TECHNICAL FIELD

[0001] The present application relates to torrefaction of pre-processed biomass such as wood chips or wood particles for the production of feedstock for biofuels and the production of fuel feedstock for a power plant.

BACKGROUND

[0002] Torrefaction, also referred to as torrification, ratification or mild pyrolysis, is a process used for the treatment of biomass which changes the chemical and physical structure of the biomass. Torrefaction usually involves heating the biomass in a torrefaction reactor to a predetermined temperature for a predetermined length of time. A heater can be used to provide thermal energy to the torrefaction reactor.

[0003] Torrefaction of pre-processed biomass can be used to produce fuel. The torrefied biomass can replace coal, or can be co-fired with coal. The torrefied biomass can also be used as a feedstock for the production of biofuels, such as syngas, bio-oils, torrefied wood pellets or briquettes.

[0004] The moisture content in wet basis (WB) (i.e. the weight percentage of water in the wet material) of pre-processed biomass is usually used to determine the market value of the biomass as feedstock because higher moisture content reduces capacity and increases energy costs of torrefaction. Fresh wood in a forest has a moisture content (MC) of about 50%. If the wood is allowed to dry in the forest, for example if dead wood is harvested, the moisture content is about 35% or lower.

[0005] Pre-processed biomass can have a moisture content and density that results in a net calorific value (e.g. GJ/tonne) which makes the pre-processed biomass an economically undesirable alternative to coal or other fossil fuels. Net calorific value is also referred to as "lower heating value" (LHV). For example, pre-processed wood chips can have a moisture content of 30-70 wt% and a density of 250 to 450 kg/m ³ . As a result, pre- processed wood chips of the trunk wood with bark of a pine tree can have a net calorific value of 6.2 GJ/tonne at 60% moisture. In contrast, coal can have a net calorific value between 15 and 27 GJ/tonne, while wood chips of trunk wood with bark of a pine tree that has been dried to 0% moisture can have a net calorific value of 19.3 GJ/tonne.

[0006] In an example of a torrefaction process, torrefaction of wood at a temperature of 180-220°C results in wood which is hydrophobic and resistant to microbial growth and decomposition. This temperature range may be used to improve the qualifications and color of solid lumber and the resulting product is sometimes referred to as ThermoWood. Torrefaction of wood particles or wood chips at a higher temperature, such as 260°C, results in wood particles or wood chips which have about 70% of their original mass, are brittle, hydrophobic, and which have a 15-25% greater net calorific value than wood on dry basis. Torrefaction of wood chips or wood particles at a temperature between 200 and 300°C for 15-30 minutes can result in processed wood chips or wood particles with fuel properties comparable to coal. Such torrefied biomass is also referred to as BioCoal. Torrefied biomass can have a net calorific value over 23 GJ/tonne.

[0007] Torrefied biomass coming out of torrefaction reactor is usually at a high temperature and low density and is typically dusty without further processing. If the material is released into oxygen rich atmosphere it can catch fire due to self ignition. Torrefied biomass is therefore usually cooled and compacted to ease further handling and to reduce dust content.

SUMMARY

[0008] In a first aspect, the present application provides a process for producing compacted torrefied biomass from pre-processed biomass, the process includes: drying the pre-processed biomass in a dryer with heater exhaust gas from a heater, yielding dried biomass; torrefying dried biomass with the heat coming from heater and pre-cooling and compacting torrefied biomass in a torrefaction processor, yielding pre-cooled compacted torrefied biomass; and further cooling torrefied biomass, yielding compacted torrefied biomass. Drying the pre-processed biomass can include pre-drying the pre- processed biomass in a pre-dryer with the exhaust gas from the dryer. Pre-drying the pre- processed biomass with the exhaust gas from the dryer can filter dust fines from the exhaust gas from the dryer, condense volatile organic compounds from the exhaust gas from the dryer, or both filter dust fines and condense volatile organic compounds from the exhaust gas from the dryer.

[0009] Drying the pre-processed biomass can include flash drying the pre- processed biomass in a flash dryer with the heater exhaust gas. The exhaust gas from the flash dryer can be used for pre-drying the pre-processed biomass in a belt dryer prior to flash drying. Pre-drying the pre-processed biomass with the exhaust gas from the flash dryer can filter dust fines from the exhaust gas from the flash fryer, condense volatile organic compounds from the exhaust gas from the flash dryer, or both filter dust fines and condense volatile organic compounds from the exhaust gas from the flash dryer. [0010] The heater can be a combustion heater, torrefying the dried biomass can yield syngas, and the syngas can be used as a fuel in the combustion heater. Natural gas and/or syngas can be used as fuels in the combustion heater.

[0011] Torrefying the dried biomass in a torrefaction processor can include heating the dried biomass to temperature between 220 and 280°C for between 15 and 30 minutes in a low-oxygen environment.

[0012] The torrefaction processor can include processing torrefied biomass into pre-cooled compacted torrefied biomass in a cooling compactor directly connected to the torrefaction reactor maintaining the low-oxygen environment through the process.

[0013] The heater can include a burner, where the burner generates combustion gases, and dilution air is added to the combustion gases to yield the heater exhaust gas.

[0014] The heat generated by the heater can be delivered to the torrefaction reactor by a thermal transfer fluid.

[0015] The pre-processed biomass can be any suitable biomass such as, for example, agricultural waste, wood or woody biomass. The woody biomass can be any suitable material such as, for example, wood or wood waste. The process includes processing the wood or wood waste into wood chips or hog fuel, and sizing the wood chips or hog fuel into wood particles.

[0016] The process can be a continuous process and operating parameters of the process can be determined based on measurements obtained from at least one gauge in the process. The process can be automatically controlled with Programmable Logic Control.

[0017] The heater exhaust gas and the dryer exhaust gas can be in fluid communication and can be moved using a fan and a booster fan.

[0018] A process for producing compacted torrefied biomass from pre-processed biomass, the process including: pre-drying the pre-processed biomass in a belt pre-dryer with exhaust gas from a flash dryer, yielding pre-dried biomass; flash drying the pre-dried biomass in a flash dryer with heater exhaust gas from a heater, yielding flash dried biomass; torrefying the flash dried biomass in a torrefaction reactor with heat generated by the heater, yielding torrefied biomass; and pre-cooling and compacting the torrefied biomass in a cooling compactor, yielding the pre-cooled torrefied biomass and further cooling the pre-cooled torrefied biomass in a cooling tunnel; yielding compacted torrefied biomass.

[0019] This summary does not necessarily describe all features or aspects of the invention. Other aspects and features will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the application in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Embodiments of the present application will now be described, by way of example only, with reference to the attached figures, wherein:

FIG. 1 is a simplified flow diagram illustrating a process to torrefy pre-processed biomass according to an embodiment of the present application;

FIG. 2 is a flow diagram illustrating a process to torrefy pre-processed wood particles according to a specific embodiment of the process illustrated in FIG. 1 ; and

FIG. 3 is a flow diagram depicting an example of the process illustrated in FIG. 2.

DETAILED DESCRIPTION OF EMBODIMENTS

[0021] The present application provides a process of torrefaction of pre- processed particles of biomass. The biomass can be any suitable such as agricultural waste (e.g. straw or bagasse) woody biomass, or the like. Examples of woody biomass include wood chips, hogfuel, pre-processed wood waste, and the like. Woody biomass can be sized into wood particles. Wood waste can include urban wood waste from demolitions or renovations, process waste from sawmills or pulp mills, or waste from harvesting trees, such as tree branches. Wood chips can be produced using a wood chipper, which takes wood logs and chips the logs into the wood chips. Hog fuel can be produced using a "hog" or a "crusher", which takes woody biomass, such as bark or wood waste, and produces chips of bark and wood fiber. Wood particles can be produced by taking a woody biomass, such as wood chips or hog fuel, and breaking the woody biomass into wood particles.

[0022] The process equipment can be located near a utility power plant and used as a fuel refining process, where the utility power plant co-burns BioCoal with fossil coal using existing equipment. The process equipment can be located near a source of pre- processed biomass and compacted torrefied biomass can be more economically shipped by train or truck than the pre-processed biomass due to the higher net caloric value, higher density and low moisture content of the compacted torrefied biomass. The higher net calorific value, higher density, low moisture content and hydrophobic nature of material supports the more economically-viable storing and handling of the compacted torrefied biomass. [0023] FIG. 1 shows a simplified flow diagram illustrating a process according to the present application. Pre-processed biomass 101 is dried in a two-stage dryer 104, generating dried biomass. The second stage of the dryer 104 , the main dryer 103, is heated using a heater exhaust gas 106 from a heater 108. The heater exhaust gas 106 entering the main dryer 103 is exhausted from the main dryer 103 as a dryer exhaust gas 110. The dryer exhaust gas heats a pre-dryer 102 and is released into atmosphere as vent gas 130. The dried biomass is torrefied in a torrefaction processor 105 using heat generated by the heater 108. The heater 108 provides the heat to the torrefaction reactor 112 using a heat transfer fluid 114. The heat transfer fluid 114 can be thermal oil. The heater 108 is fueled using a syngas 116, produced by the torrefaction reactor 112 and by supporting fuel 118. The supporting fuel 118 and syngas 116 are combusted using oxygen from atmospheric air 120.

[0024] The pre-cooled torrefied biomass is produced by the torrefaction processor

105 consisting of the torrefaction reactor 112 and the cooling compactor 122. FIG. 2 shows a flow diagram that illustrates a process according to a specific embodiment of the process illustrated in FIG. 1 . While the description of the flow diagram shown in FIG. 2 refers to wood particles 202, other biofuels can also be used.

[0025] In one example of the embodiment illustrated in FIG 2, the process takes

1 1 tonnes per hour of wood particles that have a moisture content of 35 wt% (WB), and produces 5.0 tonnes per hour of BioCoal that has a moisture content of 2.8 wt%(WB).

Pre-processing

[0026] Sand, grit and metal are removed from the wood particles 202. The wood particles 202 are stored in a pile or in storage bins or in silos until they are used in the process illustrated in FIG. 2. The wood particles 202 are fed on a conveyor leading into the dryer 104 using a feeder, such as a reclaimer.

Sizing

[0027] Smaller wood particles facilitate drying and torrefaction. However, size reduction creates dust and fine particles (fines), and more power is utilized to make smaller particles, thus adding to process material loss and cost. Small amount of moisture is evaporated during sizing. Optimum size of wood particles may require screening and depends on a number of variables, such as the species of wood, the moisture content of the material, and the equipment available for sizing (e.g. hammer mills, knife grinder or rotary hogs). In the present example, the wood particles 202 are size reduced to particle size less than 6 mm, with moisture content of 34.7 wt% (WB) after sizing.

Drying

[0028] Wood particles can have a moisture content between 20% and 65% (WB) and can be dried to a moisture content between 6 and 20 wt%. In the present example, the wood particles 202 have a moisture content of 34.7 wt% moisture and are dried to a moisture content of 8.2 wt% moisture. The wood particles 202 are dried in the dryer 104, which includes a belt dryer 204 and a flash dryer 206. Alternatively, the flash dryer 206 can be a rotary dryer which allows longer drying time for the wood particles. A flash dryer and a rotary dryer are air dryers that use hot gas and separate the dried wood particles with a cyclone.

Drying - Belt dryer

[0029] The wood particles 202 are conveyed, for example, on a conveying wire mesh belt through the belt dryer 204 as a mat having a material thickness from 10 to 30 cm. The mat is leveled, for example, with a leveling screw at an infeed of the belt dryer 204. The mat is moved through the belt dryer 204 and the wood particles 202 are heated by the exhaust gas 110 from the flash dryer 206. The dryer belt speed can be varied, based on the moisture content of the material, to adjust the residence time of the wood particles in the belt dryer 204. The residence time can be between, for example, 10 and 60 minutes, and is varied to result in pre-dried wood particles having a moisture content of less than 50%. The hot exhaust gas 110 can be distributed into the belt dryer 204 through a manifold on top of the conveying mesh belt. The hot exhaust gas 110

penetrates through the mat of the wood particles 202 and the conveying mesh belt. The hot exhaust gas 110 heats the wood particles 202 and removes moisture from the wood particles 202, resulting in the pre-dried wood particles. The pre-dried wood particles are collected from the conveying belt at an outfeed of the belt dryer 204 using, for example, a conveying screw, chain or belt.

[0030] The mat of wood particles filters dust and condenses volatile organic compounds (VOCs) present in the hot exhaust gas 110. The hot exhaust gas 110, which has been filtered by the mat, is collected with a fan from under the conveying belt and is vented as the vent gas 130 into the atmosphere through a stack. The vent gas 130 temperature is kept above the dew point to prevent moisture from condensing inside the dryer. [0031] The hot exhaust gas 110 can be between 100 and 230 °C. The vent gas

130 can be between 60 and 130 °C. The pre-dried wood particles can be between 50 and 1 10 °C and can have moisture content between 15 and 50 wt% (WB). In the present example, the hot exhaust gas 110 is at a temperature of 178°C; the vent gas 130 is at a temperature of 102°C; and the pre-dried wood particles are at a temperature of 60°C and have a moisture content of 27% (WB).

Drying - Flash dryer

[0032] Flash drying, also known as flash tube drying, is a method of removing surface water from a material in an enclosed ducting, where escape of dust and VOCs is reduced. To flash dry the pre-dried wood particles at 206, the pre-dried wood particles are fed from the pre-dryer 204 into a flash drying duct of the flash dryer 206 through a rotary feeder, where the pre-dried wood particles are transported airborne in the heater exhaust gas 106 from the heater 108. The heater exhaust gas 106 evaporates surface water on the wood particles and is exhausted from the flash dryer 206 as the exhaust gas 110. The wood particles are separated from the exhaust gas 110 using a flash dryer cyclone and airlock at the end of the dryer ducting. The exhaust gas 110 from the flash dryer cyclone of the flash dryer 206 has a higher moisture content (e.g. vol water vapor / vol air) than the heater exhaust gas 106 from the heater 108. The exhaust gas 110 is blown by a dryer fan into a manifold distributing air on top of the belt of the pre-dryer 204.

[0033] The dried wood particles can be between 50 and 100 °C and can have a moisture content between 6 and 20% (WB). The heater exhaust gas 106 can be between 220 and 350°C and can include water vapor between 0.5 and 5% vol water vapor / vol air. The dryer exhaust gas 110 can be between 120 and 220 °C and can include water vapor between 3 and 8% vol water vapor / vol air.

[0034] In the present example, the dried wood particles are at a temperature of

70°C and have a moisture content of 8.2 %; the heater exhaust gas 106 is at a temperature of 300°C and includes 2.2 % vol water vapor / vol air. The dryer exhaust gas 110 is at a temperature of 178°C, is pumped at a rate of 53,800 am ³ /h, and includes 5.2% vol water vapor / vol air.

Torrefaction

[0035] The flash dried wood particles from the flash dryer 206 are fed into the torrefaction reactor 112 with, for example, a screw conveyor and a rotary or dual-gate airlock. The airlock can reduce excess oxygen in the reactor. The flash dried wood particles are heated to a predetermined temperature for a predetermined length of time, yielding torrefied wood particles. The temperature can be between 180 and 330°C, and the time period can be, for example, 15, 30, 60, 120, or 240 minutes. In industrial applications, in order to reduce costs, wood particles can be torrefied at a temperature between 220 and 280°C so that the torrefaction process takes between 15 and 30 minutes.

[0036] During the torrefaction process, the wood particles are dried and heated under reduced oxygen levels, changing the physical and chemical properties of the wood particles. The torrefaction process can affect the hemicelluloses and evaporate condensable and non-condensable gases with low calorific value, thereby increasing the calorific value of the remaining material. The torrefied wood particles can have a darker color, higher carbon concentration and higher calorific value than the un-torrefied, dried wood particles. The torrefied wood particles are more hydrophobic and lose fiber structure, making them more easily pulverized by grinders. The torrefied wood particles can lose about 25 to 40% of mass and about 10% of energy content in comparison to pre-processed wood on dry basis.

[0037] In the present example, the wood particles are heated to a temperature of

260°C for 25 minutes, resulting in torrefied wood particles that have a moisture content of 2.8 wt% (WB) and a calorific value of 23.4 GJ/tonne.

[0038] The evaporated gases (e.g. the syngas 116) can be burned as fuel in a burner 208, along with the support fuel 118. The support fuel 118 can be natural gas, propane, heating oil, or any other fuel that can support combustion of the syngas 116. The burner 208 mixes the syngas 116, the support fuel 118 and the air 120, ignites the resulting mixture and generates a flame which yields heat 210 and the heater exhaust gas 106. The burner 208 can be connected to a refractory-lined combustion chamber 212 to allow additional combustion of any unignited syngas 116. Alternatively, the burner 208 can be connected directly to a heat exchanger 214 of the heater 108. The heater exhaust gas 106 is used in the flash dryer 206, as described above. The syngas 116 can be produced at a rate between 120 to 400 kg of syngas per tonne of wood particles on dry basis depending on wood species, torrefaction temperature and residence time in the torrefaction reactor. In the present example, the syngas 116 is produced at a rate of 2,283 kg syngas per 1 1 tonnes of wood particles at 35% moisture (WB), or at a rate of 320 kg syngas per tonne of wood particles on dry basis.

[0039] The heat 210 generated in the burner 208 is used to heat the heat transfer fluid 114 in a heat exchanger 214. The torrefaction reactor 112 is heated indirectly using the heat transfer fluid 114 in order to provide accurate temperature control to the torrefaction reactor 112. Accurate temperatures in the torrefaction reactor 112 controls torrefaction results and can reduce mass loss of the wood particles. The torrefaction reactor 112 can include a commercially available screw, disk wheel or paddle-type dryer adapted to the torrefaction process. In such dryers, the wood particles are convection heated by direct contact with heated rotating elements that move the material through the torrefaction reactor 112 at a predetermined rate, resulting in a predetermined residence time. The torrefied wood particles are released into the cooling compactor 122.

[0040] The reactor casing is sealed and evaporated water vapor and syngas 116 are removed from the casing and piped into the burner 208.

Cooling compactor

[0041] The torrefied wood particles can be compacted with a cooling compactor 122 connected directly to the outlet of the torrefaction reactor 112 thus maintaining the low oxygen environment of the torrefaction reactor 112. The cooling compactor 122 compacts the torrefied wood particles at an elevated temperature while the torrefied wood particles are plastic and can be bond together with the lignin forming part of the torrefied wood particles. If necessary other starch-based, lignin-based, or polymer based liquid binder can be pumped into cooling compactor to improve bonding and to reduce dust formation. The cooling compactor can be, for example, a screw compactor, where the compacting screw is water cooled with cooling water 215 to pre-cool the torrefied wood particles 215 to a temperature lower than self ignition temperature. The cooling compactor releases the pre-cooled compacted wood particles 216 in form of nuggets having density of 400 to 700 kg/m ³ and temperature 100 to 180 °C.

Cooling tunnel

[0042] The pre-cooled torrefied wood particles 216 can be cooled to a lower temperature and the heat can be recovered from a cooling tunnel 220, yielding compacted torrefied wood particles 218. The pre-cooled compacted wood particles 216 are conveyed through a cooling tunnel where material is exposed to cooling air 217. The cooling tunnel can preheats the dilution air 218. Pre-heating of the dilution air 218 leads to energy savings in the heater 108.

[0043] The pre-cooled compacted torrefied particles 216 or compacted torrefied wood particles 218 can be further processed into pellets or briquettes in a separate process to increase the density of the material. Heating

[0044] The heater 108 is used to generate the thermal energy utilized for the torrefaction reactor 112 and the dryer 104. The heater 108 includes the burner 208, the combustion chamber 212 and the heat exchanger 214. Alternatively, the burner 208 can be directly connected to the heat exchanger 214. The burner 208 accepts a fuel and combusts the fuel using oxygen from the air 120. The burner 208 can burn natural gas, propane or heating oil, the syngas 116 produced in the torrefaction reactor 112, another appropriate fuel such as wood fines, or a combination thereof.

[0045] In the specific embodiment illustrated in FIG. 2, the heater 108 uses syngas 116 produced from the torrefaction reactor 112 as a main fuel source, with natural gas as a flame carrier to sustain the combustion. Because the syngas 116 includes water vapor produced in the torrefaction process, the water vapor is also heated during combustion. Predetermined combustion parameters are used to provide appropriate retention time of the fuel for desired combustion levels. The atmospheric air 120 can be added at a rate between 600 and 1 100 m ³ /h for each megawatt (MW) heat created by burner. The natural gas can be added at a rate between 1 and 40 % of syngas depending on wood species and the moisture content of the wood particles.

[0046] In the present example, the burner 208 creates 4.75 MW heat, the air 120 is added at a rate of 5173 m ³ /h, and 67 kg of natural gas is used per 2,283 kg of syngas (i.e. 2.9 wt% natural gas).

[0047] Dilution air 218 is blown into an inlet of the heat exchanger 214 and mixed with the hot gases from the combustion chamber 212. Alternatively, dilution air 218 can be blown into a mixing box following the heat exchanger 214. The dilution air 218 adds to the drying capacity of the heater exhaust gas 106. The hot gases from the burner 208 are used in the heat exchanger 214 to heat the heat transfer fluid 114 that is circulated through the heated rotating elements in the torrefaction reactor 112, as described above. The heat exchanger 214 is sized to provide the heat transfer fluid 114 to the torrefaction reactor 112 at a suitable temperature and flow rate. The heater exhaust gas 106

exhausted from the heat exchanger 214 is used to dry the wood particles in the flash dryer 104, as described above. Exhaust gas ducting from the heat exchanger 214 is connected to a material infeed point of the flash dryer 206 and provides the heater exhaust gas 106 to the flash dryer 206.

[0048] The heat transfer fluid 114 can be pumped at a rate between 800 and 900 kg/min for each MW heat used for material drying and torrefaction in the torrefaction reactor 112. The dilution air 218 can be added at a rate between 6800 and 13600 m ³ /h for each MW net heat used for the dryer 104. In the present example, the torrefaction reactor 112 consumes 1 .08 MW heat and the heat transfer fluid 114 is pumped at a rate of about 900 kg/minute, or 1 .4 m ³ /min. The dryer 104 consumes 2.85 MW heat. The dilution air 218 is added at a rate of 30700 m ³ /h while a portion of the heat is transferred with combustion gases and water vapor from the burner 208. Certain part of heat 210 is covering process and stack losses.

[0049] Because the heater 108 of the present example uses the syngas 116 produced in the torrefaction reactor 112 as the main fuel source, and because the torrefaction process of the present example uses wood particles, the cooled torrefied wood particles 216 are nearly C0 ₂ neutral. For each kWh produced in a power plant, the C0 ₂ emissions of the cooled torrefied wood particles 216 can be about 1.5% of the C0 ₂ emissions of fossil coal.

Gas flow

[0050] Because the heating gases (106, 110, 130) are propelled along a continuous, enclosed flow path, once the gases are flowing at the desired flow rate, booster fans with reduced electric power requirements can be utilized to overcome system resistance and maintain the flow rate. The desired flow rate can be set based on the flow rate of the dilution air 218 resulting heater exhaust gas 106. Booster fans can be positioned after exhaust gas 110 leaving the flash dryer 206 and after the pre-dryer 204. Booster fans can additionally be utilized to adjust the flow rate in a particular process step. The flow rate could be adjusted to account for the thermal expansion of a gas in the particular process step.

[0051] Additionally, because the gases are in a closed system and the exhaust gas 110 is filtered using the mat of wood particles in the pre-dryer 204, the amount of dust and volatile gases escaping into the atmosphere can be limited. Filtering the exhaust gas 110 using the mat of wood particles in the pre-dryer 204 can result in an process where the vent gas 130 from the pre-dryer 204 is not further cleaned with an air cleaning device, such as with a cyclone, or a baghouse, or a wet scrubber, or a wet electrostatic precipitator, or a combination thereof.

Thermal Energy

[0052] By utilizing one or more of: (a) efficient heat transfer in the torrefaction reactor 112, (b) syngas 116 as the main fuel, (c) efficient drying system, (d) thermal energy recovery, and (e) closed gas flow through the system, the thermal energy efficiency of the process is improved and use of the support fuel 118 is reduced.

[0053] For example, in the process illustrated in FIG. 2, the combustion chamber

212 operates at a temperature which allows clean burning of the syngas 116. The heat exchanger 214 heats the heat transfer fluid 114 which heats moving parts of the torrefaction reactor 112. The moving parts are in direct contact with dried wood particles and heat the wood particles using convection heating and accurate temperature control. The torrefaction reactor 112 generates the syngas 116 and torrefied wood particles. The cooling tunnel 220 recovers heat from the torrefied wood particles and pre-heat the dilution air 218. The heater exhaust gas 106 is used in the flash dryer 206. The thermal energy of the resulting exhaust gas 110 is used in the pre-dryer 204 to reduce the amount of usable thermal energy being released to the atmosphere by the vent gas 130.

[0054] Thermal energy efficiency is also improved in the process illustrated in

FIG. 2 by using a multi-stage drying process. In the multi-stage drying process, the wood particles 202 are slowly heated in the pre-dryer 204 to move free water and capillary water to the surface of the wood particles and partly evaporate the moisture, thereby reducing the moisture content of the wood particles; the pre-dried wood particles are rapidly heated in the flash dryer 206 to evaporate the surface water; and the flash dried wood particles are heated in the torrefaction reactor 112 to remove free and cellular water from the wood particles.

Process Controls

[0055] Material and gas of the process illustrated in FIG. 2 are moved from one portion of the process to another portion of the process in a continuous manner. A control point for the material is the torrefaction reactor 112. The amount of wood particles is metered through the flash dryer 206 to allow adequate processing time in the torrefaction reactor 112, where the processing time in the torrefaction reactor 112 is determined based on the desired torrefaction results. Similarly, the flow rate and the temperature of the heat transfer fluid 114 are determined based on the desired torrefaction results.

Temperature gauges in set location inside the torrefaction reactor 112 can be used to collect control information. The pre-dryer 204 can be used as an intermediate storage location by varying the speed of the conveying mesh belt. Temperature measurement at the end of the flash dryer 206 can be used to adjust the air flows and rate of cooling water addition of the cooling compactor 122. The process controls can be automated with Programmable Logic Control (PLC) or similar technology. [0056] The flow diagram shown in FIG. 3 is an example of the process illustrated in FIG. 2.

[0057] In the example illustrated in FIG. 3, wood chips 302 are stored in a wood chip silo 304. The wood chips 302 have a moisture content of 35% (WB) and a density of 250 kg/m ³ . The wood chips 302 are delivered to a chip sizer 306 at a rate of 1 1 tonnes/hour.

[0058] The chip sizer 306 generates 44 m ³ of wood particles 202 per hour. The wood particles 202 have a density of 250 kg/m ³ and a moisture content of 34.7 % (WB).

[0059] The wood particles 202 are delivered to the pre-dryer 204. The wood particles 202 are conveyed on a conveying mesh belt through the pre-dryer 204 as a mat having a material thickness of 20 cm, and which is 4 m wide and 15 m long. The total volume of the mat is 13.5 m ³ and the conveying mesh belt travels at a rate such that the wood particles 202 have a residence time in the pre-dryer 204 of 0.3 hours.

[0060] The mat is moved through the pre-dryer 204 and the wood particles 202 are heated by the exhaust gas 110 from the flash dryer 206. The exhaust gas 110 is blown from the flash dryer 206 by a dryer exhaust fan 316 that provides 53,770 actual cubic meters per hour (am ³ /h) at 178 °C temperature. The exhaust gas 110 includes water vapor at a rate of 2,790 kg water per hour. The pre-dryer 204 dries the wood particles 306 and generates pre-dried wood particles 310.

[0061] The mat of wood particles filters dust and condenses volatile organic compounds (VOCs) present in the exhaust gas 110. The exhaust gas 110, which has been filtered by the mat, is vented as vent gas 130 at a flow rate of 41 ,900 am ³ /h, and at a temperature of 100 °C. The vent gas 130 includes water vapor at a rate of 5,000 kg/h. The vent gas 130 is propelled by a vent gas fan 312.

[0062] The pre-dried wood particles 310 are delivered to the flash dryer 206 at a rate of 9.8 tonnes/h. The pre-dried wood particles 310 have a moisture content of 27%, resulting in 2661 kg of water per hour entering the flash dryer 206.

[0063] The pre-dried wood particles 310 are transported airborne in the heater exhaust gas 106 from the heat exchanger 214. The heater exhaust gas 106 evaporates surface water on the pre-dried wood particles 310, generating flash-dried wood particles 314. The heater exhaust gas 106 is exhausted from the flash dryer 206 as exhaust gas 110. [0064] The heater exhaust gas 106 is blown from the heat exchanger 214 at a rate of 83,500 am ³ /h and at a temperature of 300°C. The heater exhaust gas 106, including water vapor at a rate of 1690 kg water per hour, enters the flash dryer 206.

[0065] The exhaust gas 110 is blown from the flash dryer 206 using a flash dryer exhaust fan 316. The flash dryer exhaust fan 316 provides the exhaust gas 110 at 56,600 am ³ /h and at a temperature of 178 °C. The exhaust gas 110, including water vapor at a rate of 2,794 kg water per hour, enters the pre-dryer 204

[0066] The flash-dried wood particles 314 are at 70°C and have a moisture content of 8.2%. The flash dryer 206 provides 7.8 tonnes of the flash-dried wood particles 314 per hour to the torrefaction reactor 112. The torrefaction reactor 112 produces 5.0 tonnes of torrefied wood particles 318 per hour. The torrefied wood particles 318 have a moisture content of 2.8% and are at a temperature of 260°C.

[0067] The torrefaction reactor 112 also generates, at a temperature of 260°C, the syngas 116 at a rate of 2,280 kg/hour and water vapor at a rate of 500 kg/hour. The syngas 116 is burned in the burner 208 and the combustion chamber 212 along with natural gas 320. The natural gas 320 is burned at a rate of 70 kg/hour.

[0068] Heat generated in the combustion chamber 214 is used to heat thermal oil

322 in the heat exchanger 214. The thermal oil 322 circulates between the heat exchanger 214 and the torrefaction reactor 112 at a rate of about 900 kg/min, which corresponds to1.4 m ³ /min. The thermal oil 322 enters the heat exchanger 214 at a temperature of 265°C and exits the heat exchanger 214 at a temperature of 290°C. The combustion air 120 is blown into the burner 208 using an air fan 324 at a rate of 5,200 m ³ /h. The hot gases from the combustion chamber 212 are mixed in the heat exchanger 214 with the dilution air 218, which can be pre-heated by sucking air through the cooling tunnel 220. The dilution air 218 is blown into the heat exchanger 214 using a dilution air fan 326 at a rate of 30,700 nm ³ /h.

[0069] The torrefied wood particles 318 are pre-cooled and compacted in the cooling compactor 122. The cooling compactor 122 receives the torrefied wood particles 318 at a rate of 5.0 tonnes/hour. The torrefied wood particles 318 have a moisture content of 2.8%.

[0070] The cooling compactor 122 produces pre-cooled compacted wood particles 216 with density of 500 kg/m ³ and temperature of 150 °C. The cooling tunnel 220 yields compacted torrefied wood particles 218 at a temperature of 50°C. The dilution air 218 is pre-heated from 20 °C to temperature of 40 °C. Example 1

[0071] Mass and energy balances of a process according to the present application is shown in Table 1 , below.

[0072] In the preceding description, for purposes of explanation, numerous details are set forth in order to provide a thorough understanding of the embodiments of the application. However, it will be apparent to one skilled in the art that these specific details are not required in order to practice the application.

[0073] The above-described embodiments of the application are intended to be examples only. Alterations, modifications and variations can be effected to the particular embodiments by those of skill in the art without departing from the scope of the application, which is defined solely by the claims appended hereto.