WO2008156397A1

WO2008156397A1 - Method for recovering chemicals and production of steam

Info

Publication number: WO2008156397A1
Application number: PCT/SE2007/050443
Authority: WO
Inventors: Ulf BERGSTRÖM
Original assignee: Metso Power AB
Current assignee: Valmet Power AB
Priority date: 2007-06-20
Filing date: 2007-06-20
Publication date: 2008-12-24
Anticipated expiration: 2009-12-20

Abstract

The invention relates to improved steam generation in a chemical recovery process in a recovery boiler. The black liquor obtained from a kraft pulping process is subjected to a lignin separation process forming a lignin rich fraction and a lignin lean fraction. The lignin rich fraction is burnt in a parallel furnace having superheaters, while the lignin lean fraction is burnt in the ordinary recovery boiler. The resulting flue gases from the parallel furnace are merged with the flue gases in a later flue gas position in the recovery boiler, reducing the harsh chemical conditions in the recovery boiler. This results in more efficient steam generation and possibilities of generating more high pressure steam for production of electricity.

Description

Method for recovering chemicals and production of steam

Technical Area

The present invention relates to a method for recovering chemicals and production of steam in a chemical pulp mill.

State of the art

In chemical pulping is the residual alkali recovered from the spent cooking liquor using the Tomlinson Process. The spent liquor with its content of organic material is subjected to evaporation reaching a dry-matter content of about 80% or more, and is thereafter burnt in a recovery boiler. Sometimes the recovery boiler is also fired with other waste streams from the pulp mill, such as NCG gases, or by adding other fuels.

The process for converting the chemicals requires combustion temperatures around 1200°C, and during the process a chemical smelt is formed in the bottom of the boiler. This smelt is fed from the furnace floor out of the boiler to a dissolver where a green liquor is formed, which thereafter is subjected to a causticising process producing new white liquor to be used in the cooking process.

In the Recovery boiler steam is also produced for primary use in the pulp mill for heating purposes, and a secondary use of the steam is for power generation purposes in a steam turbine driven generator. However, the primary use as a recovery process for chemicals dictates the operating conditions, as the black liquor combusted contains contaminants like potassium and chlorine. This creates a corrosive environment for the parts of the recovery boiler being subjected to high temperatures and these residual chemicals in the flue gases. For the superheaters this will set upper limits of the steam temperature in the range 450-500 °C, and some 525 °C at the most if supreme materials are used in the superheaters.

The generation of high steam temperatures in the recovery boiler could be optimised in several ways. In US 2.606.103, already published in 1952, is the steam generation improved by adding a parallel furnace in the lower part of the recovery boiler, having the final superheater surfaces in the parallel furnace. As explained here is the parallel furnace fired with oil or other fuel chosen in such way that the products of the combustion will neither cause objectionable deposits upon the tubes of the superheater nor will injure the structure of the tubes at the high temperatures required to achieve the higher steam temperatures. Using oil as the fuel will increase costs for operation as this is not a fuel available in the pulp mill.

Further proposals in recent years have been shown with parallel furnaces. In March 1998 was published a report by Varmeforsk, Number S6-604, "Kraft recovery boilers with external superheaters", written by Johan Nygaard. In this report a detailed comparison between wood, oil or gas fired parallel furnaces with super heaters was made. The interest in increasing steam production has risen considerably in recent years as the option for production of electricity from steam have been commercially interesting, especially as the by-product electricity from a pulp mill is often considered as "green" electricity, i.e. an environmental friendly electricity obtained from energy recovery in industrial processes otherwise wasted.

From one and the same applicant, Ahlstrom OY, now operating under the name

Andritz OY, have in total 4 different patent applications been filed focusing upon parallel furnaces with super heaters. In WO 92/18690 (published 1992) is shown a parallel furnace explicitly fired with a gasified part of the black liquor.

In EP 614500 (published 1994) is shown yet another parallel furnace explicitly fired with a gasified part of the black liquor.

In WO 03/095738 (published 2003) is shown yet another parallel furnace explicitly fired with a gasified solid wood-based material, instead of a part of the black liquor.

In WO 03/104547 (published 2003, and corresponds to SE 527390 granted

Dec. 2005 despite above prior art) is shown yet another parallel furnace explicitly fired with a gasified part of the black liquor. Examples of other fuels are given such as natural gas, LPG, oil, methanol, liquefied biomass.

Even tough these solutions and proposals disclose the advantages with parallel furnaces with final super heaters in aspects of better conditions for the super heater tubes, and proposals for different fuels, these fuels are either an expensive external fuel or an internal (available at the mill) resulting in large energy losses or losses in pulp yield, as either gasification is needed or wood material is used.

Objective with the invention The primary objective with the invention is to enable a more energy efficient operation of the recovery boiler and its production of steam, not necessitating additional external fuels nor any wood material that better could be used in the pulping process. The goal is to increase the amount of electricity that can be produced from black liquor available at the mill without sacrificing economy or loss of original wood.

Another objective is to enable an increase of the recovery boiler capacity while burning the main part of the black liquor with its main part of residual content of reusable cooking chemicals in the bottom part of the recovery boiler at less flue gas flow rates. If the flue gas velocity could be reduced in the lower part of the recovery boiler above the smelt bed, then the risks for withdrawing droplets of smelt and black liquor droplets in the flue gases is reduced.

These objectives are met by the inventive method as defined in the characterising clause of claim 1.

Description of the invention

The present invention relates to a process where the black liquor from the kraft pulping process is introduced to the recovery boiler in at least two forms: a lignin rich form and a lignin lean form where the method to introduce the two forms of liquor provides for the possibility to increase the inorganic recovery capacity of the boiler for a given size of furnace and at the same time provide for the possibility to increase the steam parameters for the boiler and also to reduce the super heater area that is needed.

A recovery boiler is a boiler where spent liquor from chemical pulp digestion is burnt. The main objective of burning liquor in the recovery boiler is to recover the cooking chemicals but also to convert the organic substance in the spent liquor to steam. The chemicals (inorganic matter) are reused in the chemical pulping process and the steam that is produced is used in the cooking process for heating and is also often used for electricity production in a steam turbine generator.

A recovery boiler is a vital part of the kraft pulping process. The recovery boiler is also one of the most expensive individual machinery in a kraft pulp mill. For that reason there is normally a big commercial interest from pulp mill owners to maximise the capacity of the recovery boiler. Also due to increasing costs for energy most customers in the kraft pulping business seek for new solutions to maximize the amount of electricity that can be produced from the steam generated in the recovery boiler.

One of the methods to increase the electricity production in steam boiler processes is to increase the steam parameters. This means that the steam pressure and the steam temperature is increased. With higher steam parameters it is possible for the steam turbine to generate more electricity from the steam that is fed to the turbine. However increased steam parameters normally increases the corrosion rate in the superheater. The corrosion rate in recovery boiler superheaters is a well known problem and has forced recovery boiler manufactures to use more exotic superheater materials with higher material costs as a consequence.

One of the most important design aspects of a recovery boiler is the so called specific hearth heat rate. The specific hearth heat rate in a boiler is defined as the ratio between heat input to the boiler and lower furnace cross section area. The specific hearth heat ratio has by its definition a close relation to the flue gas velocities in the lower furnace. Flue gas velocities in the lower furnace has a direct relation on the amount of carry over that can be expected from the lower furnace to the superheater region. For that reason it can be assumed that if the specific hearth heat ratio becomes too high the carry over situation in the boiler will be worse and this will result in various problems such as reduced steam temperature, enhanced corrosion and general fouling of boiler heating surfaces. In many cases high carry over will lead to reduced availability of the boiler and imposes the need to shut down the boiler for manual cleaning or water wash.

As described above the recovery boiler is a vital part of the kraft pulping process. The spent liquor that is burnt in the recovery boiler is called black liquor. Black liquor is the residue from the pulping process where the fibres has been separated from the lignin that is contained in the wood chips that forms the raw material in the pulping process. The lignin contains mainly hydro carbons and is an energy rich substance. The black liquor is a mixture of the lignin and the chemicals that has been used in the pulping process to achieve the right condition for separation of fibres and lignin.

Lignin act as the glue in the wood raw material and together with the fiber builds up the strength in wood. It has been well known that lignin as a refined product can be used in many applications. Lignin was refined from sulfite liquors during the 60s and was used as ingredient in many applications in oil industry, animal food industry and building material industry. Because the lack of sulfite liquor as raw material for lignin production, a process was developed during the 70s to extract lignin also from kraft pulp liquors. In this new process only part of the lignin was extracted hence forming both the lignin product and the remaining "lignin lean" product which also contains the inorganic compounds that is recovered in order to produce new chemicals for the cooking process. By this extraction process the "lignin lean" liquor would have a lower heating value than the original black liquor and the lignin product would have a higher heating value than the original black liquor.

Today with increased energy cost the lignin product is also thought of as a potential energy resource for various applications. However because the origin of the lignin product there are contaminations in the product. Normally inorganic material originating from the kraft pulping process is part of the lignin product. Since sulphur is a major compound in kraft pulping this will call for desulphuhsation need of the flue gases if lignin is burnt in a normal power boiler.

The invention

The present invention relates to a process where a "lignin lean" fraction of the black liquor is introduced in to the lower furnace in the traditional way while the "lignin rich" fraction is introduced to a separate or parallel furnace. Due to the fact that the "lignin lean" black liquor has a lower heating value more "lignin lean" liquor can be fired compared to if normal black liquor is fired, provided the same specific hearth heat rate.

In a parallel furnace that can be attached to the recovery boiler furnace, the extracted lignin rich fraction is fired. The extracted lignin contains lower levels of K and Cl compared to untreated black liquor. This means that the flue gas is not as corrosive as flue gas from untreated black liquor is. The flue gas from the separate furnace can then be introduced to and mixed with the flue gas in the recovery boiler furnace. The mixing point is preferably located at a position high up in the furnace so the carry over situation in the lower furnace is not affected. The separate furnace can be equipped with final super heater elements that form part of the recovery boiler super heater system. The steam in the super heater panels in the separate furnace is heated by the flue gases produced by the combustion of the extracted lignin rich fraction. By arranging the final super heater elements which have the highest temperature in the separate furnace where the flue gas environment is less corrosive the corrosion rate of the final super heater elements will be lower than if the final super heater elements would have been placed in the recovery boiler furnace. Also with the positioning of the super heater elements in the separate furnace where the fouling situation is better compared to the recovery boiler, the total installed superheater area can be reduced.

The present invention brings the following advantages to the recovery boiler process: • Steam parameters can be elevated since the final super heater stage will be heated with lignin fuel in the separate furnace. With lignin fuel in the separate furnace there is a much better possibility to control the flue gas chemistry close to the super heaters and hence reduce the corrosion rate.

• Steam temperature control can with the present innovation be achieved in a much better way than in a traditional recovery boiler. In a traditional recovery boiler super heaters has to be "oversized" in order to account for gradually increased fouling of the super heater surfaces. The steam temperature control is in traditional recovery boilers achieved with means for throttling the capacity that is extensive when the boiler is clean and which throttling is reduced the more fouled the super heater gets. With the present invention the final steam temperature can be controlled by controlling the amount of lignin that is fired in the separate furnace. Therefore with the present invention the total installed super heater area can be reduced compared to what is needed in a traditionally recovery boiler.

• When the lignin rich fraction is burnt in a parallel furnace and joining the flue gases with the flue gases from the main furnace in the recovery boiler, then the sulphur content in the lignin from the burnt lignin rich fraction is not a problem since the flue gases from the main furnace in a recovery boiler normally has a high degree of evaporated sodium which will absorb the sulphur compounds and form sodium sulphate. Therefore there is no need of separate desulphurization equipment after the combustion of the lignin rich fraction.

List of figures

Figure 1 Show the principal layout of a system capable of operating according to the inventive method

Embodiment of the invention In figure 1 is a principal layout shown of a system wherein the inventive method could be implemented. The heart of the invention lies in the separation process of the lignin rich fraction from the black liquor emanating from the kraft cooking process. The kraft cooking process is visualised as the stage 1 and the flow of black liquor is labelled BL.

The black liquor is first passing some evaporation stages 2a, and subsequent to this is the at least partially evaporated Black liquor subjected to a lignin separation process in stage 3.

The lignin separation process 3 is preferably a precipitation process where the lignin is made to precipitate by lowering the pH by addition of acid/Ac. This type of precipitation process is not requiring a lot of energy and requires only an addition of chemicals.

Alternatives to the precipitation process could be found in similar non energy consuming processes like filtration or any other chemical process. Gasification is not to be used as this process requires addition of some 20-40% of the energy value obtained from the gasified part of the black liquor. Some further evaporation stages could also follow the separation process 3, as indicated by 2b.

The lignin separation process results in a lignin rich fraction L_RICH and a lignin lean fraction L_LEAN- The lignin lean fraction L_LEAN is sent to the conventional black liquor guns 10 in the bottom area of the main furnace 20 recovery boiler 30. The lignin rich fraction L_RICH is sent to the parallel furnace 21 where a lignin burner 1 1 is installed.

The lignin separation process is separating the lignin lean fraction having the major part of the residual cooking chemicals left in this lignin lean fraction.

Typically is more than 75%, and preferably more than 90%, of the residual cooking chemicals from the original black liquor still kept in the lignin lean fraction, and in the lignin rich fraction is more than 75%, and preferably more than 90% of the content a lignin content, besides residual liquids from the separation process such as acids.

In the conventional recovery boiler are the flue gases passing a first heating surface HE1. This heating surface HE1 is cooled by circulating boiler water from the drum D and could preferably be the water cooled walls of the recovery boiler. The flue gases thereafter pass a first superheater SH1 in the recovery boiler. This superheater contains steam collected from the drum D heated by the flue gases and approaching a temperature in the range of 310-500°C. This high pressure steam could typically be sent to the high pressure steam net of the pulp mill, to be used in a power turbine.

For less demanding heating purposes in the pulp mill is often a low pressure steam net available at pressures in the range of 2-5 bar. This steam net is most often supplied by the pressure reduced high pressure steam, which pressure reduction is an effect after having passed the power turbine. When the ordinary flue gases has passed the first heating surfaces, HE1 , and the superheater, SH1 , then the flue gases passes the economiser, ECO, generating bank.

In the economiser feed water FW is heated in a first step before it is supplied to the drum D compensating for the steam taken out from the system. Finally the flue gases passes different cleaning steps such as electric filters wet scrubbers etc., indicated by filter F in the figure.

In this example is only one first heating surface HE1 , one superheater SH1 and one economiser ECO shown, but the number of heating surfaces, superheaters and economisers could be larger than one, and connected in a sequence differing from this embodiment. For example could in the direction of flue gases following sequence be installed; 1 ) a first heater surface, 2) a first superheater, 3) a second heater surface, 4) generating bank and economiser.

Now, according to the invention is a parallel furnace 21 arranged close to or integrated with the ordinary recovery boiler structure. In this parallel furnace 21 is the lignin rich fraction L_RICH burnt in a dedicated lignin burner 1 1 in the bottom of the parallel furnace. The flue gases from this combustion are passing a superheater SH2 that is integrated with the steam generation system of the recovery boiler. In this case is the superheater SH2 in the parallel furnace the second superheater of the steam generation system, and the steam already heated by the superheater SH1 in the recovery boiler is passed in series in this second or final superheating stage. The steam parameters could be raised considerably in this final superheater, and the steam temperature could be elevated to 450-550 °C without risking the superheater tubes even if ordinary material is used in the tubes.

Depending on the ratio between lignin rich fraction and lignin lean fraction from the process, more or less of the total superheating of the steam will be done in SH1 or SH2. In an extreme case could also all superheating of the steam take place in SH2, and the superheating surfaces SH1 in the main furnace could be omitted.

After the flue gases in the parallel furnace has passed the final superheater SH2 could the flue gases be merged into one flow with the remaining flue gases coming from the main furnace of the recovery boiler. The merger could take place in a position after the first heating surfaces of the main furnace but before the final gas treatment stages of the main furnace (i.e. somewhere before final cleaning F). In the embodiment shown in the figure is the flue gases merged before the merged flue gases passes the superheater of the recovery boiler.

As a further option could also an additional fuel 40 be used as a additional or supplemental fuel in the parallel furnace as indicated in the figure. This additional fuel could be burnt in a dedicated burner as shown, or possibly use the lignin burner 1 1 , and be activated if the steam heating requirements exceed the possibilities obtained from burning lignin only.

The invention is not limited to the embodiment shown, and several differing embodiments could be possible within the scope of the subsequent claims.

Claims

PATENT CLAIMS

1. A method for recovering chemicals from black liquor obtained from a chemical pulp mill in a recovery boiler having a main furnace while producing steam from heating surfaces in the recovery boiler, characterised in that

• At least a part of the black liquor is subjected to a lignin separation process, resulting in a lignin rich fraction and a lignin lean fraction

• At least a part of the lignin rich fraction is burnt in a parallel furnace having at least one final heating surface for steam, and • The remaining part of the black liquor and the lignin lean fraction is burnt in the main furnace,

• And the steam produced is first heated in the heating surfaces of the main furnace, and finally heated by the final heating surfaces of the parallel furnace.

2. A method according to claim 1 characterised in that the lignin rich fraction is obtained from a precipitation, filtration or a chemical reaction process.

3. A method according to claim 1 characterised in that the flue gases from the main furnace and the flue gases from the parallel furnace is merged into one flue gas flow in a position after the first heating surfaces of the main furnace but before the final flue gas treatment stages of the main furnace.

4. A method according to claim 1 characterised in that the parallel furnace is also fired with an additional support fuel besides the lignin rich fraction