the fuel-water mixing barrel. Therefore, in order to obtain uniform distribution of the water particles in the fuel, the diameter of the water injection apertures is progressively reduced from the upper to lower portion of the water injection tube. From a practical point of view, it is preferred, for realizing approximately uniform distribution of water particles, to change linearly the diameter of the aperture from 2mm. at the highest apertures to 1mm. at the lowest apertures. According to a combustion experiment conducted with such a water injection device, the fuel burns with highly stable flames, without fluctuation of the combustibility of the fuel with the passage of time, and the water particles are distributed uniformly in the fuel and move constantly while continuing random motions.

An effective fuel supply system comprising the wateradding device described with reference to FIGS. 3 and 4 is illustrated in FIG. 5. According to this system, the water-adding device can be incorporated simply in the fuel supply systems of a conventional combustion chamber. A fuel supply tube 21 leading from the outlet of a fuel pump (not shown) is closed by a blind flange 22 and the fuel-water mixing device 12 described above is provided in a branch tube 23 which diverges from the fuel supply tube 21. Water is supplied from a water injection pump P through a manually operated valve 24 to the water injection tube 14 of the mixing device 12. A water pump proportional to fuel flow may be used for the water pump P to supply water at a rate proportional to the flow rate of fuel. A fuel-water mixture supply tube 25 leading to a fuel injection nozzle is slightly inclined over a length of about IO meters to prevent the separation of fuel and water and said portion of the mixture supply tube should be free of pockets and concaves. A drain valve is preferably provided at the lowest portion of the system for draining the 7 water from the system when the combustion chamber is out of operation. Reference character A designates combustion chambers.

At the start of combustion, fuel only is supplied. Water is added to the fuel in a controlled amount at the point when smoke and oxides of nitrogen start to be discharged in the exhaust gas as the combustion load increases. By the addition of 6-87! of water. the B.S.N. value of the smoke can be reduced by 1 unit and the concentration of oxides of nitrogen to a value of the order of 30 PPM from 75 PPM. The amount of water to be added is subjected to limitation. The addition of an excessively large amount of water will tend to result in an increasing amount of smoke. although the amount of oxides of nitrogen decreases. A suitable amount of water to be added is up to about 10% of the amount of fuel, which is only l/lO-l/ of that used in the conventional method for reducing formation of oxides of nitrogen by the injection of aqueous vapor. This is one of the advantageous features of the present invention.

Although the fuel-water mixing device 12 is provided on the discharge side of the fuel pump in the fuel supply system shown in FIG. 5, it may of course be provided on the suction side of the pump. In this case. a circulating pump needs to be provided for continuously stirring the fuel in the fuel tank because otherwise the fuel would be returned to the fuel tank by the action of a pressure regulating valve provided on the discharge side of the fuel pump and the water mixed in the fuel would settle at the bottom of said tank.

I claim:

1. Apparatus for burning fuel, comprising:

a. an elongated combustion chamber; including means for mixing hydrocarbon and water, comprising b. means for injecting a hydrocarbon fuel mixed with 5 to 10 percent water into the combustion chamber as a spray at one end thereof.;

c. means for admitting air to the combustion chamber concentrically with and from upstream of the fuel/water spray. for supporting combustion thereof;

d. means for admitting additional'combustion supporting air to the combustion chamber from downstream of the fuel/water spray, for travel radially inwardly and upstream to support combustion;

e. means for admitting air to the combustion chamber downstream from where the fuel/water mixture is being combusted, for cooling and diluting the hot gases generated during combustion;

f. the combustion chamber including wall means defining an intermediate zone of the combustion chamber. between where. during combustion, the gas stream temperature is'below l500, but combustion is still occurring, and where the cooling and diluting air is admitted by means (e), said wall means foreclosing the admission of air to the combustion chamber in said zone. for decreasing the concentration of the oxides of nitrogen in the exhaust gas stream. and

g. outlet means for exhausting the gas stream from the combustion chamber downstream from where air is admitted by means (e).

a fuel supply tube incorporating an enlarged mixing chamber;

a water injector tube place crosswise of the fuel supply tube within the mixing chamber. and including a plurality of openings sized and spaced along the length of the water injector tube to provide uniformity of amount of injection of water from each of openings. the water injector tube including means for supplying water through one end thereof;

the fuel supply tube, where the mixing chamber is incorporated. extending horizontally;

the water injector tube extending vertically within the mixing chamber;

the supplying means supplying the water injector tube through the upper end thereof; and

the cross-sectional areas of the openings of the water injector tube progressively decreasing proceeding down the water injector tube.

A 2. The apparatus of claim 1 wherein:

the ratio of cross-sectional area of the largest to the smallest of the openings is 2:1.

3. The apparatus of claim 2 wherein:

the diameter of largest of the openings is 2mm.

4. The apparatus of claim 1 wherein the openings are arranged in three vertical rows, including one row directed upstream of the fuel supply tube and two rows oppositely laterally outwardly directed with respect to the fuel supply tube.

5. The apparatus of claim 1, wherein:

the combustion chamber is surrounded by a jacket over the length thereof,

the air admitted to the combustion chamber by means (d) and (e) passes from within the jacket, through means defining respective ports, into the combustion chamber; and wherein the means (c), (d) and (e) are sized relative to one another to provide that 1. about 10-14 percent of the air put through the apparatus in use is admitted by means (c); 2. about 25-30 percent of the air put through the apparatus in use is admitted by means ((1); 3. about 25-30 percent of the air put through the apparatus in use is admitted by means (e); and 4. about 3035 percent, which is the remainder, passes along the outside of the combustion chamber, within the jacket, without entering the combustion chamber, for cooling the outside of the combustion chamber.

Claims

1. APPARATUS FOR BURNING FUEL, COMPRISING: A. AN ELONGATED COMBUSTION CHAMBER; INCLUDING MENS FOR MIXING HYDROCARBON AND WATER, COMPRISING B. MEANS FOR INJECTING A HYDROCARBON FUEL MIXED WITH 5 TO 10 PERCENT WATER INTO THE COMBUSTION CHAMBER AS A SPRAY AT ONE END THEREOF; C. MEANS FOR ADMITTING AIR TO THE COMBUSTION CHAMBER CONCENTRICALLY WITH AND FROM UPSTREAM OF THE FUEL/WATER SPRAY, FOR SUPPORTING COMBUSTION THEREOF; D. MEANS FOR ADMITTING ADDITIONAL COMBUSTION SUPPORTING AIR TO THE COMBUSTION CHAMBER FROM DOWNSTREAM OF THE FUEL/WATER SPRAY, FOR TRAVEL RADIALLY INWARDLY AND UPSTREAM TO SUPPORT COMBUSTION; E. MEANS FOR ADMITTING AIR TO THE COMBUSTION CHAMBER DOWNSTREAM FROM WHERE THE FUEL/WATER MIXTURE IF BEING COMBUSTED, FOR COOLING AND DILUTING THE HOT GASES GENERATED DURING COMBUSTION; F. THE COMBUSTION CHAMBER INCLUDING WALL MEANS DEFINING AN INTERMEDIATE ZONE OF THE COMBUSTION CHAMBER, BETWEEN WHERE, DURING COMBUSTION, THE GAS STRE M

2. about 25-30 percent of the air put through the apparatus in use is admitted by means (d);

2. The apparatus of claim 1 wherein: the ratio of cross-sectional area of the largest to the smallest of the openings is 2:1.

3. The apparatus of claim 2 wherein: the diameter of largest of the openings is 2mm.

3. about 25-30 percent of the air put through the apparatus in use is admitted by means (e); and

4. about 30-35 percent, which is the remainder, passes along the outside of the combustion chamber, within the jacket, without entering the combustion chamber, for cooling the outside of the combustion chamber.

5. The apparatus of claim 1, wherein: the combustion chamber is surrounded by a jacket over the length thereof, the air admitted to the combustion chamber by means (d) and (e) passes from within the jacket, through means defining respective ports, into the combustion chamber; and wherein the means (c), (d) and (e) are sized relative to one another to provide that