SE542799C2

SE542799C2 - Integrated thermal treatment of acid sulfate soils

Info

Publication number: SE542799C2
Application number: SE1850665A
Authority: SE
Inventors: Markus Broström
Original assignee: Brostroem Markus
Priority date: 2018-06-01
Filing date: 2018-06-01
Publication date: 2020-07-07
Also published as: SE1850665A1

Abstract

The present invention provides a method of thermal treatment of acid sulfate soil (ASS), a method to inhibit corrosion in a combustion plant by using the sulfurous gaseous released in the thermal treatment. The invention further provides an integrated system comprising a thermal treatment plant and a combustion plant wherein the heating in the thermal treatment is effected by heat from the combustion plant, and the sulfurous gases released in the thermal treatment is used as corrosion inhibitor in the combustion plant, and related aspects.

Description

Integrated thermal treatment of acid sulfate soils Technical field of the invention The present invention relates to the field of environmental technology, and in particular to decontamination of soils containing sulfur and sulfurous compounds. The invention also relates to plants for performing the decontamination in a resource effective way.

Background of the invention Acid sulfate soils (ASS) are soils which contain significant amounts of iron sulfide minerals, predominantly as the mineral pyrite, their oxidation products, and/or organically bound sulfur. They are naturally occurring in e.g. river- and sea-beds and are commonly found in proximity to coastal areas. In an undisturbed state in oxygen free environments the sulfate soils are stable and have no impact on the surrounding environment but if such soils are exposed to air, for example by lowering of the groundwater level or excavation, the sulfides are oxidised by atmospheric oxygen thus transformed into FexOy or FeS04. If also moist is available, sulfuric acid is formed, causing the problems associated with ASS. 4FeS 9O2 + I0H2O -> 4Fe(OH)3 + 4H2SO4 4FeS2 +15O2 + 14H2O -> 4Fe(OH) 3 + 8H2SO4 The sulfuric acid released from the soil then causes acidification of the soil and release of metals, including heavy metals within the soil. Once mobilized, the acid and metals can seep into and acidify groundwater and other proximal bodies of water with killing of fish and other aquatic organisms as a consequence. The acid leakage may also cause degradation of concrete and steel structures to the point of failure.

Besides the problematic sulfur content, ASS usually also contains organic material which contributes to making the soil unsuitable as construction material or as foundation for new buildings or roads.

The problem with acid leakage from ASS is generally solved by burrowing the soil in oxygen free deposits, typically below the groundwater level for example in moors, hoping that the sulfides will remain stable in that environment.

Another method to handle the problem with acid sulfate soils is to neutralize the acid with a basic material, typically lime such as agricultural lime or lime in any other form such as hydrated lime or mixtures of hydrated lime and agricultural lime or lime slurries. The lime is then mixed with the acid sulfate soil. The neutralization process is typically performed in situ.

Combustion plants have long been used for the combustion or gasification of material containing combustible components typically fossil fuels such as coal, with a relatively high sulfur content. The heat from combustion plants is typically captured by letting exhaust gases from the engine flow through one or more heat exchangers. When burning fuels with a low sulfur content and fuels containing chlorine in heat producing plants, a sulfur containing additive can be added to the flue gas in order to inhibit corrosion and ash deposits on the superheater tubes and other parts of the plant.

AU763820 describes a method of combustion or gasification in a circulating fluidized bed wherein one or more dechlorinating agent is added to a dechlorination chamber.

EP1354167 discloses a method of obviating or abating chlorine induced corrosion in heat producing plants by injecting a sulfur containing additive into the flue gas of the heat producing plant.

US 5,011,329 discloses in situ treatment of contaminated materials to remove unwanted substances there from, particularly in situ treatment of soil to remove or obviate the effects of toxic or undesirable substances.

US5,264,654 discloses a method of and an apparatus for decontaminating contaminated soils wherein the contaminants are volatilizable at or below about 1 200 °C.

Summary of the invention In one aspect, the present invention provides a method for reducing sulfur content in Acid Sulfate Soil (ASS) by thermal treatment of ASS, comprising the steps of i) drying and comminuting the ASS to obtain ASS powder; ii) heating the ASS powder to a temperature of 150-800 °C; and iii) removing gaseous SO2 and SO3 released from the ASSpowder in step ii).

In one embodiment of this aspect, the ASS powder is heated at a temperature of 150 - 600 °C.

In one embodiment of this aspect, the ASS powder is heated at a temperature of 200 - 400 °C.

In one embodiment of this aspect, the ASS powder is heated at a temperature of 200 - 300 °C.

In one embodiment of this aspect, the method further comprises the step iv) leaching sulfate ions from the heat treated ASS powder.

In one embodiment of this aspect, heat from a combustion plant of temperatures in the range 70 °C -400 °C is used for heating in the thermal treatment.

In an additional aspect, the invention provides a method to inhibit corrosion in a combustion plant by using gaseous SO2 and/or SO3 comprising steps i) drying and comminuting the ASS to obtain ASS powder; ii) heating the ASS powder to a temperature of 150-800 °C; and iii) removing gaseous SO2 and/or SO3 released from the ASS powder in step ii) wherein the gaseous SO2 and/or SO3 removed in step iii) are collected and fed into the combustion zone of said combustion plant.

In this aspect, the step of iv) leaching sulfate ions from the heat treated ASS powder may optionally be added.

In a representative embodiment of this aspect, the ASS powder is heated at a temperature of 200 - 400 °C.

In a representative embodiment of this aspect, the ASS powder is heated at a temperature of 200 - 300 °C.

The invention further provides the method of thermal treatment wherein the thermal treatment is performed in an integrated system comprising a plant for thermal treatment of ASS and a combustion plant.

In an additional aspect, the invention provides a system for combined thermal treatment of ASS and generation of heat and/or power, said system comprising a plant for thermal treatment of ASS and a combustion plant for generation of heat and/or power by combustion of fuel, said system comprising a gas feeding system for feeding gaseous SO2 and SO3 released in the plant for thermal treatment of ASS into the combustion zone of the combustion plant and a heat transfer system for transferring heat from the combustion plant to the plant for thermal treatment of ASS.

In one embodiment of this aspect, the invention provides a system wherein the plant for thermal treatment of ASS is heated with heat having a temperature of 70 - 400 °C from the combustion plant.

Brief description of the drawings The invention is illustrated in the drawings as follows: Figure 1 is a flow scheme showing the general concept of the invention Figure 2 shows a phase equilibrium diagram for a diluted Fe-S-0 system.

Figures 3a and 3b shows a phase stability diagram at 300 °C.

Figure 4 shows TG-FTIR curves of an ASS sample Definitions The expression "Acid sulfate soils" and "ASS" are used interchangeably herein and refer to soils which contain mineral iron sulfides. Typically the mineral iron sulfides are FeS and Fe2S. A part of the sulfur in ASS can also be present in organically bound forms.

The expression "Waste fuels" as used herein should be understood as materials that already have been used for another purpose. Examples of waste fuels include but is not limited to municipal solid waste, industrial waste and demolition wood.

The expression "Bio fuels" refers to fuel from organic matter, obtained directly from plants or indirectly from agricultural, commercial, domestic, and/or industrial wastes. Generally, biofuels refers to materials from the nature that are utilised mainly without other purposes than producing energy.

The expression "combustion plant" as used herein is meant to include plants producing power, plants producing heat and combined heat and power (CH P) plants i.e. plants producing heat and power by combustion. Typically in connection with the present invention, the fuel used for the combustion plant is a biofuel or coal.

The term "pSCV' stands for the partial pressure of SO2. Similarly, PSO3 means partial pressure of SO3.

The expression "sulfurous gases" refers to SO2 and SO3.

The term "SOx" stands for SO2 and/or SO3.

The term "FeSx" stands for FeS and/or FeS2unless otherwise defined.

Detailed description of the invention The present invention relates to thermal treatment of sulfur containing soils in order to reduce or eliminate the amount of sulfur in the soil, thus preventing acid leakage to nearby recipients.

The invention further provides an integrated system comprising a plant for thermal treatment of sulfur containing soil and a combustion plant wherein gaseous sulfuric dioxide and trioxide released during the thermal treatment is used as anti-corrosive agents in the combustion plant.

Additionally, the invention provides the use of heat from a combustion plant for the thermal treatment, wherein the thermal treatment plant and the combustion plant constitute parts of an integrated system.

The invention provides a method to decontaminate sulfur containing soil by thermal treatment of the soil. In the thermal treatment according to the invention, the soil is heated to temperatures high enough to effect oxidation of the sulfur present in the soil to gaseous sulfuric dioxide (SO2) and trioxide (SO3). The present invention is based on the surprising finding that sulfur containing soils release sulfur to the gas phase at considerably lower temperatures than what is previously described for the corresponding minerals FeS and FeS2, i.e. the predominant form of the sulfur in the soil A. Boman, M. Åström, S. Fröjdö, Chemical Geology, 255 (2008) 68-77. This finding indicates that the sulfur is not exclusively present in these or corresponding forms but also, and to a higher extent than previously assumed, in much less thermally stable forms as organic bound sulfur, or sulfur existing as an excess in non-stochiometric compounds or in significantly smaller grain sizes than typically used in previous studies. Temperatures as low as approximately 150 - 250 °C have proven sufficient to bring about oxidation of the sulfur of the soil and release of SO2 and SO3 in contrast to 600 - 800 °C as would have been expected if the sulfur of the soil was present mainly as large crystals of FeS and Fe2S, A. Boman, M. Åström, S. Fröjdö, Chemical Geology, 255 (2008) 68-77, H. Y., F. B., Berichte der Bunsengesellschaft för physikalische Chemie, 101 (1997) 1870-1881. Accordingly, the invention provides a method for thermal treatment of sulfur containing soils wherein the treatment is performed at temperatures in the range 150 - 250 °C. Suitably, reactors for the thermal treatment of ASS according to the present invention are dimensioned for temperatures in the interval 200-400 °C The sulfur containing soils have shown to be of varying thermal stability, implying that FeS and Fe2S are present together with sulfur in other forms in the soils to a varying extent, and in the more thermally stable soils, i.e. larger grains of FeS and Fe2S, may remain in the soil after thermal treatment according to the invention. At the temperatures used in the thermal treatment according the invention any remaining FeS and Fe2S will, if not fully oxidized releasing the sulfurous gases, be transformed to FeS04 which is an easily water-soluble compound that can be removed by washing. The thermal treatment according to the invention therefore opens for a simple and rapid washing/leaching as a final cleaning step. The leaching step is thus especially suitable for more thermally stable soils, where the thermal treatment according to the invention may be insufficient to remove all the sulfur present. Once free from sulfur, the remedied soil can be replaced at the removal location, or alternatively it can be used as filling material for various constructions, such as roadworks, railworks, river banks.

Organic materia, typically the remaining of plants and animals decomposed in sediments, that may be present in the ASS to a various extent will make the remedied soil difficult to use for constructions e.g. as filling material as it is and thus restrict the value and utility of the soil. The present invention is in part based on the finding that at certain temperature intervals used in the thermal treatment according to the method of the invention, also organic materia present in the soil is combusted. It has been found that the organic materia is incinerated at temperatures in the interval 200-400 °C. Accordingly, the present invention provides thermal treatment of soil containing sulfur and organic materia wherein the soil is heated at 200 - 400 °C, thus providing a soil which is substantially free from sulfur and organic materia.

Accordingly, in one aspect, the invention provides a method for reducing the sulfur content in Acid Sulfate Soil (ASS) by thermal treatment of ASS, comprising the steps of i) drying and comminuting the ASS to obtain ASS powder; ii) heating the ASS powder to a temperature of 150-800 °C; and iii) removing gaseous SO2 and/or SO3 released from the ASS powder in step ii).

Optionally, the removal location is subsequently replenished with purified soil.

The thermal treatment according to the invention can be performed in any suitable plant available on the market or described in the literature. Suitable plants constitutes part of the prior art and are well known to a person skilled in the field. For example, rotary kilns or fluidized beds can be used. In addition to efficient heating of the material, good mixing of the ASS powder and the supplied air in the reactor is required for efficient sulfur removal.

In one embodiment of the invention, the thermal treatment plant is a mobile unit which can be placed and used at or nearby the site of the ASS.

As stated above, the thermal treatment according to the invention is performed at temperatures in the interval 150-800 °C.

In one embodiment of the invention, the ASS powder is heated at temperatures in the interval 150-600 °C.

In a further embodiment of the invention, the ASS powder is heated at temperatures in the interval 200-400 °C.

Typically the ASS powder is heated at temperatures in the interval 200-300 °C.

In one embodiment, the method of the invention provides a step of leaching sulfate ions from the heat treated ASS powder.

Parts of the sulfides of the ASS may remain after the thermal treatment according to the invention due to high mineral stability or poor gas exchange in the reactor. In such cases the sulfides may be transformed to water soluble FeSO4, a compound which is highly water soluble and can be removed by leaching. Accordingly, in one embodiment, the method according to the invention provides a step of leaching sulfate ions from the heat treated ASS powder.

In one embodiment, the method of the invention provides use of heat from a power and/or heat plant having temperatures of about 70-400 °C for the heating in the thermal treatment of the invention.

In one embodiment, the use of heat from a power and/or heat plant having temperatures of about 70-150 °C for the heating in the thermal treatment is provided.

In one embodiment, the use of heat from a power and/or heat plant having temperatures of about 90-120 °C for the heating in the thermal treatment is provided.

When heat from a power and/or heat plant in the lower range is used, such as in the interval of about 70-150 °C, or about 90-120 °C heat exchange techniques is conveniently used to produce heat of the temperature required for the thermal treatment. For the first part of the heating, wherein mainly drying of the ASS is effected i.e. from ambient temperature up to about 150 °C, heat from low value excess heat sources, e.g. condensing heat in a power plant in combination with heat exchange techniques can be used. Heat exchange techniques and useful heat exchangers for this purpose are known to the skilled person and are available on the market and/or extensively described in the literature.

In one embodiment of the invention, the thermal treatment further comprises the step of iv) leaching sulfate ions from the heat treated ASS powder.

A stand-alone unit can be used in the method of thermal treatment according to the invention, but the temperature range required makes it suitable for system integration with a combustion plant. The combustion plant in the method can be a heat plant, a power plant or a heat and power (CHP) plant wherein heat from the combustion plant is used for heating the soil of the thermal treatment.

Accordingly, the invention provides a method of thermal treatment of ASS wherein the thermal treatment is performed in an integrated system comprising a plant for thermal treatment of ASS and a combustion plant. In one embodiment, the combustion plant is a CHP plant.

Combustion plants, e.g. CHP plants have long been used for the combustion or gasification of material containing combustible components, typically fossil fuels such as coal with a relatively high sulfur content. In recent years however, combustion of solid fuels in the form of bio and waste fuels have become an established technique for combined heat and power production. Bio and waste fuels though have in several aspects proven more difficult to burn than traditional hydrocarbon fuels and one of the major and most costly problems is corrosion and ash deposits on the superheater tubes, exhaust pipes and other parts of the combustion plant. Solid bio- and waste fuels typically have a lower sulfur content and a higher chloride content than traditional hydrocarbon fuels which make them highly prone to cause corrosion. Responsible for the corrosion when burning bio and waste fuels is mainly the higher chloride content in the flue gas. The corrosion may be inhibited by addition of sulfur at the burning e.g. by co-burning a fuel with a low sulfur content with a fuel of higher sulfur content such as coal. A drawback with this method is that the share of coal has to be relatively large (20-50%) in order to achieve a pronounced reduction of chlorine to the superheater, which is not always consistent with environmental requirements.

An alternative method to reduce the problem with chlorine induced corrosion is to add an anti-corrosive agent, e.g. ammonium sulfate, ammonium bisulfate or any other sulfur containing additive to the fluegas flow of the plant. This method is also connected with drawbacks, for instance the significant cost associated with use of additives when operating a heat and/or power plant.

The invention further provides the use of the gaseous SO2 and SO3 released in the thermal treatment of the ASS as anti-corrosive additives in a combustion plant, such as a heat and/or power plant e.g. a CHP plant. According to the invention, the gaseous SO2 and SO3 released in the thermal treatment are captured and fed into the combustion zone of the combustion plant. Apart from being readily available additives, the use of gaseous SO2 and SO3 according to the invention removes the need of separately taking care of and destruct the liberated gases since if SO2 and/or SO3 is added in surplus, it will be captured in the sulfur removal parts of the flue gas cleaning system.

Accordingly, in one aspect, the invention provides a method to inhibit corrosion in a combustion plant by using gaseous SO2 and/or S03, the method comprises the steps of i) drying and comminuting the ASS to obtain ASS powder; ii) heating the ASS powder at a temperature of 150-800 °C; and iii) removing gaseous SO2 and/or SO3 released from the ASS powder in step ii), wherein the gaseous SO2 and/or SO3 removed in step iii) are collected and fed into the combustion zone of said combustion plant.

In one embodiment according to this aspect, the ASS powder is heated at a temperature of 150-600 °C.

Typically according to this aspect, the ASS powder is heated at a temperature of 200-400 °C.

In one embodiment of this aspect, the combustion plant is a combined heat and power (CHP) plant.

Appropriate combustion plants can be of any suitable type and are generally commercially available and constitutes a part of the prior art.

In one embodiment of this aspect, the plant for thermal treatment of ASS is heated with heat having a temperature of 70 - 400 °C from the combustion plant.

The general concept of the system according to the invention is shown in Figure 1 and detailed below: Soil (1) containing FeS, non-stochiometric FeSx (l Figure 2 shows a phase equilibrium for a diluted Fe-S-O system.

The outcome of the thermal treatment of FeSx containing soil can be predicted using thermodynamic chemical phase equilibrium calculations. Assuming that a treatment process includes air (O2) in sufficient amounts for oxidation of sulfides and organic material and to dilute the partial pressures of any SO2/SO3 formed, and also that S is available in surplus (i.e. organic sulfur), then the phase diagram shown in Figure 2 is obtained. Besides illustrating the thermal stability of the sulfate, it has also been shown that SO3 is the most likely form of sulfur leaving the ASS. SO3 is the preferred form of sulfurous gasses for use as corrosion inhibitor or for condensation to form H2SO4. The temperature for the transition between iron sulfate and iron oxide is limited by pSO2 or pSO3, and will be shifted to lower temperatures for more diluted systems. The system shown in Figure 2 is a system with low air dilution, resembling the conditions in a packed bed with limited access to air; in such cases SOx vapor pressures will be high, thus stabilizing the sulfate and limiting successful SOx removal. Even though the Fe2(SO4)3 produced in a system with low air dilution is easily water soluble and could be removed from the treated soil by simple washing, it is preferred that the whole cleaning procedure can be made in one step, i.e. that the FeS and Fe2S contained in the soil is completely oxidized to SO2 and SO3 in one single step. A one step procedure is beneficial also considering possible system integration.

FeSx not being present at any of the temperatures under oxidizing conditions is true from a chemical equilibrium point of view. However, the oxidation, or roasting, of FeSx is generally very slow and is limited by the properties of the mineral particles. At increasing temperatures, the sulfate formation rate is increased and can be expected to be significant - but the sulfate formation is not necessarily complete in low temperature process (200-300 °C). It is well known that roasting rates of pyrrhotites (Fe(1-x)S, x = 0, and up to 0.2 for non-stochiometric crystals) are higher than that for pyrite (FeS2) K. Niwa, T. Wada, Y. Shiraishi, JOM, 9 (1957) 269-273.

Figures 3a and 3b shows phase stability diagrams at 300 °C.

Under the diluted and well mixed conditions at the lower part of the diagrams, Fe2O3 is the stable phase even at temperatures for low temperature treatment (200-300 °C), meaning that the process is possible from a chemical phase equilibrium point of view at this temperature. At higher SO2 concentrations, as can be the case in large sulfide crystals or in packed beds, the sulfate is stabilized (upper parts of the diagrams). This highlights the importance of providing well mixed conditions in the reactor.

Organically bound sulfur, or sulfur existing in excess in non-stochiometric compounds, are much less thermally stable than the mineral compounds. The exact stability in ASS cannot be predicted since exact compounds are unknown and probably varying. Therefore, thermogravimetric analysis (TGA) experiments were performed coupled to FTIR analysis of the gas formed during heating of an ASS sample collected from an infrastructure project close to Umeå, Sweden. The sample was dried in nitrogen to avoid wet oxidation and to produce a sample for analysis with the non-oxidized forms which is the form most likely to be delivered to a treatment plant. The sample was heated at 10K/min, purged with air, while recording weight changes and composition of the outgoing gas. Results are shown in figure 4.

From the thermogram and gas release profiles of Figure 4 it can be seen that the main weigh loss rate (dW, i.e. the time derivative of the sample weight loss at constant heating rate) for the soil occurs already from 100 °C, increasing to reach a maximum weight loss rate at app 270 °C, followed by a decrease, except for a second maximum at app 750 °C. From the gas release profiles it can be seen that SO2 has its maximum already at 200 °C followed by two other peaks; one at 380 °C and a weaker one at 780 °C. This means that the main SO2 release occurs at even lower temperature than the temperature region for the main degradation of organic matter, seen as the CO2 peak with maximum at 300 °C. SO3 was probably also present in the released gas, but could not be distinguished from SO2 with the FTIR method used. The SO2 signal can therefore be interpreted as SOx = SO2 + SO3.

Claims

1. A method for reducing sulfur content in Acid Sulfate Soil (ASS) by thermal treatment of ASS, comprising the steps of i) drying and comminuting the ASS to obtain ASS powder; ii) heating the ASS powder at a temperature of 150-800 °C; and iii) removing gaseous SO2 and/or SO3 released from the ASS powder in step ii).

2. The method of claim 1, wherein the ASS powder is heated at a temperature of 200 - 400 °C.

3. The method according to claim 1 or 2, wherein heat from a heat plant and/or power plant of temperatures in the range 70 °C - 400 °C is used for heating in the thermal treatment.

4. The method according to anyone of claims 1 to 3, further comprising the step iv) leaching sulfate ions from the heat treated ASS powder.

5. A method to inhibit corrosion in a combustion plant by using gaseous SO2 and/or SO3 characterized in that the method comprises the method according to any one of claims 1-3, and wherein the gaseous SO2 and/or SO3 removed in step iii) are collected and fed into the combustion zone of said combustion plant.

6. The method according to claim 5, characterized in that the combustion plant is a combined heat and power (CHP) plant.

7. The method according to any one of the preceding claims, wherein the thermal treatment is performed in an integrated system comprising a plant for thermal treatment of ASS and a combustion plant.

8. A system for combined thermal treatment of ASS and generation of heat and/or power, said system comprising a plant for thermal treatment of ASS and a combustion plant for generation of heat and/or power by combustion of fuel, said system comprising a gas feeding system for feeding gaseous SO2 and SO3 released in the plant for thermal treatment of ASS into the combustion zone of the combustion plant and a heat transfer system for transferring heat from the combustion plant to the plant for thermal treatment of ASS, wherein the plant for thermal treatment of ASS is heated with heat having a temperature of 70 - 400 °C from the combustion plant.