EPSRC Thermal Management of Industrial Processes

Transcription

1 EPSRC Thermal Management of Industrial Processes A Review of Drying Technologies (February 2010) Report Prepared by: SUWIC, Sheffield University Researchers: Dr Hanning Li, Dr K Finney Investigators: Professor Jim Swithenbank Professor Vida N Sharifi Sheffield University Waste Incineration Centre (SUWIC) Department of Chemical and Process Engineering Sheffield University

2 Executive Summary In accordance with the EPSRC grant proposal, Sheffield University has conducted an extensive literature review on biomass drying and an evaluation of the drying process. This report presents the results obtained from our literature review work. Various sources of information were used in order to compile this report. These included websites, journal publications, reports and communications with manufacturers and industry. The main topics covered in this review work include: Biomass Drying (benefits and drawbacks) Drying equipment and processes Assessment of drying technologies when using flue gases Assessment of drying technologies when using superheated steam Costs and environmental impacts of drying biomass Recent technological developments when using low temperature heat sources for drying biomass This report present the findings from the above review work. Acknowledgements: The authors would like to thank the Engineering and Physical Science Research Council (EPSRC Thermal Management of Industrial Processes Consortium) for their financial and technical support for this research work.

3 List of Contents 1. Introduction Classification and Properties of Biofuels Benefits of Drying Biomass for Combustion and Gasification Drawback of Dried Fuel Background of Biomass Drying Drying Process Stages of Drying Drying Equipment and Processes Rotary Dryer Flash Dryer Fluidized Bed Dryer Sprout Bed Dryer Belt Dryer Assessment of Dryer Technologies using Flue Gas Performance Heat Recovery Fire Safety in Dryers Environmental Aspects Cost Dryer Selection Superheated Steam Systems Dryer Type Advantages and Disadvantages Capacities of Dryers Performances Heat Recovery Cost Environmental Aspects Recent Development in Low Temperature Heat Sources for Biomass Drying DRY-REX SRE- Renergi LTD Dryer Microwave Conclusions References... 48

4 1. Introduction Dry biomass fuel provides significant benefits to combustion and gasification, but they must be balanced against increased capital and operation costs. Currently, several methods have been established and some promising technologies are being investigated. 2. Classification and Properties of Biomass Material of biological origin, excluding material embedded in geological formations and transformed to fossil fuels, is called biomass (Holmberg, 2007). As a renewable energy source, biomass is derived from living organisms, such as wood, herbaceous crops and waste. Woody biomass is composed of bark, forest residues, sawdust and cutter shavings. Herbaceous biomass is grown from numerous types of plants, including miscanthus, switch grass, hemp, corn, poplar, willow, sorghum and sugarcane. Biomass waste includes construction wood, crushed or chipped used wood, used paper, pulp and paper sludge, municipal solid waste (MSW), manufacturing waste and landfill gas. Figure 2-1 shows some samples of solid biomasses. Biomass is carbon based matter and is composed of a mixture of organic molecules containing hydrogen, oxygen, nitrogen and also small quantities of other atoms, including alkali, alkaline earth and heavy metals. Table 2-1 presents the properties of biomass fuels. 4

5 Bark Wood chips Pulp and paper sludge Crushed construction wood Sawdust Sugarcane bagasse Figure 2-1. Examples of biomass materials. 5

6 Table 2-1. Typical physical and thermochemical properties of selected wet biomass fuels (Bruce and Sinclair, 1996). Pulp and Wood Residues Milled Lignite Sugarcane Paper sludge Bark Fuel Chips Peat Bagasse Moisture % wet basis HHV, dry basis MJ/kg HHV, wet basis MJ/kg 11@50% 10@50% 10@50% 12@40% 10@50% Bulk Density (wet basis) kg/m Volatiles % dry wt Ultimate analysis Carbon % dry wt Hydrogen % dry wt Oxygen % dry wt Nitrogen % dry wt Sulphur % dry wt trace Ash % dry wt

7 2.1 Benefits of Drying Biomass for Combustion and Gasification It is commonly accepted that wet fuel consumes some heat of combustion to evaporate water in the fuel. As shown in Table 2-1, the average moisture content of biomass is typically between 55-60% (wet basis) depending on the biomass type. In addition to the high average moisture content, the weather, season and storage time may cause drastic deviations in the bark moisture. The moisture content of sawdust may vary from air-dry to 70% wet basis (Holmberg, 2007). The high moisture in the biomass will have a negative effect on combustion and gasification operations. The water should be removed from the biomass to improve the quality of combustion and gasification. The heating value (i.e. HHV, as shown in Table 2-1) is dependent on biomass moisture, where a lower moisture content in the biomass increases the heating value. As a result of drying, energy input into the boiler may be increased without increasing the fuel input, or the fuel input into the boiler may be decreased to get the same energy input, as in the case of moist fuel. The basic functions of the combustion control and burner management systems are to maintain constant steam flow or pressure under varying loads, through proper input of fuel and to maintain safe and efficient operation throughout the boilers load range. Compared with several other fuels (e.g. oil, natural gas and coal), the heating value of biomass varies as a result of varying moisture content. Generally, it is possible to control large changes in fuel quality during the combustion, but such boilers set high requirements for the process control. Drier biomass will significantly reduce the range of varying water content, facilitating process controls in combustion processes. With dry fuel, all the heat of combustion goes into heating the air and products, leading to a flame temperature of F ( C), while green wood has a combustion temperature of about 1800 F (982 C) (FBT, Inc., 1994). The increased flame temperature is beneficial in many aspects. First, the higher flame temperature means that there is a larger temperature gradient in the boiler for radiant heat transfer. More heat transfer takes place for the same boiler tube area, increasing steam production. In new boilers designed for dried fuel, the boiler can be smaller because less heat transfer area is needed. The high flame temperature with dry fuel could achieve more complete combustion of the fuel, 7

8 resulting in lower carbon monoxide (CO) levels and less flyash leaving the boiler. More complete combustion also means more heat is released from the fuel. In a new boiler, the fire box and the downstream ash handing system can be smaller. With better combustion, the extra air can be reduced. This reduction in excess air means less heat of combustion goes into heating air. Using less excess air also reduces sensible heat losses with the flue gases, hence increasing the boiler efficiency. The forced draft (FD) fan, which provides air for combustion, will consume less power with less excess air. Likewise, the induced draft (ID) fan, which draws the flue gas out of the boiler and through the pollution control equipment, will require less power. There will be an increase of up to 5-15% in the overall thermal efficiency, and possibly up to a 50-60% increase in steam production (Wade, 1998). 2.2 Drawback of Dried Biomass Fuel Drying biomass is expensive and the additional costs may discourage the use of dried biomass fuel. Figure 2-2 presents the costs for wood (ELECTROWATT-EKONO, 2003). The total production cost of pellets ranges from 52.2 to 81.3 /t (without drying) and from 73.5 to 94.6 /t (with drying). The drying costs are dependent on the technology used and range from 25 to 29 /t pellets. The production costs are also dependent on the annual operational time. If a plant is run in a 3 shifts, 7 days a week mode, the production costs are approx 84 /t pellets. Other operational modes and the associated costs include: 3 shifts and 5 days a week = 90 /t pellets, 2 shifts and 5 days a week = 101 /t pellets and 1 shift and 5 days a week = 133 /t pellets. The drying is an additional operational cost in the process and power industries, even though this may be offset by using smaller boilers, air emissions equipment and fuel handling equipment. If the required biomass is over dried, the energy consumption and maintenance costs could be significantly increased. The additional complexity may also affect the overall system operation. 8

9 Without drying Figure 2-2 Pellet costs without/with drying With drying In addition to the cost concerns, there are other operational and environmental issues, which must be addressed when using wet biomass material. Ash deposition on heat exchanger tubes is commonly known to reduce the heat transfer, which will subsequently result in a reduction in plant thermal efficiency. Ash deposition on heat exchanger tubes is also a precursor for corrosion and fouling processes in heat exchangers. Shao, et al. (2009) found that the dried biomass increased the ash deposition rate during biomass combustion. Such ash deposits from dried biomass decreases the amount of biomass available for the combustion and increases the maintenance costs (i.e. plant downtime to remove deposits). Burning dried biomass fuel results in higher combustion temperatures in the boiler. However, as the flame temperature increases, it approaches the fusion temperature of the ash. If the ash starts to flow and form slag, this can seriously affect the boiler operation. Usually the flowing temperature of the ash is safely above the flame temperature, but when contaminants from construction debris or salts are mixed with the fuel, the flowing temperature can be lower. The major problems of slagging are associated with alkali metal content, principally sodium and potassium, both common in biomass fuels with high ratio of alkaline metal oxides to silica. As dried fuel is burnt in the furnace, the increased ash leads to high concentrations of sodium and potassium and thus a tendency to slagging at lower temperatures. 9

10 Slagging can seriously affect the boiler operation. is reduced and maintenance costs are increased. resistance, but the cost is a concern. As a consequence, heat transfer tube life-time New material can be designed for corrosion With drier fuel, the higher furnace temperatures will tend to increase the formation of nitrogen oxides, though modern over fire air systems could minimise NOx concentration (compared to older designs). When NOx emissions from the firing of biomass fuel (even after drying), are compared with the natural gas-fired conventional burners, the levels are quite similar. However NOx emissions from biomass fired plants are significantly lower when compared with the oil and coal fired systems. Low NOx-burners are an active area of development and some natural gas and oil burners are capable of very low emissions, significantly lower than that from a conventionally designed biomass boiler. 2.3 Background of Biomass Drying Even though biomass drying is not common before combustion at the moment, commercial biomass dryers were used at pulp and paper mills in the 1970s and 1980s. The dominant combustion technique for biofuels at that time was grate firing. This type of boiler can handle fuels with varying moisture contents, but ideally a moisture content of 30-40% wet basis should be used (Wimmerstedt 1995). The main reason for investments in manufacturing and designing dryers in the 1970s and 80s was probably the high oil prices resulting from two oil crises. In some cases, the moist fuel also decreased the boiler capacity so much that it became reasonable to install a dryer in combination with the boiler. In the 1970s and 1980s, industrial dryers were direct flue gas dryers (Holmberg, 2007). Flue gases were either taken directly from the boiler or generated in a separate flue gas burner. The most common dryer types were drum dryers and flash dryers. Since the 1970s, fluidized bed boilers have replaced grate firing as a combustion technique (Huhtinen, 1999). Compared with grate firing, the fluidized bed boiler is a more suitable combustion technique for moist biofuels. However, the use of moist fuel decreases the energy efficiency of the power plant. One solution could integrate the existing flue gas dryers with fluidized bed boilers. Bad operating experiences, environmental considerations and economic 10

11 factors may block the integration development. Biomass is a burnable material with a heterogeneous particle size. It is also typical that the biomass flow contains stones and sand. It is obvious that the properties of biofuel set tough requirements for the operation of the dryer. For some dryer types (e.g. flash dryers), the maintenance cost may also be high. It is presumable that the operational experiences of the flue gas dryers have not always been satisfactory and bad experiences have supported the decision to develop the dryer. All types of wood material contain volatile organic material that may be emitted together with the water vapour, as shown in Table 2-1. The emissions of biomass drying are greatly affected by the drying temperature, when it exceeds 100 C (Holmberg, 2007). Below 100 C, emissions are reported to be low (Spets, 2004). The drying temperatures of flue gas dryers are clearly higher than 100 C. Exhaust gases or unclean condensates must be treated after the dryer if they contain high concentration of emissions. Treatment increases drying costs and the treatment of the exhaust gas may be a cost issue. 3. Drying Process Biomass drying systems consist of three principal factors: drying medium, heat supply and dryer type. Drying medium are mostly flue gas, air and steam. Heat supplies into the biomass are operated through convection (direct dryers), conduction (indirect dryers) or the combination of direct and indirect dryers. The type of dryers includes rotary dryers, flash dryers, belt dryers and fluidized bed dryers, among others. Rotary and flash dryers are mostly used in industry today. In direct-heated dryers, hot air, flue gas or superheated steam is in contact with the biomass material. The hot air, flue gas or superheated steam loses its sensible heat and provides the latent heat of evaporation to dry the materials. The air also removes the water vapour. The material can be agitated by mechanical devices or by fluidized air. The super-heated steam remains above its saturation without condensation. Indirect drying separates the biomass material from the heat source (either hot air or superheated steam) by a heat exchange surface. As a result, the latent heat of evaporation of the water vapour 11

12 is easy to recover since the water vapour is not diluted by air. the dryer can be designed to produce steam at a desired pressure. With super-heated steam drying, 3.1 Stages of Drying There are several steps for drying. First, the material must be heated up to the wet bulb temperature to produce a driving force for water to leave the wet material. Next, any surface moisture is evaporated very quickly. Once all the surface moisture is removed, the material must be heated to drive water from the inside of biomass to the surface so it can evaporate. This happens when the rate of drying drops as the material remains close to the wet bulb temperature. Once the material is completely dry, it begins to heat up to the surrounding temperature, because water is no longer present to keep the temperature low. These steps are important for drying a combustible material. High temperatures are desirable to increase heat transfer and minimize equipment size; but at same time, the fuel ignition could be a concern. By understanding these steps involved with biomass drying, fast drying at high temperatures can be exploited with minimal fire risk. Significant fire risks mostly occur in two instances. The first is after the surface moisture has been evaporated, but before an appreciable amount of water has been driven out from inside the biomass. During this very short period, no water vapour at the surface keeps the fuel particle cool, leading to its surface quickly heating up while the inside remains cool. If the surface remains hot for a long enough time, the biomass can ignite even if it is not completely dry. However, once the inside of the biomass starts to drive water to the surface, this supply of moisture will keep it cool until it is completely dry (Intercontinental Engineering, Ltd. 1980). Over-dried biomass is also a fire risk. If the biomass loses all its moisture, it will begin to warm and can ignite when it reaches its combustion temperature. Generally speaking, dryers are not designed to completely dry material. When a material still has moisture associated with it, its temperature will be very close to the wet bulb temperature of air as evaporation occurs, regardless of air temperature. Therefore, a very hot air stream can be used to dry biomass in a co-current flow process because the hot air is introduced to the dryer along with the wet biomass. 12

13 In super-heated drying, there is no wet-bulb temperature because only steam is present. The water in the fuel must instead be heated to its saturation temperature before it evaporates from the biomass, but once it turns to vapour, it does not need to diffuse through the air to get out of biomass or have saturated air removed from the surface to promote evaporation. As long as material temperature is higher than the saturation temperature, the vapour pressure of the water will cause the moisture to flow out of the material. In superheated process, the material will stay at its saturation temperature until it is completely dry, then the temperature will start to increase, the same as the air-dried case. Because no oxygen is present in super-heated steam drying, the fuel cannot burn, even at elevated temperatures. However, there is one potential risk of fire, i.e if the material is allowed to dry completely and heats up above its ignition temperature of about 260 C. Despite the medium used in the drying process, potential fuel ignition should be a concern in the equipment and process design, as well as the selection of dryers. 4. Drying Equipment and Processes Currently, various dryers are used in industry. The following descriptions introduce those that are mostly used in industry, including rotary, flash, belt, sprout bed and fluidized bed dryers. 4.1 Rotary Dryer Rotary dryers are the most common type for biomass drying. Despite the introduction of new technologies, the long-applied rotary dryer is still widely regarded as the workhorse of the drying process industries. The robust yet simple construction combines flexibility with reliability, enabling this type of dryer to handle a vast range of materials and to operate continuously under the most arduous conditions. The design also permits the use of the highest possible drying temperatures and, in contrast to other dryers, is not sensitive to wide variations in material size, moisture content or throughput. In its simplest form, the rotary dryer consists of a slightly inclined rotating cylinder, fitted with a series of peripheral flights arranged to lift, distribute and transport the material. The flights are designed to suit the particular handling characteristics of the material, which may vary with 13

14 increasing dryness. Figure 4-1 shows internal structure of flights in a typical rotary dryer made by Aeroglide Corporation and GEA Barr-Rosin. Figure 4-1. Internal construction of a rotary dryer made by Aeroglide Co. and GEA Barr-Rosin There are several types of rotary dryers, but the most widely-used is the directly-heated single-pass rotary dryer. In this type of dryer, hot air or gas is in contact with biomass material inside a rotating drum to induce the evaporation of the moisture. The rotation of the drum, with the aid of flights, lifts the solids in the dryer so they fall down through the hot gas, promoting better heat and mass transfer. If contamination is not a concern, hot flue gas can also be fed directly into the dryer. The heat in the hot air or gas evaporates the water and consequently, the gas temperature is rapidly reduced as it leaves the dryer. The exhaust gases leaving the dryer may pass through a cyclone, multi-cyclone, gashouse filter, scrubber or electrostatic precipitator to remove any fine material entrained in the air. An ID fan may or may not be required depending on the dryer configuration. If one is needed, it is usually placed after the emission control equipment to reduce erosion of the fan. It may also be placed before the first cyclone to provide the pressure drop through downstream equipment. Figure 4-2 is a schematic diagram of rotary dryer and its 14

15 associated processes. The biomass and hot air normally flow co-currently through the dryer, so the hottest gases come in contact with the wettest material, as shown in Figure 4-2. For materials where temperature is not a concern, the flue gas and solid flow in opposite directions, called counter-current flow, so the driest solids are exposed to the hottest gases with lowest humidity. This counter-current flow configuration produces the lowest moisture in the biomass as it leaves the dryer, but this exposes essentially dry material to a high flue gas temperature, which could increase the fire risk. Figure 4-2. Schematic diagram of co-current operation in a rotary dryer. The basic, single-pass rotary dryer design can be modified to allow three passes of the air and biomass through the dryer. The material first enters an inner cylinder with the hot air. Smaller and drier material is quickly blown through the cylinder into a larger concentric cylinder for the second pass. Larger material is moved and tumbled with the aid of flights. After the second pass, the air and material pass back up the outermost cylinder of the dryer and are removed. The triple-pass design works best with biomass smaller than an inch, because larger material can cause plugging. Single-pass dryers can take larger material. Another commonly applied technology is indirectly heated rotary dryers, which use a heat source steam or hot air passing through the outer wall of the dryer or through an inner central shaft to heat the dryer by conduction. This is more common with materials that would be contaminated by direct contact with flue gases or with materials that react with air. A hybrid direct/indirect 15

16 rotary dryer also exists where very hot flue gases enter the dryer through a central shaft and initially provide heat indirectly by conduction, then the same gases pass through the dryer coming into direct contact with the wet material. During the second pass, the indirect heating warms the flue gas and solids. In this way, a high flue gas or burner temperature can be used for heating, while reducing the fire risk by limiting the temperature of the gas in direct contact with biomass. The inlet gas temperature to rotary biomass dryers can vary from 230 to 1100 C and the outlet temperatures vary from 70 to 110 C. Most dryers have outlet temperatures higher than 100 C to prevent the condensation of acids and resins. Retention times in rotary dryers can be less than a minute for small particles, and minutes for larger materials (Intercontinental Engineering, Ltd., 1980; Haapanen, et al., 1983). The efficiency of the dryer is largely dependent on the differential temperatures between the inlet and exhaust gas, although the heat transfer rate is also influenced by the relationship between the design of flights and the speed of rotation. Irrespective of the gas and material temperatures, however, the drying (or residence) time may be critical, as this is governed by the rate of diffusion of the water from the core to the surface of the material. Numerous applications of rotary dryers have been made in the drying of sludges from municipal waste water treatment plants. Relative to other sludge drying processes, the dryer produces large quantities of exhaust gases containing odorous compounds. Other applications include pulp and paper, saw mills and board mills. The manufacturers supplying various rotary dryers include M-E-C Inc, Rader Inc., Raytheon (Stearns-Roger Division) and ABB Raymond (Bartlett-Snow). 4.2 Flash Dryer The pneumatic or flash dryer dries biomass rapidly owing to the easy removal of free moisture or where any required diffusion to the surface occurs readily; drying takes place in a matter of seconds. Wet material is mixed with a stream of heated air (or other gas), which conveys it through a drying duct where high heat and mass transfer rates rapidly dry the product. Flash or pneumatic dryers are followed by a cyclone. The gas passes through a scrubber to remove entrained particulate material. A simplified process of a flash dryer (without a scrubber) is shown in Figure

17 The flash dryer provides short drying times and the equipment is more compact than a rotary dryer. The flash dryer requires smaller sizes of biomass so as to transport and suspend it by an air stream. Some dry biomass can be mixed with the wet inlet material to improve material handing. Flash dryers have been used not only for wood waste, but also for peat, bagasse, lignite and other solids. Gas temperatures are slightly lower for flash dryers than for rotary dryers, but they still operate at temperatures above the combustion point. The solids retention time in a flash dryer is generally less than 30 seconds. Manufacturers include Flakt, of Sweden, Raymond Division of ABB Raymond, Ahlstrom of Finland, Williams Patent Crusher and Pulverizer Co., Inc. of St. Louis, MI, F.L. Smidth, of Denmark (Figure 4-4). Figure 4-3 flash dryer configuration 17

18 M.E.C. flash tube dryer Figure 4-4. Constructed flash tube dryer. Two flash dryers (15t/h evaporation. Bar-rosin) 4.3 Fluidized Bed Dryer In a fluidised bed dryer, as shown in Figure 4-5, the hot air is supplied to the bed through a special perforated distributor plate and flows through the layer (or bed) of solids at a velocity sufficient to support the weight of particles in a fluidised state. Bubbles form and break within the fluidised bed of material promoting intense particle movement. In this state, the solids behave like a free-flowing boiling liquid. As a result, the gas phase achieves good contact with the biomass particles, leading to well mixed gases and solids. Thus, fluidized beds operate at high heat and mass transfer rates to provide uniform and fast evaporation. In fluidized beds, the distributor plate is required to allow fluidising gas to be evenly distributed across the area of the bed. During processing with fluidized beds, many biomass materials may become a sticky or cohesive. Those with vibrating equipment, however, could be effective in reducing agglomeration. In addition, a rotating agitator can be incorporated within the first drying section of the bed. This slow-moving device serves to gently agitate the wet material, encouraging even fluidisation and eliminating rat-holing without causing particle degradation. The agitation also helps the gas phase to break up into bubbles that will improve heat and mass transfer. 18

19 Fluidized bed dryers are mostly used in steam drying processes and co-location near process plants or power plants. The manufacturers include GEA Barr-Rosin Inc, Canada; GEA Process Engineering Inc; Anhydro. Figure 4-5. Schematic diagram and manufactured fluidized bed dryers Sprout Bed Dryer Sprouted dryers (cascade dryers) are commonly used for dying grain, although they can be used for other types of biomass. The material is introduced by a flowing stream or screw driver and the drying media is introduced at the bottom (Figure 4-6). The feedstock then falls down to the bottom and is lifted again. The material is let out through the opening holes on the side of chamber. The residence time is generally a couple of minutes. The original cascade dryer design was by Bahco of Sweden. The original Bahco group is now part of the ABB group which have retained the rights for the rest of the world. Others include Hercules, Canada and ESI Inc. of Kenneshaw, GA. The cascade dryer is mostly be used for wood waste. 19

20 Figure 4-6. Sprouted dryer (Berghel et al., 2008). 4.5 Belt Dryer In belt dryers (Figure 4-7), the feedstock is spread onto a moving perforated conveyor to dry the material in a continuous process. Fans blow the drying medium through the conveyor and feedstock. If multiple conveyors are used they can be in series or stacked (i.e. multi-pass). Belt dryers are very versatile and can handle a wide range of materials. Recently, belt dryers are frequently used in low temperature operations to save energy, as well as to reduce air emission and fire hazards. Belt dryers are provided by Swiss Combi, Bruks Klöckner, Mabarex, Andritz Fiber. Figure 4-7 Drying plant in Nufri, Spain. Plant type: Belt drying plant. Input: Sludge from fruit juice plant. Heat Generation: waste heat, hot water. Evaporation: t/h. Start up:

21 5. Assessment of Dryer Technologies using Flue Gas Thermal drying systems for moist fuels can be grouped into two categories, flue gas/air type and steam type. Flue gas/air dryers use the sensible heat in the products of combustion to accomplish the drying and are a well established technology. The heat supplied in the gases, together with the moisture driven off, is directed first to the emission control equipment and then to the stack for release. Steam dryers use the latent heat in steam to accomplish the drying. The steam originating from the moisture driven off from the fuel is recirculated as the heat transfer medium. They are a more recent development, and come in a variety of forms. Their assessment will be discussed in next section. The following introduces the advantages, limitation, manufactures, capacity, operational conditions and costs of each dryer under flue gas/air as drying medium. 5.1 Performance Rotary Dryer Rotary dryers are less sensitive to particle size and can accept the hot flue gases. They have low maintenance costs. The material moisture is hard to control in rotary dryers because of the long lag time (Fredrikson, 1984). Rotary dryers also present a fire hazard and require the most space. Compared with single-pass dryers, triple-pass dryers have higher capital costs, higher maintenance costs, higher blower costs and pose more of a fire risk (Intercontinential Engineering, Ltd. 1980). The retention time for the smallest particles can be as short as 30s, while for most of the material it is in the order of minutes. Common design features affecting retention time include single and triple pass, and dense versus open internal flighting. The single pass dryer is less prone to plugging and can handle a wider range of particle sizes; a triple-pass dryer uses concentric cylinders to increase the gas path length and generally cost less. Some designs recirculate 55-65% of the flue gas to lower the gas temperature at the inlet to the dryer and reduce the generation of blue haze fine particulates composed of the more volatile components of the material being dried, which is particularly problematic in the case of wood. Rotary dryers are suitable for materials which are in the form of free flowing solids and fibrous particles of up to 125mm (5"), granules, pellets, broken filter cakes and powders. Additionally, 21

22 these materials must be moderately insensitive to heat (Bruce and Sinclair, 1996; Wade, 1998). Evaporation capacities are mostly in the range of 3-23t/h, corresponding to dryer sizes of 3 feet diameter x 8 feet to 13 feet diameter x 60 feet long. Quebec, Canada, has a rotary dryer with an evaporative capacity of 23 t/h of 18.5 feet diameter x 85 feet long for a board mill. The rotary dryer performance is summarized in Table 5-1. Table 5-1. Typical range of design and performance data for various dryers. Rotary Flash Sprouted Belt Evaporation (t/h) Temperature ( C) Capacity (t/h) Feed moisture (%) Moist. Discharge (%) Typical feed discharge 55,12 55, 35 58, 25 Pressure drop (kpa) Particle size (mm, opt) Particle size (mm, max) Thermal Requirement (GJ/t_evap) Flash Dryer Flash dryers are much more compact than rotary dryers, but have higher installation costs (Fredrikson, 1984). They can be used on most types of biomass, but have high blower power costs in addition to the heat requirements for drying. The particles being dried must be small enough to be suspended in the air stream. Heat recovery is difficult due to the fact that the air mixed with the water vapour. Flash dryers have a lower risk than rotary dryers due to the shorter retention time and lower operating temperature. The final moisture is normally in the range of 10-20% wet basis. Retention time is in the order of 2-10s, providing the potential for rapid response to control signals. Size reduction is required prior to the flash dryer. In flash dryers, oven dry throughputs ranging from t/h and evaporation is around 17 t/h. Flash dryer performance is summarized in Table

23 Sprout Dryer Sprout dryers are similar to flash dryers, except they can handle slightly larger particles. However, for a good cascading effect in the dryer, the particle size must be fairly uniform. Like other direct air-heated dryers, heat recovery is difficult and expensive. Moisture reduction is normally similar to that with rotary dryers, from 50-60% down to 20-40% range, wet basis. Retention time is in the order of 2 minutes. Based on 4 cascade dryer applications, Bruce and Sinclair (1996) summarized oven dry throughput rates ranging from t/h. The dryer performance is summarized in Table 5-1. Belt Dryer Belt dryers are better suited to take advantage of waste heat recovery opportunities because they operate at lower temperatures than rotary dryers. Rotary dryers, for example, typically require inlet temperatures of at 260 C for drying, but more optimally operate around 400 C. In contrast, the inlet temperature of at least one commercially available vacuum dryer can be as low as 10 C above ambient, although more typically belt dryers operate at higher temperatures between about 90 C and 200 C. Because of their lower temperatures, belt dryers can even be used in conjunction with a boiler stack economizer to take maximum advantage of heat recovery from boiler flue gas. In this scenario, an economizer first recovers heat from the boiler flue gas, then the exhaust from the economizer is used for fuel drying. Their lower temperature also means that there is a lower fire risk. Emissions of volatile organic compounds (VOCs) from the dryer will also be lower. An advantage of belt dryers over many other dryer types is that the material is not agitated. This means there may be fewer particulates in its emissions. On the other hand, fines may need to be screened out first and added back into the dryer at a later point, since they can fall through the belt s perforations. 5.2 Heat Recovery Energy efficiency in air drying can be improved by using heat exchangers, recirculating exhaust gases, multistage drying and heat pumps. In heat exchangers, the heat is transferred from the exhausted gas through the wall of a heat 23

24 exchanger to the inlet gas. The exhaust gas is usually not saturated, so part of it can be recirculated into the inlet of the dryer. Because the exhaust gas is generally warm, the energy is not needed for heating recycled exhausted gas. Overall, the drying efficiency can be improved. Besides, the exhaust gas, if high enough in oxygen, can be also used as preheated burner air. The latent heat of water vapour in exhaust gases can be recycled to heat inlet material. Wimmerstedt (1999) studied a case that a flue gas dryer was not co-located with other plants. The dryer was operated with a dew point for exiting gas of about 80 C. The gas can be used for material pre-heating in a direct contact process. If there is a demand for the material to be heating to a low temperature, a considerable part of the latent heat can be recovered. An assessment of the heat consumption in one such plant gave a specific heat consumption of 3140 kj/kg evaporated water. About 80% of the waste energy could be recovered for district heating but the outlet water temperature was as low as 50 C. Multistage drying can be used when high inlet temperatures are a concern. Instead of diluting the entire hot gas stream with cold air to reduce the temperature, some of hot gas can be introduced to later stages of the dryer to boost the air temperature. As a result, less dilution air is used. A run-around coil can be used where the physical layout of the dryer does not allow the exhaust gas to be close to the inlet gas. A heat carrier, such as antifreeze, oil, or any kind of heating fluid is first pumped through a heat exchanger coil in the exhaust gas duct, then through a heat exchanger in the inlet air duct to release its heat to the inlet air. The disadvantage is that two heat exchangers are needed, but this is sometimes cheaper than running extra duct work. A heat pump is similar to a run-around coil. Because the heat pump uses a refrigerant and compressor, it can recover part of the latent heat of vaporization by condensing or dehumidifying the exhaust gas and then provide this heat to the inlet air at a higher temperature. Heat pumps could improve the efficiency, but capital costs for the compressor can be very high with significant compressor energy requirements. Heat pumps are also generally limited to providing heat at no more than C. 24

25 5.3 Fire Safety in Dryers Fire safety refers to precautions that are taken to prevent or reduce the likelihood of a fire. Fires start when a flammable and/or a combustible material with an adequate supply of oxygen is heated to an ignition temperature. Combustible organic vapours are generally released from wood at temperatures from 200 C, where auto-ignition temperatures are generally C. Most dryers however can operate at much higher temperatures to keep the air temperature greater than the biomass surface temperature. This increases the drying rate, but also increases the fire risk in the dryer. For this reason, all dryers are designed to minimize fire risk and are equipped with fire suppression systems. In general, air drying has a potential high fire risk, because of the high amounts of oxygen in the air supply. This can be reduced by limiting the amount of excess air or by recirculating exhaust gases to the dryer inlet. Recirculation of exhaust gas also increases the thermal efficiency of the dryer. Flue gas dryers can operate at higher temperatures than air dryers, because flue gases have a lower oxygen content (Intercontinental Engineering, Ltd, 1980). Compared with air or flue gas dryers, superheated steam drying processes have a low fire risk. One risk however is the hot dried biomass coming into contact with air after drying (Haapanen 1983, Wardrop Engineering, Inc. 1990). As mentioned above, one important issue causing a fire hazard is the high temperature. One effective method is a reduction in the operational temperature. For example, an operation temperature of less than 100 C could significantly reduce the likelihood of a fire. Various drying processes at low temperatures will be discussed in Section 9. The maximum temperature that can be used at the dryer inlet is limited by the burning temperature of dry wood in air around C. The flue gas can reach high temperatures, since they are depleted in oxygen relative to air (typically 5-10% O 2 by volume). 5.4 Environmental Aspects The exhaust gas from a biomass dryer may need to be treated before release to atmosphere. 25 If

26 flue gas is used for drying wet fuels, the outlet gas from the dryer may contain SO 2, CO 2, CO, particulate matter (PM) and hydrocarbons. Table 5-2 shows emissions data after secondary cyclones for drying softwood. Table 5-2. Softwood dryer emissions data (Wade, 1998). A survey by Bruce and Sinclair (1996) indicated that for rotary dryers, total PM leaving the dryer is lower than entering. In some cases, the reduction is as high as 50-80%. To achieve a reduction in air emissions, the equipment is required to associate with the process. The first piece of equipment after the rotary or flash dryers is a primary cyclone to separate the biomass from exhaust gas stream. A set of multicyclones can follow the primary cyclone to remove some PM. Cyclones, however, are not very effective for very small particles and thus a baghouse or wet scrubber may be needed to remove small particles. Some of particles may be removed in part by cyclones and wet scrubbers. However, condensable volatiles can not be removed by dust collectors and appear in the stack emissions and result in undesirable blue haze. At dryer temperatures higher than C, condensable resins and organics will be released. After leaving the dryer, the condensed resins and organic acids form aerosol, called as blue haze (Wade, 1998). These condensable organics are also counted as particular matter (Bruce and Sinclair, 1996). The most effective method to control fine PM and aerosols are wet electrostatic precipitators. The others include filter bed and electro-filter bed scrubbers. The volatile emissions mostly consist of monoterpenes. Monoterpenes are naturally emitted from wood and have boiling points of C. The major components are α-pinene and β-pinene. 26

27 Photochemical reactions of monoterpenes with nitrogen oxides form low level ozone. Ozone is a strong oxidant and a component of smog. In high concentrations, ozone is responsible for impaired lung function in human populations, crop damage and is believed to be responsible for forest damage in Europe and North America. Volatile emissions control can be achieved by lowering peak temperatures, recycling exhaust gases leaving the dryer and reducing the amount of fine material in the dryer feed material and its residence time. 5.5 Cost In general, three kinds of cost information are available in the literature. The first kind of cost information is the dryer equipment alone, i.e. flue gas dryer equipment: rotary, cascade, and flash, and so on. The second kind of cost information is the complete costs, including equipment and installation costs. The third one is the operating cost, such as costs by electricity, water and gas, etc. The costs may change according to the different dryer types, installation and the retrofit of the dryer. During the course of this study, the costs for different dryers were identified: Rotary Dryer - Single pass rotary dryer = $20/kg/h, three pass rotary dryer = $18/kg/h. - The dryer cost for 7.6 t/h wet wood chips = $ (included installation cost); - The dryer cost for MW = $ million (complete unit); - Steam & Roger rotary single pass dryer = $26-47 /kg/h (complete unit); - MEC rotary single pass dryer = $24-65 /kg/h (complete unit); - Aeroglide rotary dryer = $17-31 /kg/h (complete unit); - Heil rotary dryer = $32-88 /kg/h (complete unit); - Biomass dryer = $38,000 /t/h(42/kg/h) (included installation cost); - Biomass flue gas dryer for 55t/h = $5.4 million(complete unit); The heat requirements were 3,000-8,1000 kj/kg of water removed., with most estimated in kj/kg Flash Dryer - For a disk dryer, the cost was estimated at $5.4; the capital cost of a flash dryer was $18-35 /kg/h. 27

28 - The total cost of both the equipment and installation of a MW flash dryer for was $ /kg/h. - The cost of the dryer for bark = $350 /kg/h. -There are two line Flakt flash dryer installed at the Assi Lövholmen Linerboard mill in Pitea, Sweden. One unit supplies a lime kiln, the other, a recovery boiler converted to a power boiler. The units cost 17.5 M$US in 1980, or the equivalent of 24.5M$US at the end 1995 for both the 6 and a 13 t/h capacity, or about 9.5 and 15MUS$ respectively. A second article describes a Flakt flash dryer installation at E. B. Eddy Ltd, in Espanola, Ontario, which cost 14MCdn$ in 1986 and handles about 18 t/h of wood waste (Wade, 1998). Cascade Dryer During the course of this study, technical articles providing capital costs for three cascade dryer installations were identified: Cascades Inc., East Angus, Quebec cost 36M$Cdn in 1992, or the equivalent of 32M$US at the end Included were both cascade and flash drying and suspension firing of part of the fuel. The cascade dryer with a throughput of about 9.0BDt/h accounted for about 5M$US; Fletcher Challenge, Crofton, BC, cost 8.5M$Cdn in 1986, or the equivalent of 7.2M$US at the end The cascade dryer had a throughput of about 36BDt/h. Alabama River Pulp, Claiborne, AL, cost 6.3M$US in 1992, or the equivalent of 6.6M$US at the end The cascade dryer had a throughput of about 32BDt/h. Bruce and Sinclair(1997) summarized the costs for three kind of dryers and found that the total installed costs for the complete dryer systems were very similar, if the wood fuel handling and storage is excluded. This cost information is summarised in table 5-3. It should be emphasised that the material handling equipment such as conveyors, feeders and bins is not included in any of the cost information presented. These costs and retrofits where space is limited can add substantially to the total cost of an installation. 28

29 Table 5-3. Capital Cost of Flue Gas Dryers (a) ( Bruce and Sinclair, 1996). Type Moisture Content In,% - Out,% Equipment Cost k$/t/h Total Installed Cost (b) k$/t/h Rotary Sprouted Flash Notes: a - based on all the boiler flue gas entering at 300 C and leaving the dryer at 105 C. b - the first value being for about 4 t/h, the second about 35 t/h. Besides equipment and installation costs, the operating costs are important concerns. The main components to the operating costs are those for power and maintenance. Power consumption based on oven dry throughput are: 8-14 kwh/t for rotary dryers; kwh/t for cascade; and kwh/t for flash dryers, the latter depending on the size reduction required, which is a large consumer of power. Size reduction power is highly variable depending on the equipment used, size distribution of the feed, product size, type of material being pulverised, species and initial moisture content. Summary data presented in a reference indicates a range of kwh/t or more. In the absence of data encountered in the literature on the maintenance costs of equipment, an allowance of 2% of total installed cost of the drying system equipment is suggested as the basis for preliminary evaluation purposes. 5.6 Dryer Selection The selection of dryers depends on a particular application. In general, water evaporation rate, biomass property, biomass size, operation temperature and heating resource availability are important in the selection. Meanwhile, environmental controls and safety are important considerations in the dryer design. For flue gas/air dryers, the selection of dryers is highly depended on the size of the biomass material. For flash dryers, a small particle size is needed for moving air or steam to suspend the particles. Triple-pass rotary dryers will accept larger material, but may experience plugging by 29

30 very large material. For large or variable material, a single-pass rotary dryer might be the best option. In general, reducing the size of the material may be a good option for the drying process, but it is an energy-intensive operation. In the selection of dryers, the advantages and disadvantages of various dryers are always considered. The significant advantages of rotary dryers include the fact that they are less sensitive to material size, they operate at high temperatures to reduce drying time, their wide range of evaporation rates and their easy installation. The major drawback is that these possess the greatest fire hazard, since the high temperature operation is mostly applied in rotary dryers. Air emissions need to be controlled. Heat recovery is difficult in rotary dryer. Flash dryers are more compact and easier to control, but require a small particle size. Air emissions again need to be highly controlled. Both flash and cascade dryers are used for high capacity water removal. Belt dryers are currently adopted in low temperature operations, which present a lower fire risk, reduced air emission and low energy consumption but require a large footprint. In general, rotary dryers are the most commonly selected. Technical knowledge of equipment configurations, installations and operations could provide useful information for new users. These are summarised in Table 5-*. Table 5-4. Summary of considerations in choosing a dryer. Requires Dryer Type small particles Heat recovery Fire Hazard Air Emission Drying Temperature ( C) Evaporation (t/h) Rotary No Difficult High medium Belt No Easy Low Low Flash Yes Difficult Medium High Cascade No Difficult Medium medium The capital costs of various dryers are often comparable. However, a belt dryer operated at low temperatures may require less equipment for treatment of emissions; so for new installations the overall cost may be less. The operation and maintenance costs of belt dryer are higher than for other dryers. In general, multi-pass dryers are more complex than single-pass dryers and so have greater operation and maintenance costs. 30