WO2012113987A1

WO2012113987A1 - Method and apparatus for burning raw material

Info

Publication number: WO2012113987A1
Application number: PCT/FI2012/050175
Authority: WO
Inventors: Antero Moilanen; Pasi Vainikka; Esa Kurkela
Original assignee: Teknologian Tutkimuskeskus Vtt
Priority date: 2011-02-23
Filing date: 2012-02-22
Publication date: 2012-08-30
Also published as: FI20115174A0

Abstract

The invention relates to a method and apparatus for burning raw material (3) including biomass-based and/or waste-based raw material in a combustion reactor (1) in such a way that the raw material is fed to the firebox of the combustion reactor, wherein the raw material is burnt. According to the invention, a calcium compound (4) is provided to the firebox partially or entirely in the form of calcium carbonate, the temperature and partial pressure of carbon dioxide in the gas atmosphere of the firebox are adjusted (5) during the combustion in such a way that calcination of the calcium carbonate is hindered and the maintenance of the calcium compound in the carbonate form in connection with the combustion is furthered, in which case the calcium carbonate forms a reactive bonding surface with alkali metal based substances, and the unwanted alkali metal based substances are conveyed (7) out from the firebox by means of the calcium carbonate.

Description

METHOD AND APPARATUS FOR BURNING RAW MATERIAL

FIELD OF THE INVENTION

The invention relates to a method defined in the preamble of claim 1 and to an apparatus defined in the preamble of claim 8 for burning raw material, e.g. biomass and waste.

BACKGROUND OF THE INVENTION

Known from .the prior art are different combustion methods and apparatuses for the production of energy products from different raw materials. It is known to burn different biomass-based and waste-based raw materials for example for the production of elec- tricity and heat.

The greatest problem in the combustion of biomass-based fuels, particularly agricultural biomass- es, is often the strong slag formation and corrosion behavior of ash that impedes the functioning of the combustion process. The main reason for this is the high alkali metal, halogen and silicon content in biomass fuels. The alkali metals, such as potassium and sodium, form chemical compounds which melt and evaporate at the temperatures of the combustion firebox, thus leading to the formation of different slag depos- . its, which may hamper the operation of the process by blocking the gas flows, impairing heat transfer, etc. In addition, the deposits easily corrode those surfaces on which they accumulate. In waste-based fuels, al- so other metals, such as zinc, lead and other heavy metals, can act in an equivalent manner.

The biomass- and waste-based fuels contain alkali and heavy metals as different compounds which are typically water-soluble and which are bound to or- ganic material, as well as internal and external min- eral or material particles. The most strongly harmful ones are^ the water-soluble compounds and those bound to the organic material. These compounds r-eact or melt as eutectic mixtures with other ash components, such as silicon or bed materials of the fluidized-bed combustion, in which case agglomerates are formed, leading to malfunctioning of the fluidized bed. Some of these compounds easily evaporate in the conditions of the firebox, and the vapor migrates along with gases to cooler parts of the process, wherein the vapor condenses and may thus form deposits.

Known from publication "Effect of additives on behavior of alkali metals during straw combustion", Ma, Xiao-qin; Luo, Zhong-yang; Fang, Meng- xiang; Yu, Chun-jiang; Cen, Ke-fa, State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou,: Peop. Rep. China, Zhejiang Daxue Xuebao, Gongxueban (2006), 40(4), 599-604, is a method wherein experimental combustion of straws has been carried out without and alternatively with additives. E.g. calcium carbonate has been used as an additive. The method is not able ^■ to prevent calcination of the calcium carbonate.

It is known from publication "Combustion in bubbling fluidized bed with bed material of limestone to reduce the biomass ash agglomeration and sintering", Fernandez Llorente, M. J.; Escalada Cuadrado, R.; Murillo Laplaza, J. M. ; Carrasco Garcia, J. E. Centro de Investigaciones Energeticas, CEDER, Fuel (2006), 85(14-15), 2081-2092, that, in the method, limestone is added to fluidized-bed combustion. The end result is that the limestone particles reduce agglomeration and sintering. In the method, limestone is used as an additive, but the drawback of the method is that the limestone is calcinated in the combustion process. Known from publication WO0011115 is fluid- ized-bed _: gasification of biomasses, wherein slagging problems can be avoided by means of limestone.

OBJECTIVE OF THE INVENTION

The objective of the invention is to disclose a new type of a method and apparatus for burning biomass-based and waste-based raw materials without disturbances.. The disadvantages of fluidized-bed combustion referred to above can be prevented by conveying the alkali and other active agents out of the combustion by means of a calcium-containing agent to be fed to the firebox.

SUMMARY OF THE INVENTION

The method and apparatus according to the invention for burning biomass-based and/or waste-based raw material in a combustion reactor are characterized by what has been presented in the claims.

The invention is based on a method for burning raw material including biomass-based and/or waste-based raw material in a combustion reactor in such a way that the raw material is fed to the firebox of the combustion reactor wherein the raw material is burnt. According to the invention, a calcium compound partially or entirely in the- form of calcium carbonate is provided into the firebox, the temperature and partial pressure of carbon dioxide in the gas atmosphere of the firebox are adjusted during the combustion in such a way that calcination of the calcium carbonate is hindered, preferably prevented, e.g. the calcination is prevented substantially entirely or the calcination is considerably reduced, and the maintenance of the calcium compound at least partially in the carbonate form in connection with the combustion is furthered, in which case calcium carbonate forms a reactive bonding sur- face with alkali metal based substances, and unwanted alkali metal based substances are conveyed out of the firebox by means of the calcium carbonate.

In addition, the invention is based on an apparatus corresponding to the method for burning raw material including biomass-based and/or waste-based raw material, the apparatus including a combustion reactor and feeding means for feeding the raw material to the firebox of the combustion reactor, wherein the raw material is burnt. According to the invention, the apparatus includes feeding means for feeding a calcium compound to the firebox, the calcium compound being at least partially in the form of calcium carbonate or being provided at least partially into the form of calcium carbonate in the firebox, adjusting means for adjusting the temperature and partial pressure of carbon dioxide in the gas atmosphere of the firebox during the combustion in such a way that calcination of the calcium carbonate is hindered and the maintenance of the calcium compound in the carbonate form in connection with the combustion is furthered,: in which case calcium carbonate forms a reactive bonding surface with alkali metal based substances, and means for conveying unwanted alkali, metal based substances out of the firebox by means of the calcium carbonate.

Biomass-based raw material in this context means any_: biomass-based raw material, such as wood, wood-based raw material, agricultural mass, straw, bi- osludge, other solid biomass materials, biomass-based waste and the like and combinations thereof.

Waste-based raw material in this context means any waste material containing organic components, e.g. agricultural waste, domestic waste, industrial and commercial waste, construction waste and other useful waste according to the EU definition of waste. A calcium compound in this context means any calcium-containing compound which may contain one or more calcium-based agent components. The calcium compound may contain calcium carbonate, calcium oxide, other calcium-based agent and different mixtures thereof.

In one embodiment of the invention the reactive bonding surface is a molten and eutectic bonding surface. Eutectic in this context means a mixture having a melting point lower than the melting point of the agents included in the mixture individually.

Typically, compounds of the alkali metals included in biomass-based and waste-based raw materials cause problems in the combustion process by forming agglomerates and causing corrosion. According to the invention, this can be prevented by removing the alkali metals from the firebox by calcium being present in the carbonate form. The advantage of calcium carbonate as compared to other additives is that it forms a reactive bonding surface in the carbonate form, yet being brittle to be ground and convey the alkali metals out of the firebox along with dust. Calcium carbonate is disintegrated, i.e. calcinated, in the conventional combustion conditions. In this method according to the invention^■ the formation of carbonate is made possible and calcination prevented by adjusting the temperature of the firebox and the carbon dioxide content of the gas atmosphere, i.e. gas layer, of the firebox. In addition, it is essential of the method that calcium remains in the firebox entirely or partially in the carbonate form and that this amount of the carbonate can be adjusted.

In one embodiment of the invention the temperature and partial pressure of carbon dioxide in the gas atmosphere of the firebox are adjusted during combustion in such a way as to make the formation of calcium car- bonate from the calcium compound possible in the firebox .

: In one embodiment potassium is removed by means of calcium carbonate from the firebox. In one embodiment sodium is removed by means of calcium carbonate from the firebox.

In one embodiment of the invention the partial pressure of carbon dioxide in the gas atmosphere of the firebox is provided sufficiently high and the temperature of the firebox sufficiently low. In one embodiment the partial pressure of carbon dioxide in the gas atmosphere of the firebox is provided sufficiently high and the temperature of the firebox sufficiently low based on the reaction equilibrium representative: of calcination of calcium carbonate, but not necessarily for requiring the providing of the equilibrium. ;

Typically in the combustion conditions the calcium carbonate is disintegrated, i.e. calcinated, in which case calcium oxide and carbon dioxide are formed. The calcium oxide generated in the calcination is easily ground to dust and then removed from the firebox along with gas flows. Calcium carbonate -is maintained or the calcium compound forms a carbonate when the partial pressure of the carbon dioxide in the gas layer of the firebox is sufficiently high and the temperature of the firebox is sufficiently low. According to the reaction equilibrium representative of calcination, for example at a temperature of 850°C the partial pressure of carbon dioxide of 0.6 bar, i.e. 60 vol-% C0₂, prevents the calcination, at 800°C the corresponding amount is 0.25 bar, i.e. 25% vol-% C0₂, and at a temperature of 750°C 0.1 bar, i.e. 10 vol-% C0₂. In addition, it is to be noted that at carbon dioxide contents close to the equilibrium values the calcination slows down, which is why the carbon dioxide con- tents close to the equilibrium contents can also have such an effect as to have a sufficient degree of the calcium compound in the carbonate form present in the firebox. : This information can also be used in the method to adjust the amount of fines being removed. When calcination is partial, there may appear porosity, which is advantageous in such a way as to convey alkali metal into the particle of the calcium compound, and not to form any adhesive layer caused by the melt to the surface. By adjusting the temperature and carbon dioxide content, the calcination can be prevented and the conditions are provided suitable for removal of the alkali metals by means of calcium carbonate. In addition, calcium oxide that may be formed in calcination of the calcium carbonate or other reaction, such as when the organic calcium compound is released in the combustion, may be recarbonated in the conditions where the temperature and partial pressure of carbon dioxide are suitable. Particularly in oxygen combustion applications, such adjustment of the carbon dioxide content is possible. In addition, for example the sand seal of a circulating fluidized process can be used for the formation of carbonate, which assures the presence of the carbonate in the firebox even when calcinating conditions prevailed in the firebox per se.

Typically, the operating principle of additives preventing the slagging of ash is to raise the melting point of the ash or prevent bonding of the component ;of adhesive molten ash and thus prevent the formation of agglomerates and deposits. One bed material developed for fluidized-bed combustion, Ag- glostop™, has been developed to prevent bed agglomeration. The operation of the bed material in question is based on the fact that alkali metal silicate typically generated in the combustion and causing the agglomera- tion _. cannot be generated because the bed is quartz- free. However, alkali metal silicate is formed in the combustion of straw or equivalent silicon-containing fuel, and it will in time bind the rest of the ash, which leads to agglomeration. In the method being the object of the invention alkali metal is conveyed out in such a way as to prevent or reduce the formation of the silicate to a significant degree. In addition, in comparison to other additives, calcium carbonate is different in that it forms a reactive bonding surface while remaining as a carbonate, yet being sufficiently brittle to be ground and convey alkali metals out along with dust generated as a result of the grinding.

The uniqueness of the invention lies in the fact that the conditions in the firebox are so adjusted that the calcium carbonate is not calcinated or is calcinated in a controlled manner, i.e. the calcination rate is adjusted. The grinding of calcium carbonate can be adjusted by means of the controlled calcination reaction. The control is effected by means of the carbon dioxide content and temperature. It is possible to convey the material bound as a result of grinding to the surface of calcium carbonate out from the reaction zone. The grinding tendency is calcium carbonate specific. Adjusting effected by means of the carbon dioxide content is based on adjusting the calcium carbonate specific calcination rate. The calcina^¬ tion slows down when effected in a gas layer containing carbon dioxide as compared to a gas not containing carbon dioxide.

Calcium carbonate can be mixed with the fuel, i.e. raw material, or added to the firebox as separate feed. In one embodiment calcium carbonate is fed to the firebox. In one embodiment calcium carbonate is fed to the firebox mixed with raw material. In one embodiment calcium carbonate is fed to the ^'firebox as inde- pendent feed. In one embodiment calcium carbonate is formed from a calcium compound in the firebox or other part of the combustion reactor. Formation of the calcium carbonate in the firebox can be affected by adjusting the temperature and partial pressure of carbon dioxide.

In one embodiment of the invention the amount of calcium carbonate in the firebox is adjusted, e.g. by means of the temperature and carbon dioxide content, i.e. partial pressure of the carbon dioxide, in the firebox.

In one embodiment of the invention, wherein potassium is used as an example, the operating mechanism is realized as follows. A molten, eutectic bonding surface is formed between potassium chloride and calcium carbonate at the contact site between them, in which case the potassium chloride is bound to the calcium carbonate. Such a eutectic temperature, i.e. the lowest melting temperature, may be approximately 720°C. Also, potassium carbonate forms . a melt with calcium carbonate. The lowest melting temperatures of such mixtures may be less than 800 °C.

In one embodiment of the invention, wherein potassium . is used as an example, the operating mechanism is realized as follows. The potassium reacts at the contact site with calcium carbonate, in which case K₂Ca(C0₃)₂ is formed. The compound is known as the butschlite mineral, and it has been found in ash generated in the combustion of wood. This type of reaction may be possible when the potassium compound included in biomass reacts into potassium oxide, which happens for example as potassium nitrate used as fertilizer on fields is disintegrated at high temperatures. The compound may also be formed as a result of a reaction between potassium carbonate and calcium carbonate . ^: The method according to the invention is suitable for fluidized-bed combustion applications, and particularly for oxygen combustion, wherein the carbon dioxide content can be adjusted by circulating flue gases and/or limiting the feeding of air, e.g. entirely : refraining from the use of air.

By virtue of the invention, it is possible to maintain the carbonate form also in gasification by means of temperatures lower and partial pressure of the carbon dioxide higher than the combustion.

The method according to the invention differs from the feeding of lime designated for desulphuriza- tion .

The combustion boiler may in this context be any kind of boiler known per se, either a large size- class power station boiler or a small combustion boiler.

The method and apparatus according to the invention provide considerable advantages in comparison with the prior art.

The method makes possible the burning of bio- masses, such as agricultural waste, straw and other biomasses; cultivated for energy production as well as different: types of waste, the greatest problem of which is ^' typically their strong slag formation and corrosion behavior of ash. Such biomasses, for example in the area of the EU, constitute a significant portion of the biomasses suitable as fuels.

By virtue of the invention, a good quality product can be formed cost-effectively without the strong slag formation and corrosion behavior of ash. By virtue of the invention, it is possible to utilize different raw materials and raw material components which may not have been utilized before cost- effectively and particularly in fluidized-bed combustion. LIST OF FIGURES

Fig. 1 shows one apparatus according to the invention as a simplified schematic illustration, and

Fig. 2 to 4 show test results from combustion tests .

DETAILED DESCRIPTION OF THE INVENTION

In the following section, the invention will be described using detailed examples of its embodiments with reference to the accompanying Figures 1 to 4.

Example 1

In this example one method according to the invention and one apparatus to be used for carrying out same are described with reference to a schematic illustration (Fig. 1) .

In the method, the raw material used is bio- mass-based and waste-based raw material (3) .

The apparatus includes a fluidized-bed combustion reactor (1) and feeding means (2) for feeding the raw material (3) to the firebox of the combustion reactor (1), wherein the raw material is burnt. The apparatus includes feeding means for feeding a calcium compound (4) to the firebox and adjusting means (5) for adjusting the temperature and partial pressure of carbon dioxide in the gas atmosphere of the firebox during the combustion. In addition, the apparatus includes means (6) for conveying unwanted alkali metal based substances (7) out from the firebox.

The raw material is burnt in the fluidized- bed combustion reactor in such a way that the raw material is fed to the firebox of the combustion reac- tor, wherein the raw material is burnt. Calcium carbonate is fed to the firebox and/or calcium carbonate is formed from a calcium compound in the firebox. In the firebox, the temperature and partial pressure of carbon dioxide in the gas atmosphere are adjusted during combustion in such a way that calcination of calcium carbonate is prevented and the maintenance of the calcium carbonate during the combustion is furthered. In this case, calcium carbonate forms a reactive bonding surface with alkali metal based substances, and the unwanted alkali metal based substances, e.g. potassium, are conveyed out from the firebox by means of calcium carbonate.

Example 2

The operating effect of the invention was verified by fluidized-bed tests, wherein straw was burnt both in a_. conventional manner by means of air and in a carbon dioxide containing gas atmosphere, i.e. gas layer. The tests were carried out using natural sand as bed material, to which the straw was fed as such and mixed with calcium carbonate. The straw mixed with the calcium carbonate was burnt in a carbon dioxide containing gas layer. The straw was burnt as a pellet aggregate suitable as feed for the fluidized- bed reactor. The chlorine content of the straw was

0.7% with a potassium content of 0.7% in dry material. In the combustion tests the temperature was 850 °C both in the fluidized bed and in the top part thereof. The carbon dioxide content of the carbon dioxide containing gas layer was 80 vol-%, the residual 20 yol-% being oxygen. Calcium carbonate was mixed into the straw fuel in an amount corresponding to the ash content,

1. e. 5.9g per lOOg of moist fuel, wherein moisture of the fuel was approximately 10%. The result was that the straw without addition of the calcium carbonate quickly formed fluidization impeding agglomerates which were not formed when the straw was burnt with calcium carbonate in the carbon dioxide containing gas layer .

Example 3

In a second combustion test series straw was burnt in a fluidized-bed reactor, wherein sand was used as bed material in a first test and granular limestone in a second one. The temperature level was approximately 770 °C, and the carbon dioxide content of gas approximately 15 to 16 vol-%. In using sand as bed material, the test had to be stopped after approximately one hour and 28 minutes from the start of the test as a result of bed agglomeration. In using limestone, the combustion test was finished after 2 hours, during which time there were no signs of bed agglomeration when fluidization of the bed was monitored by means of pressure difference measurement carried out over the bottom part of the reactor.

Fig. 2 to 4 show test results in an exemplary fashion for this second test series of fluidized-bed combustion. In the test, wheat straw has been burnt, the gas atmosphere and temperature of the reactor adjusted in two different cases: i) using a natural sand bed and ii) using a limestone bed. Fig. 2 shows that the bed has agglomerated after lh and 28min from the start of feeding the fuel. Fig. 4 shows that agglomeration did. not appear after 2h in using a limestone bed in equivalent conditions. For example, due to calcination rates of different types of limestone, the tem- perature of the limestone bed may be slightly higher than suggested by the equilibrium temperature for calcium to be maintained in the carbonate form at different partial pressures of carbon dioxide, which is shown in Fig. 3. Fig. 3 shows an example of the par- tial pressures of carbon dioxide at different temperatures for forming calcium oxide and calcium carbonate. However, adjusting the temperature and partial pressure of carbon dioxide is essentially important. Fig. 4 shows that even a small increase in the temperature may cause calcination of the limestone bed, the resulting quick grinding and loss of the bed as fly ash. At the same time, carbon dioxide is released as disclosed in Fig. 4. The condition of combustion must therefore.be determined based on Fig. 3 but, on experimental bases and depending on the type of limestone, a higher temperature or lower partial pressure of the carbon dioxide can be used.

The temperature of the firebox can be adjusted for example: i) by circulating gas, wherein combustion gases to be fed to the combustion device are par- tially or entirely flue gas; ii) by thermal sizing of the firebox (load per surface area or per volume) ; or iii) by positioning of heat transfer devices. The gas atmosphere can be adjusted by providing circulating gas mixed with combustion air or by replacing the com- bustion air.

The method and apparatus according to the invention are suitable as different embodiments to be used in most different combustion processes using most dif- ferent raw; materials .

The invention is not limited merely to the examples referred to above; instead, many variations are possible within the scope of the inventive idea defined by the claims.

Claims

1. A method for burning raw material including biomass-based and/or waste-based raw material in a combustion reactor in such a way that the raw material is fed to the firebox of the combustion reactor, wherein the raw material is burnt, c h a r a c t e r i z e d in that a calcium compound is provided to the firebox partially or entirely in the form of calcium carbonate, the temperature and partial pressure of carbon dioxide in the gas atmosphere of the firebox are adjusted during the combustion in such a way that calcination of the calcium carbonate is hindered and the maintenance of the calcium compound in the carbonate form in connection with the combustion is furthered, in which case the calcium carbonate forms a reactive bonding surface with alkali metal based substances, and the unwanted alkali metal based substances are conveyed out from the firebox by means of the calcium carbonate.

2. The method according to claim 1, c h a r -. a c t e r i z e d in that calcium carbonate is fed to the firebox.

3. The method according to. claim 1 or 2, c h a r a c t e r i z e d in that calcium carbonate is formed from a calcium compound in the firebox.

4. The method according to any one of claims 1 to 3, c h a r a c t e r i z e d in that the amount of calcium carbonate is adjusted.

5. The method according to any one of claims 1 to 4, c h a r a c t e r i z e d in that the temperature and partial pressure of carbon dioxide in the gas atmosphere of the firebox are adjusted during combustion in such a way as to make formation of calcium carbonate from a calcium compound possible in the firebox.

6. The method according to any one of claims 1 to 5, c h a r a c t e r i z e d in that the partial pres- sure of carbon dioxide in the gas atmosphere of the firebox is provided sufficiently high and the temperature of the firebox sufficiently low.

7. The method according to claim 6, c h a r - a c t e r i z e d in that the partial pressure of carbon dioxide in the gas atmosphere of the firebox is provided sufficiently high and the temperature of the firebox sufficiently low based on the reaction equilibrium representative of calcination of calcium carbonate.

8. An apparatus for burning raw material including biomass-based and/or waste-based raw material, the apparatus including a combustion reactor (1) and feeding means (2) for feeding the raw material (3) to the firebox of the combustion reactor, wherein the raw material is burnt, c h a r a c t e r i z e d in that the apparatus includes feeding means for feeding a calcium compound (4) to the firebox, the calcium compound being at least partially in the form of calcium carbonate or being provided at least partially into the form of cal- cium carbonate in the firebox, adjusting means (5) for adjusting the temperature and partial pressure of carbon dioxide in the gas atmosphere of the firebox during combustion in such a way that calcination of calcium carbonate is. hindered and the maintenance of the calcium compound in the carbonate form in connection with the combustion is furthered, in which case the calcium carbonate forms a reactive bonding surface with alkali metal based substances, and means (6) for conveying the unwanted alkali metal based substances (7) out from the firebox by means of the calcium carbonate.