FI81847B - Method for treatment of combustible gas produced from waste liquors from alkaline cellulose digestion - Google Patents

Description

81 847

METHOD FOR THE TREATMENT OF FUEL GAS PRODUCED FROM THE WASTE OF ALKALINE CELLULOSE PROCESSES - FÖRFARANDE FÖR BEHANDLING AV BRÄNNGAS FRAMSTÄLLD UR AVLUTAR FRAN ALKALISKA CELLULOSAKOK

The invention relates to a process for the treatment of temporal cellulose processes, in particular sulphate cooking. for the treatment of combustion gases produced from waste liquors and suitable for energy production. In the process, the time metal components contained in the combustion gas are contacted with one or more amphoteric oxides, such as titanium oxide (TiO 2) or iron oxide (Fe 2 O 3) at high temperature. Due to their low volatility, the mixed oxide compounds formed during the treatment can be removed from the gas, which results in the amount of vapor, aerosol or par-15 precious metal compounds contained in the gas ·,; significant reduction. Hydrolysis of degassed early metal alloy oxide compounds results in ;;; the formation of an active cooking chemical (time metal 1 hydroxide), whereby the amphoteric oxides 20 are regenerated simultaneously.

The dry matter of the waste liquor (so-called black liquor) from sulphate cooking consists mainly of three fractions of almost equal content: inorganic ·: ·. compounds, lignin degradation products and carbohydrate degradation products (Al6n, R., Chemistry 15 (1988) 565). In the conventional process for recovering sulphate black liquor 2 81847, the extractants are first separated from the broth as a raw soup, after which it is concentrated to a dry matter content of 65-75% and incinerated in a recovery boiler (Virkola, N.-E. (ed.), Manufacture of wood pulp, part 1, 2).

5th edition, Academy of Technical Sciences. Turku 1983). In this case, the organic matter of the lye is converted into energy. The inorganic matter leaves the boiler mainly in the form of a melt consisting primarily of sodium carbonate (Na 2 CO 3), sodium sulfide (Na 2 S) and sodium sulfate (NassSCU). These salts are returned to the cooking through a causticization step in which Na 2 CO 3 is converted by calcium hydroxide (Ca (OH) a) to the active cooking chemical, sodium hydroxide (NaOH):

CaO + H * O - Ca (OH) =

15 Ca (OH) a + Na 2 CO 3 - CaCO 3 + 2 NaOH

The required CaO is in turn regenerated by calcination. separately the CaCOa formed in the latter reaction is called

in a lime kiln: "* CaCOa - * CaO + COa 20 Although it is reasonable to argue that in the current recovery process the constituents of black liquor are recovered in their entirety, the recovery process can be substantially improved. With the exception of extracts, black liquor organic matter is gia, the majority of which is obtained as thermal energy and in which: · correspondingly, the share of electrical energy is relatively small.

The thermal efficiency of the current boiler process (based on higher calorific values) is only 50-55%, taking into account the heat required to evaporate the waste liquor.

30 The sulphate process art intake equipment is also expensive, corresponding to about AO% of the mill's total investment, and its operation involves a risk of meltwater explosion. Above

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3 For the reasons mentioned 3 81847, research has focused in part on the application of alternative conversion techniques to black liquor.

Gasification technology offers an alternative in order to make more efficient use of 5 black liquors. In gasification, the organic material of the raw material reacts with oxygen and water vapor at a high temperature, type 11 800-1300 ° C, and is converted into a gas with a calorific value. In particular, pressurized gasification technology connected to 10 combined gas and steam turbine power plants (so-called combined gasification power plants or KKV plants) has been of interest (Kohl, A.L., Can. J. Chem. Eng. 64 tl986) -299). Figure 1 shows a simplified process diagram of a type of SME facility. Because the fuel gas produced from feedstock 15 is burned in a gas turbine. the combustion gas must be fairly pure and contain only very little solids and early metal compounds (Mojtahedi, W., Chemistry 14 (1987) 835). Particularly due to environmental emissions, the content of sulfur compounds 20 must also be quite low. For these reasons, the raw gas from the conversion reactor usually has to be purified: and in the HVAC solution, this purification is carried out specifically under pressure. In addition, the temperature of the treatment process is of great importance for the overall economy of the KKV process.

25 where, as a general rule, raising the temperature to a certain limit contributes to the economy of the process.

· ’·, Alternatives to traditional causticization have also been explored. One of them is the so-called. autokaustisointi. where amphoteric oxide suitable for the purpose is added to the black liquor ’! 30 incineration (Kiiskilä. E. & Virkola, N.-E., Paperi ja Puu 3 (1978) 129). This amphoteric oxide forms a * 81847 mixed oxide compound with sodium oxide (Na 2 O) and, accordingly, the formation of Na 2 CO 3 is considerably lower than in conventional black liquor combustion. At combustion temperatures, said mixed oxide compound also evaporates to a much lesser extent than the sodium compounds normally formed under these conditions. In the post-combustion leaching step, the mixed oxide compound reacts with water (hydrolysis reaction) directly to form the active cooking chemical. Suitable amphoteric oxides can be divided into two groups: Group I: in the hydrolysis step, a compound derived from the amphoteric oxide is formed, which is at least partially soluble in the cooking liquor. In this case, the broth differs substantially from conventional temporal broths, such as those used in sulphate soup or soda broth. This group includes alumina (AlaOa) and silica (SiO *) '. *.

- Group II: during the hydrolysis step, sodium hydroxide is formed and the amphoteric oxide insoluble in the broth is regenerated. In this case, the composition m'm 'of the cooking liquor does not differ significantly from the composition of a traditional cooking broth. Titanium oxide (TiO2) and iron oxide (FeaOa), among others, belong to this group. When using, for example, TiO 2, the main reactions can be represented in the following simplified form:

In the combustion stage: NaÄ0 + TiOa - »NaaO.TiOa In the dissolution stage: V Na2O.TiOÄ + HaO - * 2NaOH + TiOa During the development of the method presented here, 30 new and surprising observations have been made, namely that 5 81847 black liquor fuel gas vapor, aerosol or amphoteric oxides belonging to said group II can be used to remove the early metal components in particulate form at a high temperature (600 to 1300 ° C), whereby an alkali metal hydroxide, i.e. an active cooking chemical, is obtained by hydrolyzing the formed and degassed mixed oxide compound. The use of these oxides for the purification of the combustion gas produced from black liquor thus means a completely different application 10 than their use in the above-mentioned auto-causticization. The economic conditions of the new application also differ completely from the corresponding conditions of auto-causticization, as the new method applies only a small part of the total amount of time metal in black liquor.

. The main features of the process according to the invention, which are set out in more detail in the claims, are: • 1. In gas treatment, the fuel gas produced from the waste liquor is contacted with one or more amphoteric oxides belonging to said group II.

* * 2. The gas treatment uses a relatively high temperature, 600 to 1300 ° C, to form mixed oxide compounds of early metal oxide and amphoteric oxide *: ·. between. In this temperature range of these compounds -. Evaporation is less than that of sodium compounds, V. such as e.g. Evaporation of Na 2 O formed as a result of thermal decomposition of Na 2 CO 3.

3. The amphoteric oxide in question is regenerated by hydrolysis of the mixed oxide compounds formed in the gas treatment step to form alkali metal hydroxide at the same time. Both insoluble amphoteric oxide and unhydrolyzed insoluble mixed oxide compounds are separated from the solution and recycled back to the 5 gas treatment steps. The time metal hydroxide is an active cooking chemical, so that said solution can be used in the cooking of cellulose, e.g. by adding it to the main stream of cooking liquor.

By integrating the method according to the invention into a gasification combined power plant (KKV plant) based on the gasification of waste-10 broth, considerable advantages are achieved over the traditional chemical and energy art extraction process. One of the most significant is the higher electricity / heat ratio of the SME plant in energy production. In this respect, the advantages 15 increase up to a certain limit as a function of the increase in the gas cleaning temperature. As the method presented in this context uses a relatively high temperature, 600 to 1300 ° C, the method is quite suitable as part of the KKV solution.

·; ·. The two best - known Group II amphoteric oxides. are TiO 2 and Fe 2 O 3. The use of Fe 2 O 3 differs from the use of TiO 2 in that Fe 2 O 3 also reacts with any sulfur compounds in the flue gas. The iron-sulfur compounds thus formed can be converted back to FeaC 3 by oxidizing them under appropriate conditions to form sulfur dioxide gas. Thus, when iron oxide is used, it is technically possible to combine the gas degassing and desulfurization steps. On the other hand, it is clear that, for economic reasons, the use of Fea03 "· 30 is best suited for the treatment of gases containing relatively little sulfur compounds.

Il 7 81847 Such are, for example, gases produced at high temperatures and, on the other hand, gases after primary desulphurisation. TiOa, on the other hand, does not react with sulfur compounds to a significant extent and can therefore be used in the case of gases rich in sulfur compounds. When using TiO 3, it is possible to combine this gas treatment step with a primary or secondary desulphurisation step if, for example, a sulfur retention chemical, for example, is used in the reactor at the same time as TiO 2.

Since a relatively high temperature is used in the process according to the invention, a secondary gasification step of the waste liquor can also be carried out in connection with the gas treatment by passing partially unreacted materials in the primary gasification, in particular residual carbon, to the gas treatment step.

If necessary, the gas treatment step can be carried out ... · 'in the same set of equipment as the primary gasification step of the waste liquor, for example by using a reactor with 20 different reaction zones on top of each other.

The following example shows an alternative implementation of the new method. The example is intended only to illustrate the invention without limiting it in any way. Furthermore, it is self-evident to the person skilled in the art 25 that the method is applicable to the recovery of waste liquors (as well as those formed during cooking) generated by the temporal delignification processes of different types of raw materials. The method can also be applied to the treatment of gases for non-combustion applications, such as synthesis gases.

8 81847

Example

Figure 2 shows a simplified process diagram of an embodiment of the method, in which the fuel gas produced by the sulphate waste liquor (material stream 1) 5 is treated in a fluidized bed reactor. The fluidized bed material consists mainly of TiO 2 and mixed oxides such as Na 2 O. TiO 1. The temperature is maintained in the range of 800 to 1000 ° C and the pressure is 1 to 3 MPa. The fluidized bed particles agglomerate to suitably sized particles-10 e.g. because the temperature is kept close to the melting point of the mixed oxides. If necessary, air or oxygen (stream 3) is added to the gas treatment reactor to achieve a sufficiently high temperature (partial combustion). The fluidizing gas consists of the exhaust gas to be treated (stream 1), optionally said air or oxygen stream (stream 3) and, if necessary, a circulating gas (stream A).

The fluidized bed (material stream 5) is derived from the fluidized bed at a pressure of '11. to the step of hydrolyzing the mixed oxide compounds formed in the gas treatment 20 according to the following scheme:

Na10.T iOa + H 2 O -12NaOH + TiOa Insoluble TiO 3 and unhydrolyzed mixed oxides are separated from the solution and returned to the gas treatment step (stream 2). The solution, which contains not only water but also NaOH (substance stream 6), is added to the main stream of cooking liquor, i.e. the so-called the white liquor.

After the gas treatment according to the invention, other harmful substances are removed from the flue gas (stream 7).

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9,81847 compounds, solids, etc.) at a pressure of 1-3 MPa and a temperature> 600 ° C, after which the gas is used as fuel in a combined gas and steam turbine power plant.

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Claims (10)

10 81 847 A process for treating a gas produced from the effluent of temporal cellulose processes, in particular the sulphate process, characterized in that the gas is contacted with one or more amphoteric oxides such as titanium oxide (TiO 2) or iron oxide (FeaO 2) at a temperature of 600 to 1300 ° C and a pressure 0.1 to 10 MPa, wherein the vapor, aerosol or particulate time metal compounds in the form of vapor, aerosol or particles react partially or completely with the oxides in question and the reaction products formed (mixed oxide compounds) are removed from the gas; and that both said mixed oxide compounds and unreacted amphoteric oxides are then subjected to a leaching step in which the mixed oxide compounds are hydrolyzed to a significant extent to form an alkali metal hydroxide upon regeneration of the said amphoteric oxides and then the insoluble amphoteric oxides are removed: and returned to said gas treatment step, and the remaining solution containing alkai imetal 1 skin hydroxide, i.e. the active cooking chemical, is added to the cooking liquor required for the production of cellulose. 1. Raw material Method according to Claim 1, characterized in that external oxygen or air is used to achieve the required gas treatment temperature. • ♦ ··. A method according to claim 1, characterized in that the gas treatment is combined with a secondary gasification step of the waste liquor, in which the partially unreacted material in the primary gasification II 11 81847 is further converted. A method according to claim 1, characterized in that the gas treatment is combined with another gas purification step. 2. Fuel gas 3. Air / oxygen (if applicable) ’4. Circulating gas (if applicable) 81 847 3. Without 4. Water vapor (if applicable) 5. Alloy oxide compounds such as NaaO.TiOa, TiO * 5. Inorganic matter Process according to Claim 1, characterized in that the dissolution step, the post-dissolution separation and the recirculations of the different phases formed in the separation step are carried out at a pressure of 0.1 to 10 MPa, provided that said different steps do not necessarily require the same pressure. • Process according to Claims 1 and A, characterized in that if the active amphoteric oxide is TiO 2 and said second gas purification step is desulfurization, some other externally added chemical is required as sulfur retention agent. 6. NaOH solution 6. Without A method according to claim 1, characterized by '· *; from the insoluble ··· from 1 leaching step. The stream containing 25 compounds is dried before being fed to the • · gas treatment step. • »I 12 81 847 Process according to claims 1 and A, characterized in that if the active amphoteric oxide is FeaOa and said second gas purification step is desulfurization, Fea0a also acts as a sulfur retention chemical. . A method according to claim 1, characterized in that the gas treatment and the gasification of the waste liquor are performed in the same set of equipment. 7. Treated flue gas 7. Flue gases Process steps / equipment: A. Pressurized carburettor B. Gas cleaning: C. Combustion chamber: D. Gas turbine T: E. Waste boiler F. Steam turbine Figure 2. Exemplary embodiment Material flows: · '·' 1. Flue gas to be treated * · *: 2. TiO 2, a mixture of di compounds such as Na 2 O-TiOs. other Na compounds, H 2 O. A method according to claim 1, characterized in that before or after the gas treatment, those other minor substances which are harmful when using the gas as a fuel in a combined gas and steam turbine process, i.e. the so-called combined cycle power plant. li i3 81 847 8. Water or lean cooking liquor Process steps / equipment: A. Gas treatment reactor B. Dissolution step (pressurized) C. Separation of solids D. Drying (if applicable) E. Feed device li