WO2011022767A1

WO2011022767A1 - A method and system for drying carbonaceous material

Info

Publication number: WO2011022767A1
Application number: PCT/AU2010/001091
Authority: WO
Inventors: Andrew Hoadley; Anthony Campisi
Original assignee: Monash University; Hrl Limited
Priority date: 2009-08-25
Filing date: 2010-08-25
Publication date: 2011-03-03

Abstract

A method and system for drying a carbonaceous material feedstock to a power station comprises heating the carbonaceous material with boiler flue gases to a temperature sufficient to enable at least partial removal of moisture from the carbonaceous material in a drying operation. Drying carbonaceous material, such as sub-bituminous or lignitic coal, involves flue gas temperature reduction facilitating integration of a carbon capture process, for example a post-carbon capture or oxy-fuel carbon capture process with the power station.

Description

A METHOD AND SYSTEM FOR DRYING CARBONACEOUS MATERIAL

This invention relates to a method and system for drying carbonaceous material, particularly a carbonaceous material used as a feedstock for combustion in boiler(s) of a power station. The method and system are particularly useful when boilers are integrated with carbon dioxide (CO₂) capture plant where there is need to cool the boiler flue gas to near ambient temperatures before treatment for CO₂ separation.

Coal fired power stations, in which coal is burned in a boiler to heat water, generate steam and, through use of steam turbine(s), generate electricity remain an important source of energy. However, coal fired energy generation must be performed efficiently for economic and environmental reasons. In the case of lignite, brown coal or biomass fired power stations, a key source of inefficiency comes from the high water or moisture content of the fuel. Such fuels may have a moisture content in excess of 30 wt% wet basis. Brown coal fuels may have moisture content in excess of 50 wt% wet basis or even in excess of 60 wt% wet basis. While such fuels may require to be accepted, water in coal lowers the efficiency of power generation, thereby increasing the amount of coal burnt and the amount of carbon dioxide which is generated for a given power output. Pre- drying is a potential response to this issue though drying processes are typically energy intensive. Coal drying also presents potential hazards of rapid oxidation and resultant fire hazard due to spontaneous combustion of dried coals, especially reactive coals.

For example, US Patent No. 4354825 discloses a drying process for a coal , a sub-bituminous or lignite coal, which aims to remove moisture and produce a superior fuel product while minimising the above noted risk of spontaneous combustion. Coal is therefore dried with hot gas supplied to a coal dryer from a combustor which burns fuel to achieve the necessary gas heating. Drying occurs, in a conventional dryer, at a carefully controlled temperature, for example near 200⁵F (about 93⁵C) to minimise risk of spontaneous combustion. Nevertheless, at such temperature, the coal oxidises rapidly and this can bring about spontaneous combustion. Therefore, provision is made for quickly cooling the dried coal to a temperature of about 100⁵F (about 38⁵C) or lower. This quick cooling is accomplished by introducing the dried coal directly into a coal cooler in which the coal is cooled by ambient air. The dried coal may then be sent to product storage or a burner.

Carbon dioxide generated from the combustion of coal may require to be captured and stored in a carbon capture and storage (CCS) process. There are two main types of CCS process being investigated to achieve carbon capture in existing power stations: (1 ) Post-combustion capture and (2) Oxy-fuel combustion:

1. For post-combustion CO₂ capture, capture processes involve the cooling of combustion gases to as close to ambient temperature as possible. This is required in order to remove water, corrosive and pollutant SOx compounds and also to enhance the efficiency of the absorption or adsorption process used to concentrate the CO₂.

2. For oxy-fuel combustion, the combustion gases are essentially just water and CO₂ with considerably lower, by approximately an order of magnitude, lower nitrogen content than in the case of post- combustion CO₂ capture. However, these gases still need to be cooled to close to ambient temperature to remove as much of the water as possible prior to CO₂ removal operations.

Presence of water in coal and integration of a carbon capture process within a coal fired power station represent an energy penalty, in terms of gas cooling and coal drying duties, that it would be desirable to reduce or remove in order to achieve efficient power generation. It will be understood that similar issues, in terms of energy penalty, will arise for carbonaceous material feedstocks other than coal. Such alternative feedstocks may include biomass.

It is therefore an object of the present invention to reduce or remove energy penalty that may arise from use of wet carbonaceous feedstock to a power station, particularly where carbon capture and storage is to be implemented to handle carbon dioxide contained in the combustion gases from the power station.

With this object in view, the present invention provides a method for drying a carbonaceous material feedstock to a power station comprising heating the carbonaceous material with boiler flue gases to a temperature sufficient to enable at least partial removal of moisture from the carbonaceous material in a drying operation. The method is especially suitable for pre-drying of the carbonaceous material feedstock prior to its delivery to boiler(s) of a power station, such pre- drying increasing boiler and power station efficiency.

Advantageously, the boiler flue gases are directly contacted with the carbonaceous material during drying, typically in a dedicated dryer. Advantageously, the carbonaceous material is to be entrained within, and/or fluidised by, the boiler flue gases in an entrained flow or entrained bed dryer. Such direct contacting avoids the need for a more complex heat exchanger design and saves on capital costs.

The method enables reduction of the energy penalty induced by high moisture carbonaceous materials, especially those with a high moisture content in excess of 30 wt% wet basis, 50 wt% wet basis or even 60 wt% wet basis because energy requirement for drying may be obtained from waste heat present in carbonaceous material combustion gases, for example in the boiler flue gases of a coal fired power station. Further, and importantly, if a carbon capture and storage system is used, carbon capture and storage processing is facilitated in two ways. First, use of waste heat in the boiler flue gases may be used as a source of heat to dry the carbonaceous material. It is not necessary to heat drying gas by burning fuel in a dedicated combustor, unlike US Patent No. 4354825, and this saves costs. Second, the boiler flue gases contain a significant proportion of carbon dioxide and carbon capture technologies require that the temperature of the boiler flue gases be significantly reduced prior to processing by methods such as solvent absorption methodology in which a solvent, such as monoethanolamine (MEA), is used to absorb the carbon dioxide. Other carbon dioxide separation technologies may be used and are also likely to require significant gas temperature reduction.

Use of the boiler flue gases to dry the carbonaceous material results in a significant gas temperature reduction which assists in reducing or avoiding the need for complex and expensive gas cooling equipment upstream of the carbon capture stage. In this way, process integration of carbon capture plant from carbonaceous material (especially brown coal) combustion can be increased. Reduction of the amount of coal previously required to generate a required power output may also be achieved. This will also reduce the amount of carbon dioxide generated and therefore reduce the size of the carbon capture plant with capital cost benefits.

In any event, use of the method allows higher moisture lignite coals to be more economically used as a feedstock with a potential reduction in operating costs.

It is to be understood that drying, through the heating and evaporation of water present in the carbonaceous material, does not mean complete drying, partial drying will result in power station efficiency benefits. Further, the drying operations may not involve all of the carbonaceous material feed to the boiler though it would be particularly beneficial for all of the feed to power station boilers to be pre-dried. Benefits in terms of power station efficiency may be observed in any of these cases. For example, a portion of the carbonaceous material feed to the boiler could be dried to around its equilibrium moisture content. The equilibrium moisture content is the moisture content of the carbonaceous material, for example coal, when exposed to a specific humidity of air at a specific temperature and given sufficient time to reach equilibrium - typically it is ambient temperature and 60% humidity. For some lignites, the equilibrium moisture content is between 15-20wt% wet basis.. The pre-dried feed may be mixed with undried feed prior to entering the boiler; or all or most of the feed could be partially dried which could result in moisture contents between the equilibrium moisture content and the content of the raw feed. The actual degree of drying will depend on particle size with larger particles expected to be dried to a lesser extent than smaller particles particularly if coal particles are sufficiently large not to be entrained in an entrained bed dryer. In any event, the carbonaceous material may still contain some moisture at the end of the drying operation. Therefore, the material at the end of the drying operation may be considered as partially dried or semi-dried.

One or more dryers may be employed in series or parallel to enable drying to the required moisture level. In some cases, it may be more efficient to have a second drying operation after this to further reduce the moisture content. Further drying operations may be conducted if required. For example, in the case of coals - especially brown coals - such further drying operations may include steam fluidised bed drying (SFBD). SFBD processes involve evaporative drying in a superheated steam fluidised bed. In this case, the size and cost of the process plant and the steam requirements for further drying would be substantially reduced since a portion of moisture has been removed from the coal.

Dimensions of a dryer, to be used in the method of the invention, are selected - with reference to gas flow parameters, such as boiler flue gas velocity - through the dryer. The key dimensions are dryer height and cross-sectional area. Flue gas velocity, taken with dryer dimensions, dictates residence time of carbonaceous material in the dryer and degree of drying. Flue gas velocity, dryer dimensions and residence time are optimised to meet capital cost and dried coal moisture content specifications.

The flue gas velocity is affected by the number of dryers and the cross- sectional area of each dryer. Decreasing the number of dryers or decreasing the cross-sectional area will increase the flue gas velocity.

Conveniently, the flue gas is used to transport the carbonaceous material through the dryer and on to subsequent unit operations within the power station. Therefore, the dryer may be described as an entrained bed dryer through which the flue gas conveys the carbonaceous material, for example to a solid separation system - for example including cyclones - which removes the dried carbonaceous material for direction as a feedstock to the power station boiler.

The carbonaceous material may be selected from a range of materials, or fuels, including coal, such as sub-bituminous or lignitic coal, having ahigh moisture content typically in excess of 30 wt%, 50 wt% or even 60 wt% wet basis. The method is suitable for drying of reactive coals, such as brown coals as extensively used for power generation in Victoria, Australia. Brown coal reserves are also available for power generation in China, Germany, United States and other countries. Coals or biomass having moisture content of less than about 30 wt% may also be dried with benefit using the method of the invention even though the energy penalty for drying is less than for high moisture content carbonaceous materials. The particle size of the carbonaceous material is that selected for efficient combustion in the power station and will be that acceptable as a pulverised coal feedstock. However, particle size distribution is also important here as it determines the degree of entrainment and drying of carbonaceous material in the flue gas and the surface area available for heat and mass transfer during drying.

Biomass and sludges may also be processed in accordance with the method of the invention.

Boiler flue gases typically have low oxygen content and contact with a feedstock, even a reactive feedstock such as brown coal, should not present undue risk of spontaneous combustion. The method may be controlled with reference to boiler flue gas oxygen content to provide further control and risk minimisation. In particular, oxygen content of the boiler flue gases may be monitored as high levels of oxygen, above about 4 vol% may increase the risks of spontaneous combustion. Target oxygen content may be in the range 2-3 vol%. The dryer may be shut down or other corrective action taken if oxygen content of flue gases becomes hazardous. Corrective actions may include the use of water quench sprays for temperature control.

In another aspect, the present invention provides a system for drying a carbonaceous material feedstock to a power station comprising a dryer connected to a source of power station boiler flue gas for heating the carbonaceous material with boiler flue gas to a temperature sufficient to enable at least partial removal of moisture from the carbonaceous material during a drying operation.

Where the boiler flue gas is directly contacted with the carbonaceous material feedstock, the dryer may be an entrained flow or entrained bed dryer. If the dryer is a fluidised bed dryer in which the boiler flue gas fluidises the carbonaceous material, it is enabled to be transported to another part of the power station.

A dryer may have vertical sided walls and have substantially the same cross-sectional area along its length. Alternatively, the dryer may have sloped or stepped walls so that cross sectional area increases with increasing vertical height (measured from a base of the dryer). Such a design would be beneficial for increasing the residence time of carbonaceous material in the upper sections of the dryer and enabling a more complete degree of drying if required.

Such a system may be implemented in new power stations or readily incorporated within existing power stations by retrofitting, for example by inclusion of entrained bed dryers to be used in the drying system. Dependent on the moisture content of the carbonaceous material, particularly coal, the boilers of a conventional power plant, for example a sub critical brown coal fired power station, may not require any modification for retro-fit of the entrained bed drying system. For example, for a boiler operating with lignite with 62 wt% wet basis moisture, operation with coal moisture contents down to about 50 wt% et basis is expected to be possible without boiler modifications. Below that moisture level, boiler modifications may be required to handle the dried coal. The drying system is well suited to integration with a carbon capture system. A number of such dryers may be employed in series or parallel within the power station.

The method and system for drying a carbonaceous material of the invention may be more fully understood from the following description of a preferred embodiment of the invention made with the reference to the following drawings in which:

Figure 1 is a schematic of the drying system in accordance with one embodiment of the present invention.

Figure 2 is a schematic of a drying system in accordance with another embodiment of the present invention.

Figure 3 is a schematic of one type of dryer suitable for use in the drying systems of Figures 1 and 2.

Figure 4 is a schematic of a second type of dryer suitable for use in the drying systems of Figures 1 and 2.

In summary, Figure 1 shows a system 10 for drying a wet carbonaceous material feedstock to a power station in a drying system or operation 80 comprising a dryer 20 connected to a flue of a power station boiler (not shown) for heating the carbonaceous material with boiler flue gas to a temperature sufficient to enable removal of moisture from the coal.

The power station is a sub critical coal fired power station in which a carbon capture and storage (CCS) process is to be implemented and in which the carbonaceous material is a lignite or brown coal with moisture content of significantly more than 30 wt% moisture, in fact typically having moisture content just above 60 wt% (62 wt% wet basis). Such a power station would typically operate with a pulverised or milled brown coal feed having particle size distribution that enables fluidisation and required heat and mass transfer during drying and combustion. Combustion of the pulverised brown coal in the boiler furnace generates sufficient heat to generate steam which is directed to steam turbines to generate electricity. To increase the efficiency of electricity generation, the pulverised brown coal is partially dried in the manner described below.

The combustion of the brown coal in the power station boilers also generates combustion gases or boiler flue gases which have the following approximate composition dependent on mode of operation, being: (1 ) current operation (without CCS) and post - combustion capture; or (2) oxyfuel CCS.

Table 1

Composition of Combustion Gases (Boiler Flue Gases) for Power Station

The boiler flue gases 15 are directed to dryer 20 which is a vertically disposed vessel, with vertical side walls and constant cross-sectional area along its length, connected to the boiler flue. When leaving the boiler, flue gases 15 may have a temperature approaching 400⁰C though this temperature is reduced to about 270 to 300⁰C by the time the flue gases have been cooled, by a rotary air heat exchanger which also enables pre-heating of combustion air for the boiler, enter dryer 20. The pressure of the boiler flue gases 15, being from an atmospheric boiler, is atmospheric pressure. Lignite or brown coal, milled to average particle size having D₅₀ less than 2 mm, an important specification as described here, is fed to dryer 20 where the particles become entrained, to very significant extent, in the boiler flue gases 15, which are directed through distributor 22 to optimise such entrainment. Dryer 20 may therefore be referred to as a direct contact entrained bed dryer, this dryer being of low capital cost. The direct contact of hot boiler flue gases with the brown coal particles, over a sufficient residence time dictated by the volume of the dryer 20, heats them, by heat contained in the flue gases, to a temperature at which a significant proportion of water is driven off and the brown coal becomes partially or semi-dried. The removal of moisture is beneficial to the economics and efficiency of the power station. In particular, the use of drier coal results in more efficient combustion and lesser generation of carbon dioxide.

The volume of dryer 20, as dictated by its height and cross-sectional area, and the gas velocity of boiler flue gases set residence time of coal in dryer 20. This residence time, together with flue gas temperature, determine the degree of drying of coal achievable within dryer 20. Coal particle temperature increases during drying.

Various dryer designs may be used in the drying system. Figure 3 shows a dryer 200 with a stepped configuration with steps 220 and 230 being formed in side walls of the dryer 200. Dried coal is recovered through outlet 210. Figure 4 shows a dryer 300 with side walls 330 being sloped at an angle α to the vertical. Dried coal is recovered through outlet 310. In both cases, residence time of entrained coal may be increased in the upper portion of the dryers 200 and 300 since cross-sectional area of the dryers 200 and 300 is increased with consequential reduction in flue gas velocity and increase in residence time. Angle α may be selected to provide a controlled graduation in the flue gas velocity and residence time. Such dryer designs may be selected to achieve a desired degree of drying. If the dryers are retro-fitted in the power station, the degree of drying is to be selected with regard to acceptable extent of modification, if any, to boiler design.

At the same time as drying occurs, the heat exchange between boiler flue gases and brown coal particles results in a reduction of the temperature of the boiler flue gases. Such reduction in temperature is particularly beneficial, or even necessary, where - as in this preferred embodiment - a carbon capture process is used within the power station as peak temperatures of the boiler flue gases from boilers in which brown coal feed is combusted are very high, too high for conventional carbon capture processes. Such a carbon capture process may be a conventional carbon dioxide absorption process using a solvent, such as monoethanolamine (MEA), and the absorption process is a low temperature process, desirably to be conducted at near ambient temperature in a post- combustion carbon capture system. Therefore, the use of boiler flue gases to dry the coal achieves a required temperature reduction of the gases, prior to entering a carbon capture system, whilst performing a useful operation. The temperature reduction is such that requirements for expensive flue gas cooling equipment are reduced. Further cooling may be implemented, if required, to control temperature of flue gases entering the carbon capture system.

Alternatively, the carbon capture process may involve burning the coal in an oxygen stream that has been diluted by the flue gas. This process is called oxy-fuel combustion. Even in this case, it is necessary to cool the flue gas to close to ambient temperature to condense moisture, prior to compression. Therefore, just as with the solvent (MEA) carbon capture plant, part of this cooling could be achieved within the dryer 20.

The temperature reduction in the boiler flue gases, to 140⁰C, may be sufficient to take the temperature below the dewpoint at which corrosive species such as SOx, NOx and CO₂ become active in the condensate. Therefore, the dryer 20 is constructed of a corrosion resistant material to address the corrosion risk. For example, a suitable lining or coating - resistant to acidic condensates and particle erosion - could be used in the construction of dryer 20.

Saturated boiler flue gases 25 with entrained partially dried coal particles are then directed to a solid separation system made up of cyclone separators 30. Partially or semi-dried coal is separated and conveyed, in the dense phase, to the boiler for combustion. Prior to entering the boiler, the partially or semi-dried coal may be mixed with undried coal feed. It is not necessary that all coal feed be dried. The coal may be partially or semi-dried prior to delivery to the boiler or mixing with undried coal feed. Coal combustion in boiler(s) for power generation then follows. The boilers are operated under subcritical conditions.

Offgas 35 from cyclone separators 30 is directed through a dust or particulates removal system, such as a baghouse (not shown), before being directed to the carbon capture system (not shown). Carbon capture may also involve storage where a CCS system is employed. Geosequestration is a possible CO₂ storage option.

Condensates may also be recovered from the drying step, such condensates potentially having composition and calorific value sufficient to make them useful as a fuel to be used in power station or other applications.

The invention is now described with reference to the examples.

Three examples of predrying lignite for the generation of electricity in a subcritical boiler have been investigated by simulation. The cases are compared with an existing boiler with no predrying, which uses coal with a moisture content of 62wt% wet basis (wb), as the reference or base case.

Table 2

Example 1 represents the retrofit of the coal dryer to an existing power station, where the air preheat is reduced substantially to provide sufficient heat in the flue gas to dry the coal to 50wt% moisture content. The net efficiency of the process is increased by 1 .3%.

Example 2 represents the post-combustion capture of CO₂ from the flue gas, where the flue gas must be cooled in any case to 4O⁰C. The flue gas passes through the rotary heat exchanger to the dryer. Combustion air is first preheated to 8O⁰C using waste heat from the carbon capture process, before exchanging heat with the flue gas to further preheat the combustion air to 155⁰C. The use of the very low grade waste heat to preheat the combustion air provides an overall increase in efficiency of 2.6%. Note the loss of efficiency from the capture process has not been deducted.

Example 3 represents the oxy-fuel combustion case for the same boiler. In this case there is no rotary heat exchanger, but because of the different composition of the flue gas, the temperature exiting the economizer and entering the drier is similar at 283⁰C. The net efficiency cannot be compared with the three previous cases, because the oxygen plant and CO₂ compression are included in the efficiency calculation. However, the efficiency is increased by 2%, compared with oxy-fuel with no fuel dryer.

Table 3 illustrates simulated examples of the different combinations of dryer dimensions, in particular height for a fixed cross-sectional area, which can dry coal with an initial moisture content of 62wt% to 50wt% wet basis by cooling the flue gas from 277⁰C to 14O⁰C, consistent with Examples 1 to 3 - and in particular Example 2 - in Table 2.

Table 3

Dryer Height and Coal Particle Size Distribution

From Table 3, the finer the coal, as represented by lower D₅₀, the more flexibility there is with the dryer design. Furthermore, because the finer coal allows a greater fraction of coal to be entrained in the boiler flue gas, this reduces the need for the existing beater mills, which dry and simultaneously pulverize the coal. For example, with Distribution 3 coal, more than 50% coal is entrained at 5m/s, which would allow half of the beater mills of the power station, without use of the drying method and system described here, to be shutdown.

In a variant of the method, schematised by Figure 2, the lignite or brown coal drying operation 80, conducted as described above with reference to Figure 1 , is followed by a further drying stage 90. In drying stage 90, brown coal or lignite may be further dried using a steam fluidised bed with drying being achieved by evaporative drying in a superheated steam fluidised bed. Operating practice of steam fluidised bed dryers for brown coal drying is understood in the art. Combustion in boilers (unit operation 100) for power generation follows.

The power station may be adapted to accept different carbonaceous material feedstocks such as biomass and sludge which also typically have high moisture content.

Modifications and variations to the method and system for drying a carbonaceous material of the invention may be appreciated by the skilled reader of this disclosure. Such modifications and variations are deemed within the scope of the present invention.

Claims

CLAIMS:

1. A method for drying a carbonaceous material feedstock to a power station comprising heating the carbonaceous material with boiler flue gases to a temperature sufficient to enable at least partial removal of moisture from the carbonaceous material in a drying operation.

2. The method of claim 1 when used for pre-drying of carbonaceous material feedstock prior to its delivery to boiler(s) of said power station.

3. The method of claim 1 or 2 wherein said boiler flue gases are directly contacted with the carbonaceous material during said drying operation.

4. The method of claim 2 wherein said carbonaceous material is entrained within said boiler flue gases and dried in an entrained flow or entrained bed dryer.

5. The method of claim 4 wherein said dryer is retro-fitted to said power station.

6. The method of any one of the preceding claims wherein moisture content of said carbonaceous material feedstock is in excess of 30 wt% wet basis.

7. The method of claim 6 wherein moisture content of said carbonaceous material feedstock is in excess of 50 wt% wet basis.

8. The method of claim 7 wherein moisture content of said carbonaceous material feedstock is in excess of 60 wt% wet basis.

9. The method of any one of the preceding claims wherein, during drying of said carbonaceous material feedstock, temperature of the boiler flue gases is reduced prior to entering a carbon capture system.

10. The method of claim 9 wherein said carbon capture system is selected from the group consisting of a post-combustion carbon capture system and an oxy-fuel carbon capture system.

1 1 . The method of any one of the preceding claims wherein a portion of the carbonaceous material feedstock to power station boiler(s) of said power station is partially dried and mixed with undried carbonaceous material feedstock prior to entering the boiler(s).

12. The method of any one of the preceding claims wherein said drying operation is followed by a second drying operation.

13. The method of claim 12 wherein said second drying operation is steam fluidised bed drying.

14. The method of claim 4 wherein said boiler flue gas is used to transport the carbonaceous material through the dryer and on to subsequent unit operations within said power station.

15. The method of any one of the preceding claims wherein said carbonaceous material is selected from the group consisting of coal, biomass and sludge.

16. The method of claim 15 wherein said coal is a sub-bituminous or lignitic coal.

17. The method of claim 16 wherein said coal is brown coal.

18. The method of any one of the preceding claims controlled with reference to boiler flue gas oxygen content.

19. A system for drying carbonaceous material in accordance with the method as claimed in any one of the preceding claims.

20. A system for drying a carbonaceous material feedstock to a power station comprising a dryer connected to a source of power station boiler flue gas for heating the carbonaceous material with boiler flue gas to a temperature sufficient to enable at least partial removal of moisture from the carbonaceous material during a drying operation.

21 . The system of claim 20 wherein said boiler flue gas is directly contacted with said carbonaceous material in a dryer during a drying operation and the dryer is an entrained flow or entrained bed dryer.

22. The system of claim 21 wherein said dryer is a fluidised bed dryer in which the boiler flue gas fluidises the carbonaceous material for transport to another part of said power station.

23. The system of any one of claims 20 to 22 being integrated with a carbon capture system.

24. The system of claim 23 wherein said carbon capture system is selected from the group consisting of a post-combustion carbon capture system and an oxy-fuel carbon capture system.