[FIELD OF THE INVENTION]
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A biomass power production system comprising a fuel preparation part for receiving biomass fuel in solid form and an energy production part comprising boiler for generating thermal energy.
[BACKGROUND]
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In countries where renewable energy and/or renewable energy is not feasible or is not available, the main source for the production of electricity has in the past predominately been performed using fossil-fuel power stations. The fossil fuel power stations have often been powered by the combustion of coal, natural gas or even petroleum to produce energy.
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In recent years the energy source for power production has moved from fossil-fuel to biofuels, due to a national and international desire to reduce emissions of greenhouse gasses, based on non-renewable fuels. Thus, there has been an increased need for power stations that utilize biomass, as a renewable energy source, to produce electricity. Biomass may be defined as biological material from living, or recently living organism and is often seen as a carbon-neutral solution, as the hydrocarbons present in the biomass are harvested from the atmosphere during biomass production. Examples of biomass that is considered to be carbon neutral are wood, sugar cane, straw, etc., as the growing of the organisms harvests carbons from the environment, e.g. in the form of CO2.
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Energy production in the form of solid biomass fuels may be performed using a number of different biomass sources, such as wood pellets, thistle pellets, eucalyptus chips, nut shells, sorghum, olive husk logs, wood chips, palm kernel shells, straw, straw bales, etc. These types of biomass sources are often seen as residue material derived from other processes, such as corn production, nut production and so on.
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A huge advantage for energy production facilities using biomass fuels is if the production facilities are capable of receiving more than one type of biomass, and preferably more types of biomass in different forms as pellets, chips, bales. However, one of the problems with regards to receiving a number of different types of biomass is the fact that the moisture content of the biomass differs greatly between the certain types of biomass. As an example, wood pellets, which are produces from wood, are produces in a process where the wood is dried, ground, and pressed into pellets, so that it is ensured that the wood pellets have a moisture content that is no more than between 10-12 wt% moisture (water). Wood chips, which have not been pre-processed, may have a moisture content of up to 40-47 wt%.
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Such a difference in moisture content is not relevant for certain types of biomass boilers, such as grate fired boilers, as grate fired boilers can receive biomass material that has a moisture content that is relatively high, as the chips may be burnt without pulverising the material beforehand.
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However, when working with boilers, such as a pulverized fired boiler, the moisture content of the biomass must be relatively low, or below approximately 15 wt%, as the biomass has to be pulverized or powdered before the biomass is injected into the furnace. This means that wood chips having a moisture content of up to 40-45 wt% of moisture are not suitable for pulverized fired boilers, as the wood chips cannot be grated, grinded or pulverized adequately due to the moisture content.
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One of the crucial issues with regards to energy production using biomass is that the availability of biomass on offer in the region of the production facilities may fluctuate seasonally, depending both on crops and harvesting of biomass. This may result in increased prices but may more critically result in unavailability of biomass, which may lead to a reduction or complete stop of the energy production. Thus, an important issue with such energy production is the availability of alternate fuel sources for energy production, such as coal, petroleum, natural gas, and other types of biomass, when the required biomass is not available.
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The advantages of grate fired boilers in being capable of receiving a variety biomass having a high water content is challenged by the fact that grate fired boilers are limited in utilizing most types of fossil fuel as a backup source for energy production. The grate fired boiler is not capable of utilizing fluid based fuel sources, such as natural gas, petroleum, or liquid biofuels, such as methanol, bio-ethanol, bio-diesel, or vegetable- or seed oils. Thus, one of the few sources of backup energy for a grate fired boiler is in the form of non-pulverized coal.
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However, the pulverized fuel boiler is capable of substituting substantially any type of pulverized biomass fuel with a fluid based fuel, such as natural gas or petroleum, without any significant modification, as long as the fluid based fuel is introduced into the boiler in pulverized or gaseous form (e.g. through a spray nozzle). Thus, depending on the prices of the fuels, a pulverized fuel boiler may choose the most economical source of backup fuel at any specific time to compensate for the unavailability of the specific biomass.
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Thus, there is a need for a power production system that is capable of receiving a variety of biomass material having varying moisture content, in different forms and that is adapted to continue power production using a variety of backup fuel options.
[GENERAL DESCRIPTION]
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In accordance with the invention, there is provided a biomass power production system comprising: a fuel preparation part for receiving biomass fuel in solid form comprising: a drying apparatus using hot air for reducing moisture in the biomass fuel, and a grinding apparatus for pulverizing or grinding the biomass fuel into a pulverized biomass fuel, and an energy production part comprising: a pulverized fired boiler for receiving the pulverized biomass and generating thermal energy by burning the pulverized biomass where the thermal energy is in the form of pressurized steam, a steam turbine for converting the thermal energy of the pressurized steam into mechanical energy and where the steam turbine is adapted to drive an electrical generator to convert the mechanical energy into electrical energy, a flue gas cooler adapted for receiving an exhaust gas from the boiler and extracting thermal energy from the exhaust gas for reducing the temperature of the exhaust gas, wherein the thermal energy generated in energy production part is converted into thermal energy for heating up the hot air of the drying apparatus.
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Pulverized fired boilers require the biomass fuel to be pulverized before the fuel is injected into the furnace of the boiler. This pulverization is performed using a grinding apparatus that can grind wood chips, straw or other types of biomass before the biomass is introduced into the furnace. However, a significant problem with the grinding process is that the grinding cannot be performed if the moisture content of the biomass is higher than about 10-15 wt%. If the grinding is performed on biomass having higher moisture content, the ground biomass will be mushy or pastelike, and will not be optimized for use in the boiler.
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By providing a drying apparatus in the energy production system, the system is capable of receiving any type of suitable biomass, regardless of the moisture content of the biomass, as the system is capable of drying out the biomass before it is ground and before it is used to feed the boiler. This means that the energy production system is much more adaptable than conventional systems having pulverized fired boilers, as the biomass may be prepared on location. Furthermore, should the need occur, the system may be adapted to receive fossilized fluids as backup fuel for the system, should the supply of biomass be reduced or be too expensive to be economically sound.
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The conventional way of supplying biomass for systems comprising pulverized fired boilers is to provide the biomass in pellets, such as wood or thistle pellets, where the moisture content has been reduced to the required level. This means that the biomass is supplied by a provider that has at a different site dried out the biomass, prepared the biomass and shipped it to the power production facilities. This means that a significant amount of energy is used to process the biomass from its original form, i.e. from wood into the finished biomass product, i.e. wood pellets, where the energy in production is used to dry the biomass, and to form it into the finished product. Furthermore, due to the fact that the finished product is prepared at a different location, there are energy requirements and costs related to the transport of the finished product to the power production facilities.
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However, by providing drying facilities in accordance with the present invention, the biomass product can be processed at the power production facilities, and thus reducing the energy consumption for the production of each unit of biomass, as the transport between the preparation facilities and the power production facilities has been eliminated.
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Furthermore, the provision of the fuel preparation part, in accordance with the invention, means that the power production plant may receive a wide variety of solid biomass products, which previously have been unsuitable for a power production system having a pulverized fired boiler. Thus, the choice of biomass to use as fuel for the power production may be varied based on seasonal availability of biomass, the geographical availability of biomass, the cost of biomass, or the general availability of biomass. Should the power production system generally use wood to produce power, and in a certain period the supply of wood is reduced, the system is adapted to receive other types of biomass without having to adapt the energy production part of the system in any way. The fuel preparation part will be adapted to the specific type of biomass, and the biomass will be prepared accordingly to be suited to be introduced into the pulverized fired boiler.
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The thermal energy generated by the energy production part may be utilized directly or indirectly for heating up the hot air of the drying apparatus.
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In one embodiment of the invention the thermal energy from the energy production part may be residual thermal energy. By utilizing residual thermal energy from the energy production part in order to provide thermal energy to the fuel preparation part, it is possible to reduce the overall energy requirements to prepare the biomass, so that the viability of the solid biomass product is increased. Should the biomass be dried at a facility using thermal energy coming from electricity, the overall cost of the each unit of biomass would increase, as the cost of energy would be added to the unit cost of the biomass product. However, the use of residual energy from the energy production in the power production system, the preparation of the biomass fuel by drying is performed using energy that might otherwise be wasted. Thus, such production reduces the overall cost of preparing the fuel, as the energy used is a "cost effective" energy.
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In one embodiment of the invention, the fuel preparation part may further comprise a buffer unit for storing a reserve of pulverized biomass fuel. The buffer unit ensures that the supply of biomass is not solely dependent on a single source, and ensures that if there is a glitch in the biomass supply prior to the buffer, the power production will continue for a predetermined period (size of buffer) without interference.
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In one embodiment of the invention, the fuel preparation part may further comprise a dosing apparatus to collect pulverized biomass fuel and to prepare a predefined amount of fuel to be injected into the pulverized fired boiler. The dosing apparatus may be adapted to receive a number of different types of biomass fuels, after the fuel has been dried or introduced into the power production system. The energy density of different biomass fuels differs from one type to the other, and the dosing apparatus may be utilized to optimize the biomass fuel injection into the pulverized fuel boiler. One of the issues relating to the dosing regimen of fuel into the boiler is to ensure that the boiler is as energy efficient as possible, i.e. the boiler utilizes as much energy from the biomass fuel as possible.
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In one embodiment of the invention, the thermal energy from the energy production part may be the thermal energy accumulated through the cooling process of exhaust gas using the flue gas cooler. The flue gas / exhaust gas is a by-product of the burning process in the boiler, and the thermal energy of the exhaust gasses from the boiler is a residual thermal energy, which may otherwise go to waste. By cooling the exhaust gasses and harvesting the thermal energy of the flue gas, it is possible to utilize some of the thermal energy of the exhaust gas to heat up the air used to dry the biomass in the drying apparatus, and thereby reduce the energy waste during the power production. The thermal energy of the flue gas may be transferred to the air supply by introducing a first heat exchanger that intercepts the flue gas, and in a closed loop system transfers the thermal energy to a heat exchanger that intercepts the air used in the drying process. The flue gas may be cooled from approximately 140 degrees Celsius to about 80 degrees Celsius in order to obtain an optimal heat transfer from the flue gas to the air drying process.
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In one embodiment of the invention, the thermal energy from the energy production part may be extracted from a steam output of the steam turbine. It is generally considered a violation of known procedures to remove steam from a steam turbine before all the useable thermal energy of the steam has been utilized for conversion into mechanical energy. However, the inventors have surprisingly found out that in order to utilize the thermal energy of steam in order to provide thermal energy for a drying process of biomass, it may be economically sound to extract steam from a low pressure outlet of the steam turbine for use in the drying process. The pressure of the steam input into the steam turbine may often exceed 100 bars, and can preferably be around 112 bars. Thus, when the steam turbine has converted the thermal energy of the steam under pressure into mechanical energy, the energy efficiency of the low pressure range of the steam is relatively small for the production of mechanical energy. Thus, in accordance with the present embodiment, it is more energy efficient to extract steam at low pressure and utilize the thermal energy of the low pressure steam in the drying process, than it would be to provide an alternative source of thermal energy, such as providing thermal energy by electrically heating elements that provide heat for the drying process.
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In one embodiment of the invention, the steam may be extracted from the steam turbine at a steam pressure that is less than 10 bars. By accumulating the steam at pressure that is less than 10 bars, the conversion efficiency of the steam into mechanical energy is reduced to such a degree that utilization of the thermal energy for heating up the air in the drying process may be greater than the mechanical energy conversion.
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In one embodiment of the invention, the steam may be extracted from the steam turbine at a steam pressure that is less than 6 bars. At approximately 6 bars, the energy efficiency of utilising the steam for the steam turbine is reduced even further compared to the thermal energy efficiency of utilizing the pressurised steam for heating up the air for the drying process.
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In one embodiment of the invention, the steam may be extracted from the steam turbine at a steam pressure that is approximately 4 bars. At approximately 4 bars, the energy efficiency of utilising the steam for the steam turbine is very little, as the pressure of the steam is very small compared to the input pressure of the steam. Thus, the steam may be extracted without a significant reduction in mechanical energy conversion. However, the thermal energy present in the steam at approximately 4 bars is sufficient to provide an efficient source of energy in order to heat up the air used in the air drying process for the biomass.
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By utilizing low value heat from the energy production, in the form of low pressure steam from steam turbine or in the form of heat from the cooling process of flue/exhaust gasses, means that the efficiency of the system is improved significantly. The input air temperature for the drying process is between 100 - 140 degrees Celsius, and the air temperature ensures that the drying apparatus may reduce the moisture content of the biomass to a level of about 10-12 wt%.
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In one embodiment of the invention, the pulverized boiler may comprise a slacked superheater. By providing a slacked superheater to convert saturated steam into dry steam means for the power generation process means that the boiler may operate at a relatively high heat, or over 480 degrees Celsius. The preferred heat for the slacked superheater may be around 520 - 540 degrees Celsius. The slacked superheater ensures that the melting temperature of the ash inside the furnace of the boiler does not influence the energy conversion in the furnace, as the slacked superheater can tolerate that the ash forms a layer onto the superheater. Conventional superheaters do not allow temperatures exceeding approximately 480 degrees Celsius as such a temperature does not exceed the vital melting temperature of ash, and therefore a layer of ash does not form on the superheater. However, when the temperature inside the furnace of the boiler is increased, there is an increased risk that the temperature will cause the ash to melt and the melted ash will form a layer on the superheater, and reduce the efficiency of the superheater significantly.
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In one embodiment the boiler may comprise an attemperator system. The attemperator system is designed for handling large variations in the heat absorption of the boiler. The attemperator system may reduce the steam temperature by bringing superheated steam into direct contact with water. The steam is therefore cooled through the evaporation of the water.
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In one embodiment the energy production part may comprise a soot blowing system, enabling the boiler to deal with changes in the heat absorption. The soot blowing system may typically consists of the following parts: retractable steam soot blowers for cleaning of the convection surfaces; furnace wall water cannons including local control panel; steam and drainage pipe work system; and a safety valve. The steam soot blowers may be supplied with steam from the steam cycle of the boiler at suitable pressure. The soot blowers may be arranged for cleaning of both convection SH banks, and ECO. The steam soot blower control may be integrated in the DCS system.
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In accordance with the invention, there is also provided a method of producing energy from a biomass fuel source comprising the steps of: receiving solid biomass fuel, drying the solid biomass fuel so that the moisture content of the fuel is low enough to grind the biomass fuel, grinding the solid biomass fuel into a pulverized form in order to increase the surface area of the fuel, injecting the biomass fuel into a pulverized fired boiler, generating a first source of thermal energy in the boiler by burning the biomass fuel and converting the thermal energy into the form of steam, introducing the steam into a steam turbine converting the first source of thermal energy into mechanical energy and where the mechanical energy is subsequently introduced into an electrical generator producing electrical energy, cooling down exhaust gasses from the boiler by generating a second source of thermal energy from the exhaust gas, wherein at least part of the thermal energy sources generated are converted into thermal energy of air used for drying the solid biomass fuel.
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The method according to the invention may be utilized in accordance with the system in accordance with the invention. The thermal energy generated during the energy production may be utilized to provide means for heating up the air for drying the solid biomass fuel. This means that the method ensures that an external source for energy for heating up the air is not required, such as electrical energy, even though the availability of electrical energy may be sufficient. There is however always an energy loss between conversions from one type of energy to the other, such as a conversion from thermal energy to mechanical energy, or vice versa. Thus, the energy efficiency of the power production is optimized by utilizing thermal energy generated by the pulverized fired burner.
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In one embodiment of the invention, the first and/or the second source of thermal energy may be converted into thermal energy of air used for drying the solid biomass fuel. The thermal energy may, as discussed earlier, be residual thermal energy that may not contribute efficiently to the energy conversion from thermal to electrical. Such thermal energy may be extracted from the exhaust gasses, or taken at a relatively low pressure from the steam turbine.
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In one embodiment of the invention, steam of the first source of thermal energy may be introduced into the steam turbine at a pressure exceeding 100 bar, preferably at about 112 bar. In one embodiment of the invention steam of the first source of thermal energy may be extracted from the steam turbine at a pressure of less than 10 bar, preferably at about 4 bar for conversion into thermal energy of air used for drying the solid biomass fuel. At approximately 4 bars the energy efficiency of utilising the steam for the electricity production of the steam turbine is very little, as the pressure of the steam is very small compared to the input pressure of the steam. Thus, the steam may be extracted without a significant reduction in mechanical energy conversion. However, the thermal energy present in the steam at approximately 4 bars is sufficient to provide an efficient source of energy in order to heat up the air used in the air drying process for the biomass.
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In one embodiment of the invention the second source of thermal energy may be generated by arranging a heat exchanger to transfer the thermal energy in the exhaust gasses to a transport medium that transports the thermal energy from the heat exchanger to an air supply of the air used for drying the solid biomass fuel. Thus, the exhaust gasses heat up a transport medium that may be pumped to the location of the drying process, where a second heat exchanger transfers the heat from the transport medium to the air supply. Subsequently, the transport medium may be pumped back to the first heat exchanger and the sequence is started over.
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Abbreviations used in the present description:
- AFR
- Air/fuel ratio
- APH
- Air pre heater
- BWE
- Burmeister & Wain Energy A/S
- ECO
- Economizer
- FD
- Forced draft
- FGC
- Flue gas cooler
- HP
- High pressure
- ID
- Induced draft
- LFO
- Light fuel oil
- LHV
- Lower heating value
- MCR
- Maximum continuous rating
- NG
- Natural gas
- OFA
- Over fire air
- PA
- Primary air
- PF
- Pulverized Fuel
- PFD
- Process flow diagram
- PSD
- Particle size distribution
- RAPH
- Regenerative air preheater
- SA
- Secondary air
- SH
- Super heater
- TA
- Tertiary air
[BRIEF DESCRIPTION OF DRAWINGS]
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The invention is explained in detail below with reference to the drawings, in which
- Fig. 1 is a schematic diagram of a power production system in accordance with the invention.
[DETAILED DESCRIPTION OF DRAWINGS]
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Fig. 1 shows a power production system 1 using biomass fuel comprising a fuel preparation part 2 and an energy production part 3. The illustration of the system 1 is in simplified form in order to assist in the understanding of the interaction of parts in the fuel preparation part 2 and the energy production part 3.
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The fuel preparation part 2 is primarily designed to prepare the biomass fuel 4, so that the biomass fuel 4 may be utilized in the energy production part 3 of the power production system 3. The solid biomass fuel 4 is taken into the fuel preparation system, and after it has been opened from its bulk form it is introduced into a drying apparatus 5. The drying apparatus air dries the biomass fuel 4 in order to ensure that the moisture content of the biomass fuel does not exceed a predetermined moisture lever, or between 10-15 wt%.
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Subsequently, the solid biomass fuel 4 having a moisture content below the predetermined level is fed into a grinding apparatus 6, which ensures that the solid fuels are ground into small particle sizes, i.e. pulverized so that the dried ground biomass may be utilized for a pulverized fired boiler. Following the drying and the milling of the solid biomass fuel, the pulverized biomass may be transported from the fuel preparation part 2 and injected into the pulverized fired boiler 15 of the energy production part 2 of the power production system 1 via a transporting device 7.
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Alternatively, the fuel preparation part may include a buffer device 8 that allows the power production system 1 to harvest a predetermined amount of ground dried biomass fuel in order to provide a redundancy service of biomass fuel for the pulverized fired boiler 10. The buffer may provide a source for biomass fuel when the boiler 15 uses more fuel than the fuel preparation part can deliver, i.e. in peak periods, or may ensure stable provision of biomass fuel if there is a glitch in the delivery of biomass fuel to the power production system, or if there is a malfunction in the drying apparatus 5, milling apparatus 6 or any hardware preceding the buffer 10. The dried ground biomass fuel may be injected 10 into buffer 8 directly from the grinding apparatus, and fed subsequently to the transportation device 7 directly via a buffer line 11. Thus, the buffer 8 may either be utilized optionally or directly to feed dried ground biomass fuel to the pulverized fired boiler 15.
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Alternatively, the fuel preparation part 2 may be provided with a dosage device 9, where the dosage device 9 is adapted to dose the flow of dried and ground biomass fuel to the pulverized fired boiler 15. The dosage device 9 may therefore ensure that the boiler 15 is fed the correct amount of dried and ground biomass fuel in order to obtain or maintain the optimal energy conversion in the boiler 15. The usage of a dosage device may be important where the power production system 1 and fuel preparation part 2 is capable of receiving a number of different types of biomass fuel, and where the calorific value of the biomass fuel differs from one type to the other i.e. a wood based biomass fuel may have a calorific value 16-17 MJ/kg at 10 wt% moisture, where a olive cake based biomass fuel may have a calorific value at 12-17 MJ/kg at 10 wt% moisture. Thus, in order to provide a steady stream of fuel for the boiler 15 in order to maintain an even burn in the furnace, the biomass fuel may have to be dosed into the boiler 15. The dosage device may receive a biomass input either from the grinding apparatus 6 feeding line 12 or may receive a biomass input from a buffer 8 feeding line 14. The dosage device may thereafter feed the dosed biomass fuel into the transport line 7 via a dosage line 12.
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For certain types of biomass fuel, it may be advantageous to pre-treat the solid biomass source before the biomass is introduced into the drying apparatus 5. This pre-treatment of the biomass may e.g. be in the form of a bale or a package opener that removes any packaging material that is associated with the source of biofuel, such as strings for packing a bale. The pre-treatment may also be in the form of a pre-grinding apparatus, which is capable of shredding the solid biomass to smaller particles prior to drying, in order to facilitate the drying process by increasing the surface area of the biomass prior to drying. The increased surface area of the solid biomass ensures that smaller particles of biomass are dried at any one time, which may make the drying process quicker, i.e. the increased surface area allows moisture to be extracted quicker from the biomass.
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Furthermore, should the biomass source 4 enter the fuel preparation system having a moisture content that is between 10-15 wt%, the biomass may be diverted past the drying apparatus 5 and introduced directly into the grinding apparatus 6 in order to ensure that energy is not wasted on drying a biomass source 4 that is already prepared for grinding into a pulverized mass.
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The Energy production part 2 includes a pulverized fired boiler 15 that receives dried and ground biomass from the fuel preparation system. The biomass is injected into the boiler, where the boiler generates thermal energy by burning the biomass that is injected into the furnace of the pulverized fired boiler. The boiler 15 has at least two outputs 16,17 in the form of a first steam output 16 and a flue/exhaust gas output 17.
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The first steam output 16 is fed into a steam turbine 18, which may drive an electricity generator to convert the thermal energy of the steam into mechanical energy, which the electricity generator 19 converts subsequently into electrical energy via a mechanical connection between the steam turbine and the generator 19.
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The flue/exhaust gas output 17 of the boiler 15 comprises the exhaust gasses from the furnace, which is often in fluid connection with a chimney for venting the flue gasses to the outside atmosphere. However, the flue gas may also include a significant amount of redundant thermal energy in the form of heat, which through a release into the atmosphere becomes useless for the energy production process. However, the amount of energy that is present in the flue gas is relatively low, when comparing to the thermal energy used in the steam turbine. Thus, it is not likely that the thermal energy of the flue gas can contribute directly to the energy production of the energy production part 3. However, the energy may be utilized differently, where the energy of the flue gas may be transferred as thermal energy to heat up the air of the drying apparatus in the fuel preparation part 2.
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The thermal energy of the flue gas may be extracted from the flue gas using a flue gas cooler 20 that receives the flue gas directly from the boiler 15 via exhaust line 17. The flue gas cooler 20 extracts the heat from the flue gas, cooling the flue gas from typically around 140 degrees Celsius to approximately 80 degrees Celsius. The thermal energy of the flue gas cooler 20 is subsequently transported via a heat transport 21 from the flue gas cooler and to the drying apparatus 5 of the fuel preparation part 2, where the thermal energy is used to warm up the air used to dry the biomass fuel prior to the grinding, as described above.
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In one embodiment, the flue gas 20 cooler and the air heater may comprise the same heat exchanger, so that the heat from the flue gas is converted into heated air used for the drying process.
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Alternatively, the air in the drying apparatus 5 may be heated up using thermal energy from the steam generated by the pulverized fired boiler 15. The steam may be extracted at low pressure from the steam turbine 18 and fed from the steam turbine via a second steam output 22, which is fed to a heat exchanger in the drying apparatus 5 and is used to warm up the air to its optimum drying temperature, or approximately 120 degrees Celsius.
Example
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The following description is an example of an operational system for energy production using a biomass fuel. The following example is to be seen as an example embodiment of the present invention and should not be seen as limiting to the scope of the invention.
Fuel preparation part
Pre-grinding and crushing
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Bales and chips of biomass may be handled separately as the pre-treatment machines have different functions and the materials are handled differently. The bales may be opened up to a loose material to allow drying and fine-grinding. Chips, shells and cake fuels are grinded to a finer particle size to improve drying.
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All fuels are combined on the inlet to the dryer.
String remover
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Bales are received on a bale conveyor. This leads to a string cutter and remover. Strings are removed to protect the subsequent rotating machines against strings being wrapped around shafts.
Bale crusher
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The purpose of the bale crusher is to crush the compressed bale strokes. The bale conveyor drops the bales into the bale crusher where two rotating shafts tear the strokes apart and reduce the particle size/straw length.
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The bale crusher operates with a robust principle also used in waste processing. The two shafts have knives attached and rotate in opposing directions. If a knife is blocked by e.g. a foreign element, the rotation will be reverted shortly.
Chips pre-grinding
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Wood chips and similar sized biomass is pre-ground in a "pan-grinder". The purpose is to reduce to a uniform particle size, where the increased particle surface is suitable for fast and effective drying and the subsequent fine-grinding.
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The pan-grinder is similar to a roller-mill, where a grinding table is rotated under a number of rollers. The grinding table has holes, where the ground material is let through. The hole size determines the fineness of the output material, and also the capacity of the grinder.
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The grinding principle is a combination of pressure, shear forces and cutting over the hole edges, and this is suitable for chips grinding where fibres are being torn. It makes both drying and fine-grinding easier. High moisture content is not a problem.
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Advantages of the pan-grinder are:
- Low specific power consumption
- Low wear of grinding parts
- No need for ventilation air through the grinder.
- High moisture content allowed.
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Both wood chips, bark chips, shells and exhausted olive and grape cake is suitable for the pan-grinder.
Drying
Belt dryer
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Drying of biomass fuel is necessary as the inlet moisture for fine grinding must be <15%. The dryer is integrated with the plant process to get the heat needed for drying, which can be extracted at suitable temperature levels. The belt dryer is a standardized machine, which can utilize low temperature heat and this makes it well suited for biomass fuel drying.
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The drying principle is a perforated belt that transport the material inside a dryer tunnel. Ambient air is heated and sucked down through the material and the perforated belt, where the air may be discharged to atmosphere from the suction blowers.
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The inlet air can be heated up to 120°C. The heat is provided with feed water that is heated from a turbine outlet and pumped to the belt dryer heat exchangers.
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The output product is dried down to 10% moisture. Because of the varying biomass specifications, the dryer is controlled to give feasible drying. Both belt speed, layer thickness and individual drying zones can be adjusted. The last zone can be used for cooling to lower material outlet temperature to minimize risks in the following process steps.
Heat exchangers
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Two heat exchangers are provided to extract heat from the plant process for the dryer. The first heat exchanger is a flue gas cooler. A shell-and-tube type with external PTFE coating will be used to protect against corrosion. Heat is transported to the dryer in a closed water loop. This heat will be used for pre-heating the drying air and is optional.
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The main heat source is by turbine steam extraction. Feed water is taken before the feed water tank, heated by a closed preheater with extracted steam and pumped to the dryer heating sections. This feed water is returned to the process.
Silo
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The silo provides storage capacity for stable operation of the firing system. The capacity can be adjusted to meet the specific plant needs e.g. fuel reserve for a start-up sequence.
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The silo is emptied and material is distributed with screw conveyors to each of the mills via pneumatic gates. The dust extraction filters minimize dust formation at all transfer stations and in silo to lower the risk of dust explosions and remove any excess air.
Fine grinding and dosing
Milling plant
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The purpose of the milling plant is to provide PF at the right particle fineness for the dosing and combustion. By running the mills in parallel it is possible to absorb variations in fuel calorific value and outage of any of the mills without consequences for the boiler. If a coarser fuel is used, the capacity of the mills decrease, and boiler MCR could require both mills in operation. The system allows for this flexibility.
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The milling plant will be located in a dedicated milling house .The feeders provide protection from foreign materials and dose into the mill. The purpose is to distribute material evenly across the hammer mill rotor to maximize capacity and counteract unnecessary wear of the hammer mill parts.
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It is the inventors experience that biomass dust particle size distribution (PSD) is critical for combustion stability and the resulting operation stability, emissions control and ash properties amongst others.
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During the commissioning of the firing system, the pulverized fuel PSD will be optimized to achieve this combustion stability and hammer mill operation (mainly screen size) will be adjusted to provide the required fineness. The power consumption of the hammer mill motor will be a result of this. Hammer mills are chosen for their flexibility and will be with an amount of over-capacity to allow for the optimization.
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An amount of air must flow through the mill for proper function. This is provided with the filter and suction fan. The mill bin with extraction screw extracts the pulverized fuel.
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The chain conveyors transport the PF into the buffer/dosing unit.
Rotary valves
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Rotary valves will be installed on the inlet of the buffer/dosing units. This ensures that the different pressure levels do not disturb the dust dosing and prevents explosions from propagating.
Buffer/dosing units
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The purpose of the buffer/dosing unit is to absorb variations in fuel supply and provide even and precise dosing of PF to each burner. Stable and precise PF flow is critical for burner stability and control. With the designed buffer/dosing unit the particular properties of biomass PF and the needs for control of the combustion is specifically addressed. The buffer/dosing unit will have a capacity for a few minutes of operation. The unit is designed to withstand a burst pressure - for that reason the unit is made in a heavy design.
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The bottom of the buffer/dosing unit consists of dosing screws, which will be filled close to 100 % and the dosing will be done by controlling the speed of the screws. Each dosing screw supplies one burner and enables operation modes with individual control of any of the burners.
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The biomass PF from each of the dosing screws will fall into a chamber, which leads to an ejector.
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The buffer/dosing units is located as close to the burners as possible to minimize the PF piping and bends required.
Blower and ejector - system
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The blower will have a capacity for adjusting the conveying air speed. The final setpoint will be determined during commissioning. According to the designers' experience, it is important to install a system which enables fine-tuning. The required velocity and the evenness in the flow are highly dependent on air/fuel ratio (AFR) and particle size distribution (PSD). Therefore, when optimizing the system the air flow shall be adjusted to the actual biomass particle size distribution (PSD).
PF pipes
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The PF pipes will be designed for a low air/fuel ratio (AFR) and a high velocity. Bends have larger radius and they are reinforced with ceramics or special concrete. Expansion joints are included where necessary. The possibility of installation of portable in-line mass flow instruments for commissioning purposes is foreseen. The dust pipes, upstream from the burners, will be equipped with quick closing gates.
Energy production part
BOILER DESIGN
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The boiler is a natural circulation boiler without reheat and is drainable, except for the platen SHs. The boiler is arranged as a one and a half pass boiler with modified corner firing and the FGC placed in line behind the boiler. All boiler walls form part of the evaporator system of the boiler and are all welded membrane walls. The membrane walls of the boiler are stiffened with buck stays to withstand maximum suction and puffing from the furnace chamber.
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The furnace chamber is optimally calculated, in order to ensure safe wet bottom ash disposal and sufficient cooling of the flue gases to safe level under the ash fusion point before super heater (SH) inlet.
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The arrangement of the tube bundles and the determination of the tube pitches have been carefully chosen to ensure perfect cleaning by soot blowing, easy access for inspection and overhaul, and the possibility of draining all tubes completely. The boiler is designed for indoor installation and equipped for PF firing and NG or LFO as start-up fuel.
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The pressure system is fully welded, and all walls are of the membrane wall type welded together according to the fin-tube-fin principle. The boiler is supported from bottom and placed on main structure in steel, enabling the boiler to expand freely. All internal tube bundles are suspended by steam- or feed water-cooled suspension tubes.
COMBUSTION & FLUE GAS SYSTEM
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The boiler is arranged as a one and half pass box type boiler with external economizer, flue gas cooler & modified corner firing with 4 low NOx burners at a suitable level (only one level of burners) to ensure complete combustion. The combustion & flue gas system comprises the following main components.
Combustion System
PF Burner
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The BWE Low-NOx burners are applied for biomass PF firing allowing advanced air staging and high adjustability to achieve low NOx formation and stable ignition. BWE Low NOx burners feature individual air control and adjustable SA swirl for optimized combustion.
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The burner consists of the following main parts:
- Register house with adjusting cone and SA/TA inlet.
- Adjustable turbulators for SA.
- PA pipe.
- Outer PF tube.
- Inner PF tube with PF swirl.
- Gas lance with heat resistant nozzle.
- PF-bend with retractable mechanism for the burner gas lance
- Front plate with retractable mechanism for the ignition lance
- Flame monitoring equipment
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The burner system is provided with compressed air for retractable burner mechanism as well as cooling air for burners, ignition lances and flame monitoring.
Combustion Air System
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Combustion air is led to the burners from the inlet through the following components:
- inlet duct with grid and silencer
- ground-mounted FD fan with flow control
- RAPH
- Dense flow air system for conveying of PF dust to each of the burners,
- duct with venturi and control dampers for each burner for PA.
- duct with venturi and control dampers for each burner, for SA/TA.
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Cooling air for the burner system is supplied from a common fan unit and distributed through air ducts to the individual consumers.
4.2 Flue Gas System
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The flue gases from the combustion chamber pass the convection heating surfaces in the following sequence:
- Radiant superheaters SH3/SH4
- Evaporator screen formed by upper end of intermediate wall
- SH2
- SH1
- ECO
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Outside the boiler, the flue gases pass the following components:
- SNCR
- Bag house filter
- Ground-mounted ID fan with silencer
Air & Flue Gas Ducts
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All ductwork is supplied with the necessary expansion elements. The supporting systems are dimensioned to withstand the specified wind loads. The following frame conditions are treated as the main design parameters:
- Design temperature: Maximum operating temperature
- Design pressure of air ducts FD fan to burners: Stalling pressure for FD fan
- Design pressure of flue gas ducts: Stalling pressure for ID fan
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The flue gas duct between SH and ECO is designed according to the dimensioning pressure in the boiler furnace chamber, pressure drop through heating surfaces at operation temperature and with reduced safety.
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The duct buckstays are calculated according to that of the above-mentioned cases, which produces the heaviest load.
Air Ducts
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The combustion air inlet is located near the ground by means of air suction silencer.
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Downstream the FD fan, the combustion air passes through the HP-APH. The ducts are provided with the necessary steel-made expansion joints, supports, and thermal insulation.
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After passing the HP-APH, the combustion air duct is divided into air ducts leading to the individual burners for individual air flow measurement for secondary and tertiary air (TA), and a system for over fire air (OFA). Primary air (PA) is included in transportation air in PF firing system.
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Suitable flow measuring devices are installed in the respective ducts (venturi type).
Flue Gas Ducts
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Combustion gases from the boiler will be conveyed in flue gas ducts to the interface point of the chimney, passing the FGC, the bag house filter and the induced draft (ID) fan with silencer.
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Flue gas ducts includes insulation material, expansion joints, supports etc.
PRESSURE PART
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The boiler pressure parts consist of
- Economiser & FGC
- Steam drum
- Downcomer & Headers
- Furnace & Evaporator section
- Superheaters
Heating surfaces and super heaters
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The boiler is a natural circulation boiler without reheat and is drainable, except for the platen SHs. All boiler walls form part of the evaporator system of the boiler and are all welded membrane walls.
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The feed water is divided in two streams. One stream led from the HP APH, HP FGC and the ECO to the boiler steam drum whereas the other stream leads from ECO to the boiler steam drum. The ECO and HP FGC are plain tube heating surfaces in a separate gas pass with steel casing to ensure optimal cleaning.
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The evaporator circulation system consists of a set of unheated down-comer pipes also providing support for the steam drum, connection pipes for the membrane wall riser tubes, membrane walls and overflow pipes connected to the steam drum.
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Saturated steam from the drum is led to the SH sections, which are installed as convective tube banks. The platen SH section is installed at the top of the furnace
Temperature control system
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After the screen, the flow is collected in headers and directed to the SH1 and further to first attemperators. The attemperators are with fixed nozzles. From the first attemperator, the steam is flowing through the SH2 and further to the second attemperator, arranged in counter flow with the flue gas, and further to the SH3 & SH4. SH3 & SH4 is located and arranged upstream the flue gas in radiant zone. From the SH4 outlet headers the live steam is led to the HP turbine.
Drain and venting system
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The pressure part of the boiler is equipped with drains from all low points except SH3/SH4. The drain valves are installed in drainage stations at the boiler flash tank. The drains from these areas are collected in a drain tank, and from here it can be pumped to the wastewater treatment plant.
Boiler walls and pipe connections
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The boiler is supported by the side wall bottom headers and is constructed so that the entire boiler pressure part can expand freely upwards.
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Loads caused by furnace over or under pressure or by slag formations are absorbed by the boiler reinforcements (buck stays).
STEEL STRUCTURES
General
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All steel structures are designed to give adequate support of all live loads local to the steam generating units, taking into consideration the climatic conditions on site.
Boiler Buck stays
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For the absorption of stresses arising from vibrations in the furnace chamber and from local slag deposits, the boiler walls are equipped with buckstays. The buckstays are anchored to the walls through special spacers enabling the boiler to expand freely.
BOILER AUXILIARY SYSTEMS
Air pre-heater (APH)
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A boiler cold end design has been chosen, i.e. with heating surfaces after the economizer (ECO) and with preheating of the combustion air using feed water as the heat carrier. The purpose is to keep the temperature of all heating surfaces in contact with flue gas above the acid dew point for hydrochloric acid (HCl).
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A HP by-pass branch makes it possible to use a part of the feed water to preheat the combustion air causing the feed water temperature to cool down. The cooled feed water is led to a HP flue gas cooler (FGC), which then heats up the feed water.
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The air and flue gas system of the boiler is carried out with one off radial FD fan & ID fan fitted with inlet silencer and one water/air pre-heater battery.
Soot Blowing System
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The standard soot blowing system typically consists of the following:
- Retractable steam soot blowers for cleaning of the convection surfaces
- Furnace wall water cannons including local control panel
- Steam and drainage pipe work system
- Safety valve
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The steam soot blowers are supplied with steam from the steam cycle at suitable pressure. The soot blowers are arranged for cleaning of both convection SH banks, and ECO. The steam soot blower control is to be integrated in the DCS system.
Bottom Ash Handling System
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The bottom ash handling system is wet bottom ash conveyor. It is drag chain conveyors, constructed of corrosion protected steel with 2 parallel chains on wheels with internal scraper irons for transportation of ash and slag. It consists of a horizontal water-filled part into which the ash and slag from the grate slag dump and the ash hopper under 2nd and 3rd gas passes are led, as well as a rising part with a discharge at the end.
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The purpose of the water is to ensure a cooling of the slag/ash and a sealing between boiler pressure and ambient pressure. The heavy-duty conveyor chain makes the transportation through and out of the water. Two parallel chains build the belt together with steel carriers in between. The outlet from the slag conveyor is placed above a slag pit (or sufficient corrosive resistant containers). Normally the further handling will be by a front loader.
