SE535117C2 - Gasification of sulphite thick liquor - Google Patents

Abstract

23 SUMMARY Method for recovering chemicals and energy from sulphite spent liquor, said sulphitespent liquor being obtained When producing paper pulp by chemical delignification off1brous raw material using a sulphite pulping process, said sulphite spent liquorcomprising organic and inorganic compounds; the method comprising processing ofsaid organic and inorganic compounds at a temperature above 800 °C thereby producingpartly at least one phase of a liquid material and partly at least one phase of a gaseousmaterial, Wherein said processing is carried out by gasification of said sulphite spent liquor in a reactor comprising an oxidizing medium at sub-stoichiometric conditions.

Description

The present invention relates to a method for recovering chemicals and energy fromsulphite spent liquor, said sulphite spent liquor being obtained When producing paperpulp by chemical delignif1cation of f1brous raW material using a sulphite pulpingprocess, said sulphite spent liquor comprising organic and inorganic compounds; themethod comprising processing of said organic and inorganic compounds at atemperature above 800 °C thereby producing partly at least one phase of a liquid material and partly at least one phase of a gaseous material.

BACKGROUND INFORMATION The sulphite pulping process is a chemical pulping process of Wood chips Whenproducing pulp. Sulphite pulping currently accounts for less than 10% of the World°spulp production, although it has historically been a dominating process for pulpingbefore important developments in the Kraft (also terrned sulphate) pulping processmade this process more popular. Different sulphite pulping processes exist, e.g. acidic,neutral and alkaline sulphite processes.

In sulphite pulping processes using mixtures of sulphur dioxide, sulphurous acid and/orits alkali salts, the lignins in the Wood chips are made Water-soluble through theformation of sulphonate functionalities and cleavage of bonds in the lignin structures.Typical counter-ions to the sulphurous acid and/or its alkali salts include Nal, NHÄH,Mgzl, Kl and Cazl.

A main difference between the different sulphite pulping processes are the pH of thecooking liquor Which means that delignif1cation is carried out at loW, neutral or high pHin the digester. Alkaline sulphite pulping may also be performed in the presence of sulphide, so called sulphide-sulphite pulping.

The sulphite pulping process may have certain advantages compared to Kraft pulping,such as higher yield, brighter and more easily bleached pulps and relatively easilyrefined pulps. The sulphite process may produce specialty cellulose for production ofcellulose derivatives in addition to pulp for papermaking. Certain disadvantagescompared to Kraft pulping also exist, such as Weaker pulp and diff1culties in pulping certain species of Wood and more complicated chemical recovery.

The chemical recovery process of the pulping chemicals is dependent on the alkalicounter-ion used but is generally more complex than the Kraft process recovery ofpulping chemicals. The initial steps in the sulphite chemical recovery process areseparation of spent cooking liquors from the pulp/cellulose and subsequentconcentration of the spent liquor by evaporation of Water, Which gives a liquor denotedsulphite thick liquor in this text. The sulphite thick liquor may thereafter, for mostcounter ions, be bumed in recovery boilers for energy recovery and the pulping chemicals are recovered to varying extent.

The recovery boilers used for recovering chemicals and energy from sodium-basedsulphite thick liquors are very similar to those used for recovery of black liquor from theKraft pulping, however buming sulphite thick liquor in such boilers is associated With anumber of diff1culties as compared to buming Kraft black liquor, Which is further discussed below.

The flue gases produced When buming the sulphite thick liquor are more corrosive,Which limits the efficiency of liquor energy recovery and causes elevated maintenance costs.

The losses of pulping chemicals, both sodium and sulphur, mostly as fly ash, aresignificantly higher When buming sodium-based sulphite thick liquor as compared to buming Kraft liquor, Which can lead to increased chemical make-up costs in the mill.

Buming sodium-based sulphite thick liquor to recover the cooking chemicals andenergy, is a high temperature process Where the salt melt collected in the bottom of theboiler needs to be kept at high temperatures (around 1000 °C) due to the high melting point of the sulphide/carbonate mixture formed.

Reduction of the sulphur components in recovery of spent liquors from the Kraftprocess can norrnally reach 95%. When buming sodium-based sulphite thick liquor, thereduction efficiency of sulphur species in the sulphite liquor is relatively low. Typically80-85% of the recovered sulphur is reduced to sulphide that can be converted to activecooking chemicals in subsequent cooking liquor preparation process. The non-reducedsulphur gives disadvantages in the form of dead load in the liquor cycle and a tendency to cause fouling in the process equipment of the liquor cycle.

The non-reduced sulphur is, at least partly, present as polysulphide in the salt melt,which is oxidized to tiosulphate in the green liquor formed by the dissolved saltscoming from the recovery boiler. Tiosulphate decreases pulping efficiency if present inthe cooking liquor. To avoid such effects, wet oxidation is used to convert tiosulphate tosulphate. Hence, a large amount of non-active sulphur is present in the liquor cycle,causing a lower efficiency and potential problems with scaling. In addition, thiosulphate is known to cause corrosion problems in process equipment.

Sulphite thick liquors are known to have a lower reactivity in recovery boilers comparedto spent Kraft cooking liquors, which leads to lower capacity when recovery boilers areoperated on sulphite thick liquor. A key reason why the sulphite liquor behavedifferently than the Kraft liquor is norrnally meant to be caused by less swellingbehavior of the sulphite liquor droplets during heat up before combustion, which leads to higher resistance to mass and energy transfer.

Hence the complex and relatively ineff1cient chemical and energy recovery from spentsulphite liquors is one reason why the Kraft process has become the dominating pulping process.

Furthermore, persons skilled in the art may have a prejudice against recovery bygasification of sulphite liquors based on earlier experiences. Tests have for instancebeen carried out already around 1960 by the Swedish pulp and paper company Billerudand two separate pathways were further explored. A non-slagging (low temperature)gasification process was developed and built in a few facilities (the “SCA-Billerudprocess”). The slagging (high temperature) pathway was tested in a second facility atthe Billerud Mill. Tests were ended after one year due to a combination of factors. Theprocess did not reach the low smelt sulphide content requirements set by the remainingrecovery processes available at that time. Further, problems with build-up of smeltlayers on the reactor walls were present and the wear on the reactor lining was verysevere with the ceramic materials available at that time. Also the “SCA-Billerud process” was subsequently abandoned due to poor performance.

In document US 2,285,876, a process for recovery of waste sulphite liquors is disclosed.Said liquor is sprayed into a so called Tomlinson recovery fumace chamber and bumt ata fumace temperature below the fusion temperature of the non-combustible constituents of said liquor.

Document DE 1,5 17,216 describes a process for pyrolysis of cellulose spent liquors,especially of sodium based sulphite spent liquors. Thickend spent liquor is divided intovery fine particles where the major part of the particles should not exceed 200 um, saidparticles being sprayed into a hot oxygen containing gas stream and being pyrolized.The document teaches that the pyrolysis temperature should not exceed 800 °C in orderto avoid sulphides in the solid residue that is used to make green liquor and,consequently, in the cooking liquor. Pyrolysis at as low temperatures as below 800°Cwill however lead to unconverted char in the solid residue from the gasif1cation processand necessitates a second gasification step that is performed in a fluidized bed. The hot gas into which the liquor is added comes from combustion of fuels, e.g. oil.

Document, US 3,3 17,292, describes a method of treating waste substances, such assulphite waste liquor, black liquor etc, to derive hydrogen and other gases therefrom aswell as a hydrogen-containing product. The method comprises precipitating lignin-derived components, reacting the precipitate with steam at several hundred degrees in areaction atmosphere substantially void of free oxygen to favour production of carbon monoxide and hydrogen gases.

Another document SE 526435 discloses a method for recovery of chemicals fromalkaline sulphite pulping processes. Said method comprises a gasification step and thedocument teaches that said gasif1cation shall be carried out at a temperature ofpreferably 700-75 0°C in order to keep the temperature below the melting point of the salts in the solid phase.

Still another document, CA 6l9,686, disclo ses a method for pyrolysis of waste liquors from pulp manufacturing, preferably on sodium base, by using a fluidized bed.

Taking the above into consideration there is a need to improve the chemical recoveryprocess for sulphite pulping and to increase the efficiency with regard to energy and/or chemical recovery.

SUMMARY OF THE INVENTIONIt is an object of the present invention to overcome or at least minimize at least one of the drawbacks and disadvantages of the above described chemical recovery processes for sulphite pulping. This can be obtained by a method according to claim l.

Thanks to the invention a more efficient chemical recovery is obtained. Cold gasefficiency obtainable in a commercial scale gasifier is estimated to be 65-75%, leadingto high yields of motor fuels produced from the synthesis gas, if this usage of the synthesis gas is selected.

Green liquor sulphidity is significantly loWer than for a recovery boiler due to the factthat most of the sulphur may be contained in the raW synthesis gas as hydrogensulphide. This sulphur may be retumed to cooking liquor preparation in a concentratedgas stream from an acid gas removal unit treating gas from the gasifier, Which perrnits aless complex cooking liquor preparation process. The load on the part of the cookingliquor preparation that converts sulphide to sulphur dioxide and sulphurous acid is decreased due to the lower sulphur content in the green liquor According to one aspect of the invention, the amount of unbumt charcoal in said greenliquor is loWer than 5%, preferably loWer than l% and more preferred loWer than 0,2%,of the carbon in the sulfite thick liquor, i.e. carbon conversion may be very high, resulting in a good quality green liquor.

According to another aspect of the invention, the green liquor may to an extent of atleast 90%, preferably at least 95% and more preferred at least 99%, be free from non-reduced sulphur. This means that the green liquor produced may have close to 100%sulphur reduction efficiency. A high sulphur reduction efficiency decreases the totalamount of sulphur that needs to be circulated by decreasing the so-called dead-load (i.e.inactive sulphur species such as sulphate).

All these advantages taken together leading to a more efficient and cost effectivechemical recovery process With regard to cooking chemicals as Well as energy. Saidchemical recovery process may no longer be a drawback for sulphite pulping processes compared to Kraft (sulphate) pulping processes.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing aspects and many of the attendant advantages of this inVention willbecome more readily appreciated as the same become better understood by reference tothe following detailed description, when taken in conjunction with the accompanying drawings, wherein: Fig. l shows a flow scheme of a typical chemical recovery cycle for acidicsodium sulphite pulping, Fig. 2 shows a flow scheme of a modified and more efficient cycle includinggasification of sulphite thick liquor according to the inVention, and Fig. 3 shows a general process scheme on a gasification plant of the entrained- flow, high temperature reactor type DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTSThe following detailed description, and the examples contained therein, are provided forthe purpose of describing and illustrating certain embodiments of the inVention only and are not intended to limit the scope of the inVention in any way.

In Fig. 1 is a flow scheme of a typical chemical recovery cycle for sodium-basedsulphite pulping shown. Since this is common knowledge for a person skilled in the art,said chemical recovery cycle will be only briefly described here. Wood chips (A) arepumped into a digester where the delignification/pulping process (B) takes place in anappropriate sulphite cooking liquor at eleVated temperature thereby releasing cellulosef1bres (pulp). The pulp is separated from the thin sulphite spent liquor (D), which spentliquor is a mixture of spent cooking chemicals and wood residues (e. g. lignins). The rawpulp (C) is then ready but may often be fiJrther treated in bleaching units and maythereafter be exported either as wet or dry pulp.

The spent cooking liquor is cooled and stripped of free sulphur dioxide in an evaporator.The pre-evaporated and cooled liquor is ferrnented to reduce the sugar content of theliquor (not shown). After ferrnentation the liquor is stripped to recoVer the ethanolformed and evaporated (E) to a sulphite thick liquor (F) with about 60-70% dry solids.The liquor is then combusted in one or several Tomlinson-type recovery boilers (G) to form smelt, flue gases comprising ash (H) and heat for steam generation.

The smelt substantially comprises sodium carbonate and sodium sulf1de. Said smelt isdissolved in recirculated Water. The solution, so called green liquor (I), is clarif1ed orf1ltered to remove insoluble inorganic substance and any char from incompletecombustion in the recovery boilers. The green liquor is then stripped from its content ofsulphide by contacting it in countercurrent with carbon dioxide and sulphur dioxiderecovered from the Claus plant (this and the following are not shown explicitly in Fig. lbut is a part of the cooking liquor preparation). The resulting gas, a mixture of hydrogensulfide and carbon dioxide is led to the Claus plant where it is contacted with sulphurdioxide to form elemental sulphur. The elemental sulphur is combusted in a sulphurfumace to form sulphur dioxide which through an adsorption-desorption system isconcentrated and then led to the Claus plant. The flue gas from the recovery boilerwhich is rich in sulphur dioxide is contacted in a flue gas scrubber with the sodiumsulfite solution from the above mentioned sulfide stripper column to form a mixture ofsulf1te and bisulfate. Subsequently this mixture is contacted with sulphur dioxide fromthe sulphur dioxide absorber to form a bisulphate solution. A fresh cooking liquor (L)comprising sodium sulphite, bisulphate and/or bisulphate with free sulphur dioxide maythen be prepared in the cooking liquor preparation step (J). Make-up sodium andsulphur (K) may be added to the preparation (J). Said fresh cooking liquor (L) is nowready to be conveyed to the pulping process (B).

In Fig. 2, a flow scheme of the preferred embodiment according to the invention isshown where the recovery boiler has been replaced by one or several gasif1er(s) forgasification (M) of the spent liquor thereby forrning smelt and raw synthesis gas (N).Said smelt is dissolved in recirculated water or in weak wash liquor, thereby forminggreen liquor (I), in the same way as in the conventional chemical recovery cycle shownin Fig. l. Said raw synthesis gas (N) passes through a gas cleaning plant/process (O)resulting in cleaned synthesis gas (P) and a hydrogen sulphide rich gas (Q), saidhydrogen sulphide rich gas being fed to the cooking liquor preparation (J). Anotherstream comprising mainly carbon dioxide and essentially free from sulphur compounds may also be produced in the plant/process (O).

Adding the step of gasification of sulphite thick liquor to the chemical recovery processfor sulphite pulping may potentially give a much more efficient recovery process, bothwith regard to cooking chemicals and energy. It is understood that said gasif1cation stepmay either replace the recovery boiler or be included in the already existing chemical recovery process for sulphite pulping as a booster. In many mills the chemical recovery process is a bottle neck thereby limiting the pulp production which cannot be further increased.

Since the gasification process and gasif1cation plants themselves are well-documented,they will be described only briefly here and with reference to for instance WO86/ 073 96,WO 93/24704 and US 6,790,246, although they however relate to gasification of Kraftblack liquor.

Fig. 3 shows a general process scheme of a gasif1cation plant of the entrained-flow typefor gasification at slagging conditions (high temperature) in accordance with theinvention. Said plant being a part of the chemical recovery cycle for sulphite pulpingshown in Fig. 2. The plant corresponds to the parts denoted I, M, N, O, P and Q in Fig.2.

In Fig. 3 the gasification plant is shown in more detail and is to be described below. Thedescription is to be seen as a general description of a gasif1cation plant and shall beinterpreted as illustrative and not in a limiting sense. It is to be understood thatnumerous changes and modifications may be made to the below described plant, without departing from the scope of the invention, as defined in the appending claims.

Detail number l in Fig. 3 indicates a pressure vessel which comprises a ceramicallylined gasification reactor 2. The reactor is provided with an inlet 3 for sulphite thickliquor and an inlet 4 for oxygen or oxygen-containing gas and a bumer (not shown).There may also be an inlet for atomizing medium (not shown). The opening in thebottom of the reactor chamber is in the form of a chute 5, which opens directly into acompartment 38 above the surface 35 of the liquid in a green liquor liquid chamber 6which is situated below. Said compartment 38 is a part of a quench zone, which zone isan integrated portion of the reactor 2. One purpose of the quench zone is to cool the gasleaving the reactor to a temperature at which gas phase chemical reactions does not takeplace at a significant rate. Another purpose of the quench zone is to form green liquorfrom the smelt leaving the reactor.

A number of spray nozzles 7 for cooling liquid open out into the chute 5 and thecompartment 38. Green liquor which is produced is transported from the chamber 6through a conduit 8, via a pump 9 and a heat exchanger l0, to subsequent process stages for generating white liquor, or to another process stage in which green liquor is employed. A partial flow of the green liquor transported in conduit 8 may be retumed tothe green liquor liquid chamber 6 through a conduit 81 via a pump 91.

Raw synthesis gas from the first vessel is conveyed through a conduit 11 to a second pressure vessel 12 for gas treatment and sensible heat energy recovery.

This conduit 11 opens out in the pressure vessel 12 above the surface of a liquid in a washing chamber 13 at the bottom of the vessel.

The liquid in the washing liquid chamber of the second vessel may be conveyed,through a conduit 14 via a pump 15, to the first vessel in order to serve as diluting liquid or as a cooling liquid which is provided via the spray nozzles 7.

The pressure vessel 12 may comprise an indirect condenser of the countercurrentfalling-film condenser type 16 located above the chamber 13. Other types ofcondensers may be used without departing from the scope of the invention and sincemethods for gas washing and gas cooling are well known techniques it will not be described in detail here.

An outlet conduit 17 for cooled raw synthesis gas is located at the top of the secondpressure vessel 12. The outlet conduit 17 transports the cooled combustion gas to aninlet 31 of a plant 30 for further removal of sulphurous components and most of the C02(acid gas removal, AGR). The plant 30 comprises any gas separation technology foracid gas removal. A conduit 32 of the plant 30 may transport the purified and cooledsynthesis gas, now called cleaned synthesis gas, to any field of use of the synthesis gas,e. g. chemical production, fuel production, electricity generation and/or stean1/heat generation.

The gasification-based recovery process is now to be described. The gasifying reactoris fed with concentrated sulphite thick liquor, said sulphite thick liquor comprisingorganic and inorganic compounds, together with oxygen or an oxygen containing gasthat may be pre-heated to 50-400°C. The liquor is processed by gasification in thepresence of an oxidizing medium, e.g. oxygen or air, whereby heat is released by thechemical reactions taking place to give a temperature above 800 °C, preferably above900°C, more preferred above 950°C but below 1500°C, preferably below 1300°C, andat an absolute pressure of about 1.5 to about 150 bar, preferably about 10 to about 80bar, and most preferably from about 24 to about 40 bar in the reaction zone (a so called high pressure gasification). An atomizing support medium may be used. Saidgasification takes place at reducing conditions, i.e. sub-stoichiometric oxygenconditions, thereby producing a mixture of partly at least one phase of a liquid materialand partly at least one phase of a gaseous material.

The phase of gaseous material comprising raw synthesis gas, e. g. carbon monoxide,hydrogen, carbon dioxide, methane, hydrogen sulphide, and aqueous steam, and thephase of liquid material comprising inorganic smelt and ash, e. g. sodium sulphide,carbonate and hydroxide, are cooled in the quench cooler zone by spraying coolingliquor through a number of nozzles in order to achieve maximum contact with thegas/smelt mixture. The cooling liquid principally consists of water, some of which waterwill be eVaporated when it makes contact with the hot gas and the smelt at the reactortemperature. The gas temperature drops to approx. 100-230 C in the quench coolerzone. The smelt drops are dissolved in the remaining part of the cooling liquid and fallsinto the green liquor liquid chamber (the so called quench bath) where it dissolves toform green liquor. Altematively, the smelt drops fall down directly into the liquidchamber and only then dissolve in the green liquor which is already present in thislocation. The smelt drops are then possibly cooled by the evaporation of water in the green liquor bath.

The green liquor comes out from the bottom of the quench cooler of the first pressureVessel through a conduit and may be pumped through a heat exchanger, in which heatenergy is recoVered from the green liquor by cooling the latter. Altematively, greenliquor heat energy may be recoVered by other means. A screen may be used ahead of thepump to catch small particles. It is benef1cial that the amount of unburnt charcoal in thesmelt and in said green liquor is lower than 5%, preferably lower than l% and morepreferred lower than 0,2%, of the carbon in the sulf1te thick liquor. i.e. that the carbonconversion in the reactor is at least 95%, preferably at least 99% and more preferred atleast 99,8%.

The green liquor sulphide is recoVered in the same manner as the sulphide in the greenliquor from a recovery boiler, i.e. by stripping the green liquor from its content ofsulphide by contacting it in countercurrent with carbon dioxide and sulphur dioxide,preferably in an absorption/desorption tower, and then further to the pulping chemicalssulphur dioxide and/or sulphite, but the lower sulphidity of the gasif1cation green liquor(due to the sulphur content in the raw synthesis gas) leads to lower capacity requirements in the equipment used for this purpose. In addition, a high sulphur 11 reduction efficiency decreases the total amount of sulphur that needs to be circulated bydecreasing the so-called dead-load (i.e. inactive sulphur species such as sulphate). It isbenef1cial that the green liquor is to an extent of at least 90%, preferably at least 95%and more preferred at least 99%, free from non-reduced sulphur, i.e. that the sulphur reduction efficiency is at least 90%, preferably at least 95% and more preferred 99%.

A minor part of the green liquor may be employed for wetting the inside of the chute bymeans of being retumed to the chute and being permitted to form a thin film on the inside of the chute.

The raw synthesis gas, leaving the primary quench dissolver of the reaction vessel, nowessentially free of smelt drops, is further cooled to saturation in the second vessel 12 agas cooler for particulate removal and gas cooling. Water vapour in the raw synthesis gas is condensed, and the heat released may be used to generate steam.

Hydrogen sulphide and carbon dioxide are removed from the cool raw synthesis gas in aso called acid gas removal plant - AGR. Several known commercial gas cleaningsystems comprising units for absorption of acid gas and recovery of sulphur may beused. Said removed hydrogen sulphide may then be conveyed to the cooking liquor preparation.

It is benef1cial that the hydrogen sulf1de rich stream removed from the cool rawsynthesis gas in the AGR comprises at least 25% hydrogen sulf1de, preferably at least35% hydrogen sulf1de of the total stream content, since a high concentration ofhydrogen sulphide facilitates the thereafter following steps of combustion and scrubbing.

Said carbon dioxide being removed from the cool raw synthesis gas in the AGR may beconveyed back to the mill and be used where appropriate, e. g. stripping hydrogen sulphide from the green liquor in the recovery process.

The resulting synthesis gas is a nearly sulphur-free synthesis gas comprising carbonmonoxide, hydrogen and only small amounts of carbon dioxide, and may be used as feedstock for automotive fuels, chemicals or electricity generation.

With a gasifier plant with an AGR several simplifications of the system for recovery of pulping chemicals may be made. The load on the part of the cooking liquor preparation 12 that converts sulphide to sulphur dioxide and sulphurous acid is decreased due to thelower sulphur content in the green liquor. A Claus plant would not be needed to recoverthe sulphur in hydrogen sulphide form, since the hydrogen sulf1de rich stream from theAGR may be combusted directly with air/oxygen to give sulphur dioxide of sufficientlyhigh concentration that can be absorbed from the gas in a scrubber. The AGR replacesthe recovery boiler flue gas scrubber function. Part of the carbon dioxide stream from the AGR may also be used for carbonation in the cooking liquor preparation if desired.

EXPERIMENTAL - Pilot test of gasif1cation of sulphite thick liquor In the present invention experimental tests of gasif1cation of sulphite thick liquor fromsodium based sulphite cellulose production were carried out but it is understood thatother sulphite liquors, e. g. magnesium, calcium, ammonium or potassium based sulphite liquors, may as well be used without departing from the scope of the invention.

In the test, the cooking liquor was a sodium bisulphite-sulphite solution.

Sulphite thick liquor was transported in an insulated truck from the pulp mill to the pilotplant. 62% dry solids (DS) content was used, since long term stability is not verified at aconcentration of 70% DS. Liquor was f1ltered through a 2 mm screen and kept in an agitated insulated tank from which liquor for gasif1cation was taken.

The primary parameters studied are liquor load and reactor temperature. The testprocedure used is based on stepwise increase in liquor load from a relatively lowstarting point. The reactor pressure was increased simultaneously to keep reactorresidence time comparable. Temperature Variations, induced by varying OZ/liquor ratio, were used to study the influence of this factor.

Prior to atomization, the liquor to be gasified is pre-heated to decrease viscosity andincrease reactor energy efficiency. Fouling of surfaces in a heat exchanger used for thispurpose was evident during the experiment which limited the obtainable load. Thus,testing of maximum reactor capacity could not be achieved in this experiment. Problemswith surface fouling when indirect heating is used are well known from sulphite thick liquor handling at sodium sulphite mills and not specific for gasif1cation.

Operating points according to Table I were tested. The total duration of the test was 27h. Start-up, operating point changes and shutdown constituted approximately 5 h.

Operating parameters not shown in Table I was not varied systematically; the same 13 values as for gasif1cation of Kraft black liquor were used for most parameters. The onlymajor exception is the green liquor circulation in the quench section, which was significantly higher than normal.

Table I. Operating points; some representative parameters showed.

Duration Load Load Pres. OZ/liq. Reactor Methane Liquor temp.(wet) (dry) ratioA temp.B in cold gas after pre-heath. kg/h t DS/d bar(g) kg/kg °C mo1% °CI 3 388 5.7 15 0.397 1010-1070 0.15% 128-1302 2 559 8.3 20 0.370 1010-1070 0.2% 121-1243 3 559 8.3 20 0.359 980-1010 0.6% 121-1224 2 559 8.3 23 0.359 980-1010 0.6% 1235 2 631 9.3 23 0.374 1010-1070 0.2% 1206 3 631 9.3 25 0.374 1010-1070 0.2% 119-1217 6 631 9.3 28 0.375 1010-1070 0.2% 1198 0.5 631 9.3 28 0.350 980-1010 0.7% 119 A Based on wet liquor flowB Range measured by seven temperature sensors in the reactor Liquor analysisA liquor sample for analysis was taken at the mill when the liquor was shipped. The composition, shown in Table II, is representative for normal mill operation except for the dry solids content, which as explained above is lower than normal.

Table II. Sulfite thick liquor analysis Liquor compositionmass/massHHV MJ/kg DS 17.5C kg/kg DS 43.3%H kg/kg DS 4.2%S kg/kg DS 8.7%o kg/kg Ds 339%;Na kg/kg DS 8.8%K kg/kg DS 0.23%Cl kg/kg DS 0.01%N kg/kg DS 0.9%DS kg/kg wet 61.7% B By difference, not analyzed 14 Synthesis gas Gas composition was measured by on-line analyzers and sampling followed by gaschromatographic laboratory analysis.Only the results from laboratory analysis are discussed here since they are considered more accurate.

Cold synthesis gas was sampled at the end of each operating period (Table I). Theresults are shown in Table III. The high N2 content is due to specific pilot scalesolutions and will not be present in a full scale mill. The high CO2/CO ratio is also a pilot scale effect due to high heat loss, as discussed further below.

Table III. Cold gas composition as determined by gas chromatographic analysis. operating coz Hzs oz/Af NZ CH. co H2 cos HHvB LHvBppm: % % % % % % % ppm MJ/Nnf MJ/Nfff 1 26.6 1.77 0.0 31.5 0.11 18.7 20.0 66 5.43 4.99 2 26.6 2.07 0.0 23.0 0.28 21.9 24.3 62 6.58 6.053 26.3 2.04 0.1 23.1 0.56 21.5 24.6 66 6.68 6.134 26.2 2.06 0.0 23.1 0.56 21.4 24.7 68 6.70 6.145 27.1 2.06 0.0 21.2 0.23 22.4 24.9 50 6.72 6.186 27.3 2.09 0.1 20.1 0.22 22.9 25.3 48 6.83 6.287 27.6 2.09 0.0 20.1 0.18 23.0 25.5 48 6.83 6.288 26.2 2.13 0.0 20.5 0.67 22.7 26.3 58 7.10 6.51UncerLA 0.4 0.02 0.07 0.7 0.004 0.2 0.2 2.5 A Uncertainty given as standard deviation estimated from duplicate analysis of four individual samplesB Calculated based on composition Green liquor Green liquor samples were taken for chemical analysis (Table IV) and for visualinspection and density measurement. Carbonate and hydrogen carbonate weredeterrnined by acid titration, sulphide by silver nitrate titration and total sulphur by wet oxidation followed by ion chromatography.

It should be noted that some difficulties are present when trying to correlate green liquorproperties with gasifier operating conditions. The long residence time in the quenchgreen liquor volume makes obtaining steady-state time consuming. Only operating point7 has sufficient duration to give a representative green liquor sample. All other samples are considered to be influenced by several operating points.

Carbon Conversion Was not measured explicitly but is considered to be complete oralmost complete in all operating points from the visual appearance of the green liquor.Unconverted carbon (char) is norrnally clearly visible as non-settling black particulates When present in the green liquor even in small quantities.

The green liquor concentration is lower than What is normal for the pilot plant duringKraft liquor operation and compared to What used at the cellulo se mill today, Which ismainly due to the difficulty associated With controlling total titratable alkali (TTA)during the short test duration and changing operating points. Operating at higher greenliquor concentrations is not believed to influence green liquor composition or quality significantly.

Green liquor sulfidity, measured as S/Nag ratio (mol/mol), is approximately 0.5 onaverage. This corresponds to a HS- concentration that is 25% of TTA. The C02-absorption is high due to the deliberately high green liquor circulation flow and givesHCO3" concentrations that are about 30% of TTA. It is possible to control C02-absorption to a large extent by quench design and operation, Which can be used tooptimize the green liquor for cooking liquor preparation processes. It should be notedthat COZ-absorption is not a disadvantage as is the case for Kraft green liquor since causticization is not used.

Reduction efficiency does not deviate from 100% Within measurement accuracy; cf.values in Table IV, Which are based on analyses of total and sulphide sulphur This amarked difference compared to the 80-85% reduction efficiency that are obtained in the mill recovery boilers presently according to green liquor analyses.

Table IV. Results from chemical analysis of green liquor samples. operating TTA C032' Hco; Hs' Tot-s s/Naz Red.point mol/l mol/l mol/l mol/l mol/l mol/mol efficiency 1 0.70 0.16 0.19 0.18 0.18 0.51 98%2 1.31 0.33 0.32 0.33 0.33 0.51 101%3 No analysis 4 2.21 0.53 0.64 0.50 0.49 0.46 103%5 2.61 0.65 0.72 0.60 0.60 0.46 99%6 2.40 0.57 0.69 0.57 0.55 0.47 104%7 2.21 0.49 0.71 0.53 0.48 0.48 110%8 No analysis 16 Analysis and discussion Sulphur split ratio The split of sulphur between gas and smelt may be a very important parameter for millintegration and dimensioning of downstream gas processing equipment. The sulphursplit ratio (defined here as the fraction of sulphur in the synthesis gas) may be calculatedfrom sulphur content and flows in different combinations of streams. Altematively, itmay be possible to calculate the sulphur split from S/Na2 ratios in green liquor exitingand thick liquor entering the system if it is assumed that all Na leaves the system in the green liquor stream.

The method based on S/Na2 ratio is not dependent on flow measurements, which is anadvantage. Calculations based on measured ratios indicate that 69% of the sulphur ends up in the gas phase at 28 bar reactor pressure.

The sulphur split ratio for gasification of sulf1te thick liquor is significantly higher thanthe 30-40% obtained for Kraft black liquor.

Smelt melting point Smelt composition deterrnines the physical properties of the liquid phase in the reactor.There is a risk of so lidification on “cold” surfaces in the reactor exit if meltingtemperature is too high. An approximate smelt composition can be deterrnined from thegreen liquor analysis by assuming that no sulphur is lost from the green liquor orabsorbed into it in the quench. Further, the smelt is approximated to consist of onlyNagS and NagCOg. K and Cl content is very low (cf. Table II) but hydroxide content at1000 °C may be significant.

When the smelt composition obtained from the experiment is used to predict a meltingpoint in the NagS-NagCOg phase diagram, a melting point of approximately 850 °C ispredicted compared to the 825 °C predicted for typical Kraft black liquor gasificationsmelt. The relatively low melting point is a very important and encouraging conclusion,since it indicates that the risk for operating problems due to smelt so lidification may notbe greater for sulphite thick liquor than for Kraft black liquor. No signs of problems associated with deposits caused by high smelt melting point were observed as assessed 17 from temperature measurements, pressure drop between reactor and quench and visual inspection of reactor after test terrnination.

Energy efficiency The energy efficiency can be measured by the cold gas efficiency (CGE), which isdefined as the energy in cold synthesis gas divided by the energy in the sulphite thickliquor. This measure shows how much of the chemical energy in the liquor that istransferred to the synthesis gas and is also an indication of potential biofuel yield.Higher heating values (HHV) are used for the calculation in this paper.

Table V shows CGE values with and without adjustrnent of synthesis gas flowmeasurements. The adjustment is made in the synthesis gas flow measurement to closethe mass for balance for C. The deviation, which is -6% based on the actual reading isprobably explained by synthesis gas measurement uncertainty. It is known fromexperience that the gas flow metering device can give too low readings due to cloggingof pressure sensors. This is supported by an observed continuous decrease in measuredgas flow in some of the operating points, although operating conditions were keptconstant. The gas flow reading used for balances is taken at the end of each operatingperiod and is thus probably too low. An altemative mass balance with an adjustment ofthe synthesis gas flow to close the C balance has been calculated, which is referred to asaltemative 2 when energy efficiency is discussed below. Note that reactor temperature(and thus OZ/liquor ratio) is important for CGE and is included in the table for this YCEISOII.

Table V. Gasification energy efficiency as cold gas efficiency Operating RX temp Measured gas CGE HHV CGE HHVpoint flow Alt 1 Alt 2AC Nm3/h % %1 1010-1070 410 53.3% 54.5%2 1010-1070 560 61.2% 60.7%3 980-1010 550 61.0% 63.0%4 980-1010 539 59.9% 61.7%5 1010-1070 578 57.2% 59.9%6 1010-1070 563 56.6% 59.5%7 1010-1070 552 55.4% 59.2%8 980-1010 544 568% 65.5% A Alternative 2 is after adjustment of synthesis gas flow measurement to close C balance, see text. 18 Due to pilot scale effects, the CGE values in Table V are not representative for whatmay be expected in a commercial scale gasif1er. In order to estimate CGE for a full scale plant three adjustments may be compared to values measured in the pilot tests: l. Heat losses may be decreased to a level that can be expected in a full scaleplant (ß500 kW at 500 tDS/d). 2. The energy required to heat Ng in the reactor to the reactor exit temperaturemay be subtracted since N; will not neccessary be used in a commercial plant. 3. The results may be adjusted to account for the higher efficiency reached athigher DS content (70% compared to 62%).

As in previous analyses, focus is on operating point 7 since this point has the longestduration and may be expected to represent steady-state best. Adjustments according toitem l and 2 for operating point 7 increase CGE to 61% and 65% for altemative l and 2 respectively.

Adjustment to account for DS content may be less straightforward since it may requireconsideration to the lower Og/BL ratio obtainable with higher DS content. Simulations,based on a therrnodynamic model of the gasification process, show that, at constantreactor temperature, the effect on CGE by changing from 62% DS to 70% DS may be approximately 5% at the relevant operating point.

This leads to a commercial scale CGE estimate for conditions according to operatingpoint 7 of approximately 66% and 70% for the two calculation approaches respectively.It may be noted that the operating point used has “high” reactor temperature and thatacceptable green liquor quality was observed also for lower temperature. Since CGEincreases with decreasing temperature, this indicates that even higher CGE may be obtainable but further experiments at higher reactor load is required to confirm this.

Discoveries:- Full carbon conversion may be reached at temperature and residencetime similar to what is used for sulphate liquors. This is surprisingsince sulphite liquors norrnally have a significantly lower reactivity in recovery bo ilers. 19 - 100% sulphur reduction may be achieved, i.e. significantly higher thanWhat can be expected With reference to experience from recoveryboilers - About 70% of the sulphur may be obtained in the produced syngas,Which is a surprisingly high number.

- A slagging temperature (melting temperature) of the salt formed in thereactor that may be kept lower than expected thus improving the ability to make the salt melt floW out of the gasifier.

The very good results leads to that a sulphite mill equipped With gasification of thesulphite liquor may: - Increase energy efficiency of the recovery process - Simplify the liquor cycle.

- Drastically reduce or even avoid dead load of sulphate in the liquorcycle.

- Drastically reduce or even avoid losses of sodium sulphate from theliquor cycle and decrease purchase of fresh sulphur and sodium (asNaOH.) As Will be understood by those skilled in the present field of art, numerous changes andmodifications may be made to the above described and other embodiments of thepresent invention, Without departing from its scope as defined in the appending claims.For example, an altemative process configuration and equipment design may be used toreach the same result if it is used for a slagging entrained-floW gasification process ofsulphite thick liquor. It is also understood that the liquid phase produced in thegasification process as defined in claim 1 should be constructed as also applying to aprocess comprising some minor amount of solid and/ or condensed material, that maybe present. The skilled man realizes that the concept is feasible also at atmosphericpressure and, furtherrnore, that the method also applies to booster concept Wherein the gasifier is operated in parallel With the recovery boiler.

Claims (17)

1. Method for recovering chemicals and energy from sulphite spent liquor, said sulphite spent liquor being obtained when producing pulp by chemicaldelignification of f1brous raw material using a sulphite pulping process, saidsulphite spent liquor comprising organic and inorganic compounds; the methodcomprising processing of said organic and inorganic compounds at a temperatureabove 800 °C thereby producing partly at least one phase of a liquid material andpartly at least one phase of a gaseous material, characterized in that saidprocessing is carried out by gasification of said sulphite spent liquor in a reactor comprising an oxidizing medium at sub-stoichiometric conditions.

2. . Method according to claim 1, characterized in that said temperature is at least 900°C, preferably at least 950' C and below 1300° C.

3. . Method according to claim 1 or 2, characterized in that said gasification is an entrained flow gasification.

4. . Method according to claim 1-3, characterized in that the absolute pressure of the gasification process is about 1.5 to about 150 bar, preferably about 10 to about 80 bar, and most preferably from about 24 to about 40 bar in the reaction zone.

5. . Method according to claim 1-4, characterized in that said oxidizing medium is oxygen gas or an oxygen containing gas.

6. . Method according to claim 1-5, characterized in that said sulphite spent liquor is in a droplet form when meeting said oxidizing medium, said droplets having an average droplet size below 300 um.

7. . Method according to claim 1-6, characterized in that said liquid material is in the form a salt melt, which is dissolved in a liquor thereby forrning green liquor, saidgreen liquor being drawn off from said reactor and being fiJrther processed inorder to convert sodium sulphide comprised in the green liquor to sulphur dioxide and/or sulphite.

8. . Method according to claim 7, characterized in that said sodium sulphide is converted first to hydrogen sulphide by contacting said sodium sulphide in 9. 10. ll. 12. 13. 14. 21 countercurrent With carbon dioxide and sulphur dioxide, preferably in an absorption/ desorption tower, and then further to sulphur dioxide and/or sulphite.

9. Method according to claim 7, characterized in that the amount of unbumt charcoalin said green liquor is lower than 5%, preferably lower than 1% and more preferred lower than 0,2%, of the carbon in the sulf1te thick liquor.

10. Method according to claim 7 or 9, characterized in that said green liquor is to anextent of at least 90%, preferably at least 95% and more preferred at least 99%, free from non-reduced sulphur.

11. Method according to claim 1-6, characterized in that said gaseous material is araw synthesis gas comprising hydrogen sulphide, carbon monoxide, hydrogen and carbon dioxide.

12. Method according to claim ll, characterized in that said hydrogen sulphide andsaid carbon dioxide in said raw synthesis gas are being removed from said rawsynthesis gas in an acid gas removal plant thereby forrning a hydrogen sulf1de richstream and a stream comprising mainly carbon dioxide from said acid gas removal plant.

13. Method according to claim 12, characterized in that said hydrogen sulf1de richstream comprises at least 25% hydrogen sulf1de, preferably at least 35% hydrogen sulf1de of its total stream content.

14. Method according to claim 12, characterized in that the stream comprising mainlycarbon dioxide is conveyed from the acid gas removal plant to the recovery process.

15. . Method according to claim 12, characterized in that said hydrogen sulf1de rich stream from said acid gas removal plant is being combusted directly with air oroxygen to give a combusted gas comprising sulphur dioxide, said sulphur dioxide being absorbed from said combusted gas in a gas scrubber. 22

16. Method according to c1ain1 1-6, characterized in that said sulphite spent liquor is a sodiuni based sulphite liquor.

17. Method according to c1ain1 1-6, characterized in that said sulphite spent liquor is a magnesium, potassiuni or an1n1oniun1 based sulphite spent liquor.