PROCESS FOR THE PARTIAL COMBUSTION OF CELLULOSE SPENT LIQUOR II

Abstract: The present invention relates to a process for partial combustion of cellulose spent liquors using a burner equipped with a liquor lance and an atomizing nozzle, which burner is connected to a reactor operating at pressures in the range of 1.1 to 150 bars while supplying an oxygen containing gas, whereby the weight ratio between the oxygen containing gas supplied through the burner and the spent liquor solids supplied through the burner is less than 2:1, and that at least half of the non-fuel bound oxygen required for the partial combustion is discharged into the reactor through the atomizing nozzle.

Description: PROCESS FOR THE PARTIAL COMBUSTION OF CELLULOSE SPENT LIQUOR II DESCRIPTION Technical field The present invention relates to a process for partial combus- tion of cellulose spent liquors from- the cellulose industry us¬ ing a burner connected to a reactor, whi le supplying an oxygen containing gas as the oxidant. The burner comprises a liquor lance equipped with a nozzle at its downstream end which sup¬ plies liquor and the greater part of the non-fuel related oxy- gen required for the partial combustion, wherein or in which proximity the oxygen containing gas is brought into contact with the spent liquor which then disintegrates into a divergent spray . The object of the present invention is to faci litate partial combustion of cellulose spent liquor through use of a burner creating a stable, se If-igni t i ng flame at Low ai r/fuel ratios and elevated pressures. Background of the invention The cellulose industry generates spent liquors differing in composition according to the de I i gm f i cat i on process used. Within the sulphate pulping industry, spent liquor, commonly referred to as black liquor, contains valuable chemicals and energy in the form of combustible carbonaceous compounds. At the present time these chemicals and energy are normally recov¬ ered in a recovery boi ler in which the black liquor is com- p lete ly burned . Partial combustion of black liquor in a gasification reactor as in the present invention generates a combustible gas comprising H-,, CO, CO-, and droplets of molten inorganic chemicals. In conjunction with pulp bleaching, a di luted liquor comprising organic matter and sodium salts is obtained. Mechanical and sem -che i ca I pulping processes also generate di luted liquors of different compositions. These as well as other waste and spent liquors generated in the cellulose industry can, after concentration, be used as a feedstock in the process of the present invention. Alth ' ough the foLLowing description describes the present inven¬ tion as it applies to black Liquor it is not restricted only to this particular liquor in its application. The mechanisms related to partial combustion of black liquor are fairly well understood and are applied inter alia in the Lower part of the soda recovery boiler. The difference between the present burner and a Liquor burner in a soda recovery boi ler is, however, great inter alia due to the Low degree of Liquor atomization in recovery boiler burners and the absence of a well-defined liquor flame. Another important difference between a recovery boi ler burner and the burner of the present invention is that the present burner is primarily intended for gasification at elevated pressures. A major difference between the burner of the present invention and conventional oi l burners is that a stable flame has to be formed with the use of a considerably lower amount of air or oxygen carrier. As the exemplification below show, black Liquor as a fuel is characterized by a relatively low calorific value and high wa¬ ter and ash contents. Calorific value of the dry substance 13 GJ/ton dry substance (DS) Elementary composition C 29 H 34°20 Na 9 S 2 Dry solids content 65% Viscosity at 100°C 100 cSt. The presence of sodium compounds in the black Liquor and its inherently high oxygen content make it a very reactive fuel, which means, provided an adequate burner s at hand that the carbon conversion already in the flame zone becomes high, in spite of the fact that the combustion is substoichiometric. The degree of atomization of the liquor is of great importance for obtaining a stable black liquor flame, the extension of the flame and high carbon conversion. The Theological properties of the black liquor are of sign ficant importance to the degree of atomization which can be achieved in a given nozzle. The vis¬ cosity of the black Liquor can be influenced by e.g. heating and/or the addition of additives and normally the black Liquor is being heated to above 100 C for use in the present inven¬ ion. The viscosity of the black liquor at the moment of atomi¬ zation should preferably be below 200 cSt. Atomization of the black liquor can be further enhanced by flashing the liquor into the reactor in which case the Liquor is preheated to a temperature above its boi ling point at the operating pressure of the reactor. Several types of atomizing nozzles are avai lable but only a few varieties are suitable for atomizing cellulose spent liquors, such as black liquor, in the present invention. "Twin-fluid" nozzles are most suitable for use in the present burner. A common feature of "twin-fluid" nozzles is that a rel¬ atively high gas flow rate is necessary for the supply of en¬ ergy for the atomization. Another important feature of these nozzles is that the resulting size of the droplets decrease with increasing density of the atomizing gas. Depending on how the two fluid phases are brought together several mechanisms for forming droplets, such as shearing between Ligaments, com¬ bination and formation of spheres of liquor droplets and high turbulence decompos tion of the liquor spray can be antici¬ pated. Description of the present invention The present invention describes a process for efficient substoichiometric combustion of cellulose spent Liquors, using a burner connected to a reactor, whi le supplying an oxygen con¬ taining gas, which invention is characterized in that the weight ratio between the oxygen containing gas supplied through the burner and the spent liquor supplied through the burner is Lower than 2:1, and that at Least half of the oxygen which is required for the partial combustion of the spent Liquor is sup¬ plied through the burner to the reactor as an oxygen containing gas, said gas being discharged into the reactor through the nozzle. Efficient atomization of the spent liquor is particularly im¬ portant and is achieved in the burner of the present invention by direct contact between the spent liquor and the oxygen con- taining gas at elevated pressure in or directly adjacent to a nozzle designed specifically for the purpose. The spent liquor, which should be preheated to lower the viscosity before being fed to the burner, is supplied to the reactor through the Liq¬ uor Lance. At the downstream end of the Liquor Lance the Liquor is brought into contact with an atomizing gas whereby the Liq¬ uor flow velocity rapidly increases resulting in the formation of a divergent spray of atomized spent Liquor. As is mentioned above the spent liquor is fuel with unusual properties regarding reactivity and ash and water contents. The spent Liquor solids contains a high level of bound oxygen which means that the oxygen which can be supplied through the combus¬ tion air is relatively small, particularly for partial combus- ti on . The oxygen supplied to the reactor corresponds to a stoi ch iomet ri c ratio of between 0.3 and 0.6 relative to com¬ plete combustion of the spent Liquor with a preferred ratio of between 0.35 and 0.5. The air, oxygen-enriched air or oxygen to be supplied to the reactor is low in relation to the spent liq¬ uor supplied and the weight ratio is lower than 2:1 when oper¬ ating with air as oxidant and lower than 0.4:1 when operating with oxygen as oxidant. The greater part, and preferably more than 80% of the non-fuel bound oxygen which is requi red for partial combustion of the spent Liquor is supplied to the reac¬ tor as an oxygen containing gas which gas is discharged to- gether with the liquor through the nozzle. The flow velocity of the oxygen containing gas in the nozzle should be between 40 m/s and 350 m/s. The nozzle can be designed with a ci rcular gap or a circular opening for discharge of spent liquor, wherein it is contacted with the high velocity atomizing oxygen containing gas and dis¬ integrates into small droplets forming of a divergent spray. In an alternative design of the nozzle the spent liquor is dis¬ charged together with the atomizing gas through three or more symmet r i ca L ly arranged openings. The attached drawing shows two different "twin-fluid" nozzles, whereby FIG. 1 shows an embodiment of a "twin-fluid" nozzle in an axial cross-section; FIG. 2 shows a front view of the nozzle according to FIG. 1 seen along the Line 111-111 of FIG. 2; FIG. 3 shows a second embodiment of a "twin-fluid" nozzle in axial cross-section; and FIG. 4 shows a front view of the nozzle according to FIG. 3 seen along the Line V-V of FIG. 3. FIGS. 1 and 2 show a twin-fluid nozzle where the liquor and gas are mixed and then forced under high pressure through several symmetri ca I Ly arranged ci rcular openings 3. These openings are the ends of so called Y- et' atomizing nozzles comprised of two tubes 4 and 5, the former in contact with an outer tube 6 for the purpose of adding black liquor and the Later in contact with an inner concentric annular tube 7 for the purpose of add- ing atomizing gas. The openings 3 diverge, producing divergent atomized jets from the lower part 2 of the Liquor lance 1. A hood 9 fitted to the body 10 of the liquor Lance, holds the Y- jet atomizer 8 in place. The body 10 encloses the concentric annular tubes 6 and 7. The black liquor is introduced into the liquor lance 1 through an inlet tube 20 and the air through an¬ other inlet tube 21. FIGS. 3 and 4 show an embodiment of the burner gun having three concentric annular tubes 11, 12, and 13. Air is fed through the outer and the inner tubes 11, and 13, whi le black Liquor is fed through the intermediate tube 12. The air is divided through the 18 symmetrically distributed holes 14, and 15 shown in the figures, while black Liquor is forced through an annular gap 16. The holes 14 are hereby obliquely directed in one direction and the holes 15 obliquely directed in the opposite direction. The black liquor is fed through the gap 16 and meets a lip 17 forcing it in an inward direction. Now in the form of a film the black Liquor is met by the air coming through the holes 16 and is atomized. This initial air-black Liquor mixture is met by additional air outside the lip 17, creating a diverging jet of finely dispersed black liquor. The black Liquor is added to the burner through an inlet tube 20 and the air through two in¬ let tubes 21. When designing burners great attention has to be paid to the weight ratio between the oxidant and fuel. Different fuels contain different amounts of chemically bound oxygen. Bitumenous coal usually contains between 4-10 % of bound oxygen. Fuel oi ls contain Less than 1 % of bound oxygen. Black liquor dry solids contains about 35 % by weight of bound oxygen calculated on dry matter. This affects the design of burners for combustion of black liquor since a considerably Lower amount of oxygen, air or oxygen enriched air shall be added to the reactor to obtain the desired Level of combustion. The air/fuel ratio (by weight) for some fuels at stoi ch i omet r i c combustion are exemplified below: Ant raci te Ai r / fue I 10-12:1 Ethyl alcohol- 9:1 Black I i quor 4-5:1 Diesel oi l/heavy oi l 13-15:1 The burner designed in the present invention creates a stable flame in a reactor which preferably operates at pressures in the range of 0.1 to 150 bars above ambient and at temperatures in the range of 700 to 1400°C. Depending on factors such as temperature and flow velocities in the burner and the composition of the liquor, the burner nozzle can in addition to thermal effects be subjected to oxidation and reactions with sulphur which may have a detrimental effect on the degree of atomization. The burner nozzle of the present invention should therefore preferably be cooled by a circulat- i ng I i quid. A preferred embodiment of the present invention is to use oxy¬ gen or oxygen enriched air as the oxidant. In such a preferred embodiment all or nearly all the the oxygen required for par- tial commbustion is supplied through the nozzle to support at¬ omization of the spent Liquor. Part of the oxygen containing gas may be added to the reactor through a pipe arranged coa ially around the Liquor lance or through one or several gates. To compensate for the low air/fuel ratios and to achieve reasonable gas velocities all the oxygen containing gas should be preheated to at Least 100°C, preferably to 300°C and it should further be given a vortex movement, which i.a., can be achieved by passing the gas through vortex blading arranged in the coaxial pipe. The radial flow rate of the oxygen containing gas is thereby markedly af¬ fected with a maintained axial flow rate. The main principle of a vortex burner is to recirculate a portion of gases through an internal reci rcu Lati on zone towards the Liquor lance. This in¬ ternal reci rcu Lation zone facilitates combustion and stabilizes the flame and the recirculated hot gases add energy for igni¬ tion of the liquor sray. The internal rec i rcu Lation zone also serves as a depot for heat and reactive gas components.

Claims: C LA IMS 1. Process for partial combustion of cellulose spent liquors using a burner equipped with a liquor lance and an atomizing nozzle, which burner is connected to a reactor operating at pressures in the range of 1.1 to 150 bars whi le supplying an oxygen containing gas, characterized in that the weight ratio between the oxygen containing gas supplied through the burner and the spent liquor supplied through the burner is Less than 2:1, and that at Least half of the non-fuel bound oxygen re¬ quired for the partial combustion is discharged into the reac¬ tor through the atomizing nozzle. 2. Process according to claim 1, characterized in that the weight ratio between the oxygen supplied with the oxygen con- tain ng gas and the spent Liquor is Less than 0.4:1. 3. Process according to claim 1 , characterized in that more than 80% of the oxygen containing gas requi red for the partial combustion is supplied through the nozzle. 4. Process according to claim 1, characterized in that the oxy¬ gen containing gas is substantially oxygen or oxygen enriched air. 5. Process according to one or more of claims 1-4, charactei— ized in that the flow velocity of the gas and Liquor discharged from the nozzle is between 40 and 350 m/s. 6. Process according to one or more of claims 1 to 5, charac- terized in that the spent liquor and the oxygen containing gas are discharged from the nozzle through at least three symmetri* cally arranged openings. 7. Process according to one or more of clai s 1 to 6, charac¬ terized in that the spent liquor is discharged from the nozzle through a ci rcular gap or a ci rcular opening. 8. Process according to one or more of claims 1 to 7, charac¬ terized in that the cellulose spent liquor discharged into the reactor has a temperature above its boiling point at the pre¬ vailing reactor pressure.

Inventors: STIGSSON LARS, STIGSSON, LARS
Applicants: CHEMREC AB, CHEMREC AKTIEBOLAG
Application year: 1991
Main classification: D21C11
IPC codes: C10J3, D21C11, F23D11