by A Alamia · 2015 · Cited by 48 — Belt dryer is the typology better suited to exploit low-temperature heat (130°C or lower), 27B91/$File/WHDU_R_2080_040-02.pdf?OpenElement.
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Chalmers Publication LibraryDesign of an integrated dryer and conveyor belt for woody biofuelsThis document has been downloaded from Chalmers Publication Library (CPL). It is the author´sversion of a work that was accepted for publication in:Biomass & Bioenergy (ISSN: 0961-9534)Citation for the published paper:Alamia, A. ; Ström, H. ; Thunman, H. (2015) “Design of an integrated dryer and conveyorbelt for woody biofuels”. Biomass & Bioenergy, vol. 77 pp. 92-109.http://dx.doi.org/10.1016/j.biombioe.2015.03.022Downloaded from:http://publications.lib.chalmers.se/publication/214131Notice: Changes introduced as a result of publishing processes such as copy-editing andformatting may not be reflected in this document. For a definitive version of this work, please referto the published source. Please note that access to the published version might require asubscription.Chalmers Publication Library (CPL) offers the possibility of retrieving research publications produced at ChalmersUniversity of Technology. It covers all types of publications: articles, dissertations, licentiate theses, masters theses,conference papers, reports etc. Since 2006 it is the official tool for Chalmers official publication statistics. To ensure thatChalmers research results are disseminated as widely as possible, an Open Access Policy has been adopted.The CPL service is administrated and maintained by Chalmers Library.(article starts on next page)
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1 Design of an integrated dryer and conveyor belt for woody biofuels Alberto Alamia 1 , Henrik Ström 1,2,* , Henrik Thunman 1 1 Division of Energy Technology, 2 Division of Fluid Dynamics Chalmers University of Technology *Corresponding author: [email protected] Combustion or gasification of high – moisture content biomass is associated with a number of drawbacks, such as operational instabiliti es and lowered total efficiency . The present work proposes an integrated dryer and conve yor belt for woody biofuels with steam as the heat transfer medium. The use of low – temperature steam is favorable from a heat management point of view, but also helps to minimize the risk of fire, self – ignition and dust explosions. Furthermore, the present ed dryer design represents an efficient combination of fuel transport, drying equipment and fuel feeding system. The proposed design is developed from a macroscopic energy and mass balance model that uses results from computational fluid dynamics (CFD) fue l bed modeling and experiments as its input. This CFD simulation setup can be further used to optimize the design with respect to bed height, steam injection temperatures and fuel type. The macroscopic model can be used to investigate the integration of th e dryer within a larger biomass plant. Such a case study is also presented, where the dryer is tailored for integration within an indirect steam gasification system . It is found that the exergy efficiency of this dryer is 52.9%, which is considerably highe r than those of other dryers using air or steam, making the proposed drying technology a very competitive choice for operation with indirect steam gasification units .
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2 1 Introduction The use of biomass for energy production is becoming increasingly popular due to the fact that biomass is generally regarded as a CO 2 – neutral fuel. Wood represents a major source of biomass energy, and woody biofuels are particularly interesting in countries with large forest resources. An important difference between biofuels and most conventional fossil fuels is that the former have significantly higher and more varied moisture content. In gasification, fuel drying is required to avoid the combustion of support fuel or product gas, to sustain the process. If the heat demand f or drying can be reduced, more fuel can be gasified and the efficiency of the process is increased. Furthermore, biofuel with large fluctuations in the moisture contents cause s problems with regard to the stable operation of the gasifier. T herefore in gasi fication systems, the fuel is usually dried to a moisture content below 15% on wet basis (w.b.) [1] . Combustion of biofuels with high moisture content is possible , but associated with several drawbacks. First of all, the latent heat that has to be supplied in the combustor to evaporate the water cannot be utilized for power generation, since the temperature at which it can be recovered is too low (i.e. around 100 ºC at atmospheric pressure). Furthermore, a boiler that is operated with high – moisture fuel must have larger dimensions for the same thermal output. In addition to operational instabilities, the additional heat sink provided by the moisture increases the risk for harmful emissions. Woody biofuels can have initial moisture contents as high as 50 – 65% w.b. [2 , 3] . The typical heating values of dry biomass fuels are around 15 – 22 MJ/kg dry ash – free (daf) [4] . Low – temperature drying (below the boiling point of water ) can reduce the moisture content down to 10 – 15% w.b. [1] . The focus of the current work is on the design of a combined high – temperature dryer and conveyor belt for woody biofuels. More specifically, the aim is to explore the potential of high – temperature drying for large – scale processes in regions where the use of biomass has a high economic value, thus allowing for a higher level of complexity and larger investments.
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3 The overall purpose of this paper is thus to present a general design of an integrated dr yer and conveyor belt, and calculations that support the chosen detailed dryer design. The paper includes a case study for the integration of the proposed drying system in a steam gasification plant, which represents the situation where the use of the prop osed dryer is most advantageous, showing a high potential in terms of energy and exergy efficiency. The process of biomass drying and the proposed dryer are first described in Section 2. The modeling underlying the numerical simulations of the dryer perfor mance is introduced in Section 3, and the results of these simulations are presented and discussed in Section 4. The case study for the integration of the dryer within an indirect steam gasification plant, is presented in Section 5. The paper finally concl udes with a summary of the findings and a final evaluation of the dryer belt design. 2 Biomass drying Woody biomass at the point of delivery is usually in the form of chips or chunks with the largest dimension in the range of 10 – 80 mm [5] , and a moisture content between 50 – 60 % depending on the season and the type of wood. If a biomass has a lower heating value (LHV) of 19 MJ/kg and a moisture content of 50% w.b. , the heat demand for the complete evaporation and heating of the moisture up to a gasificatio n temperature of 900°C is about 22% of the LHV of the fuel. However, if the biomass is pre – dried to 10% w.b. moisture, the heat demand is only 2.5 % of the fuel LHV . During gasification this heat is provided by combustion of the fuel or product gas. By red ucing the moisture content , a higher fraction of the biomass can be gasified and the total efficiency of the process is increased. Drying is also beneficial for decreasing the dimensions of the gasifier and the ancillary equipment.
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4 In power generation pla nts, the drying of the fuel increase s t he efficiency of the thermal conversion of the biomass. Fuels with low moisture contents can also minimize other combustion control problems caused by fluctuations in the fuel properties [1] . Nevertheless, biomass dry ing is an intense process that requires a substantial input of energy, which influences the total efficiency of the process if valuable heat is used. It is therefore advantageous to use waste heat at low temperature, and to integrate the drying system with in the heat exchanger network of the biomass plant. Sources of heat include heat exchanger exhaust, turbine e xhaust, flue gases from combustion of by – products [1] , or process steam at low temperature. Depending on the combination of the heat source and the technology employed, the drying can either be accomplished directly by heat sources such as flue gas, back – pressure steam or extraction steam [2] , or via an intermediate drying medium (air or steam). In addition to improving the efficiency of the process, the drying system should minimize the risk of fire and explosion, reduce the emissions of pollutants and ensure a homogeneous fuel feeding. A fire or explosion in the dryer can arise from the ignition of volatile organic compounds (VOC) released during th e drying. Thermal degradation of the biomass starts above 100°C and becomes significant above 120 – 130°C depending on the type of biomass [6] . The risk of fire is , however , increased during an unintended stop of the dryer when VOCs can accumulate. The main measure to ensure a safe and reliable operation of the dryer is to maintain a sufficient inert atmosphere by continuous monitoring of the oxygen level and to install emergency safety equipment [6] , especially in air and flue gas dryers. S uperheate d steam dyers require lower safety measures because they eliminate the risk of fire and explosion by guarantee ing an oxygen – free atmosphere around the biomass [5] . Where it is possible to maintain a low – oxygen environment in the dryer, the drying temperatu re can be raised to 200°C or above [4] , which reduces the drying time and the size
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5 of the equipment. However, such a high temperature could produce a significant release of VOCs, which translates into an energy loss and causes environmental problems . Sever al different types of biomass dryers are available, the most common are: rotary dryers [7][8], fluidized bed dryers (including flash dryers and superheated steam dryers) [4] and belt dryers [4]. Belt dryer is the typology better suited to exploit low – temp erature heat ( 130 °C or lower) , limiting the risk of fire, harmful emissions and in some case allowing heat recovery from the dryer . Biomass is disposed on a permeable belt (e.g. a perforated conveyor or filter mesh/mat), and transported along the dryer whi le the drying medium is blown by fans through the belt and the biomass bed. The height of the biomass bed is typically between 2 cm and 30 cm, depending on the type of biomass. Due to the low temperature used, these dryers have long retention times and con sequently require large installations. Typical temperatures of the drying medium are between 60°C and 200°C. They are safe to operate , minimizing the risk of fire and explosion, and produce low emissions of VOCs. T his type of dryer is suitable to recover w aste heat, reduce the emissions of pollutants and minimize fire hazards . I t is now used in many applications (e.g. sawdust drying in pellet production). 2.1 Proposed drying system The proposed drying system is intended mainly for gasification, but can be applied to biomass combustors as well. The dryer process design is based on the concept of a conventional belt dryer and the operation of the dryer is continuous. Known advantages of belt dryers include low operation temperature, low gaseous emissions, lo w fire hazards, high robustness with regard to varying fuel properties and high potential for heat recovery [4] . The drying system , depicted in Figure 1, consists of two consecutive belt dryers with a possibility of intermediate storage. The first stage uses a conventional belt dryer that employs low – temperature heat sources (below 100°C) and air as the drying medium. D rying can
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7 Because of the fire hazard associated with the storage of biofuel, the particles must be stored well away (50 – 300 m) from the gasifier. A long conveyer belt is thus needed to transport the particles to t he combustor. With the proposed dryer design, this distance is efficiently utilized, as the transportation time is used to decrease the moisture content of the particles. Furthermore the biomass is pre – heated and delivered to the feeding system in a steam atmosphere, avoiding nitrogen contaminations. 3) Efficient heat management As the drying of the biofuel represents a significant part of the combustor system energy utilization, efficient energy management is of utmost importance. Low – pressure steam is re adily available in combustion plants and represents a suitable choice from the heat management perspective. The steam can, for example, be generated from a combination of low – grade heat available in hot cooling water and flue gases [4] . The proposed design of the integrated steam dryer and conveyor belt is illustrated in Figure 2. The biomass is transported on a mechanical belt along the drying unit, while superheated steam is injected from above, drying the particles by supplying the energy needed for wate r evaporation. Furthermore, the steam will help remove part of the dust formed in the handling of the biomass, and this dust will be collected on the belt as the steam passes through it. In this respect, the belt acts as a filter for the dust particles. Th e dust can then be scraped off from the belt at the end of the conveyor section. The steam that has passed through the bed is led via a three – way valve to a fan and a heat exchanger. The valve is regulated to remove the flow of moisture evaporated, and mai ntain the mass balance of the steam within the dryer. The fan and the steam heat exchangers are designed to restore the steam flow pressure and temperature to the injection conditions.
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8 Air or flue gases can be used to purge the biomass into the dryer; they are evacuated at the beginning of the belt together with some steam. Therefore some water is fed in to maintain the steam balance in the first part of the dryer where any moisture is evaporated. As the biofuel storage site and the combustor are typically separated by long distances, the conveyor belt as depicted in Figure 2 would necessarily also be long. However, an alternate, module – based design could be conceived, in which the total length of the conveyor belt is split into smaller sections. This varia tion of the original design has several advantages: 1) the injection temperature of the steam can easily be varied along the dryer; 2) the packing of the bed of biofuel can be adjusted, by varying the height and velocity of the different belt sections; 3) additional locations for dust removal are introduced. 2. 2 Potential for integration within an indirect steam gasification system Although the proposed belt dryer offers the possibility of heat integration through indirect heating and recirculation of the d rying medium, the enthalpy stored in the evaporated moisture leaving the dryer cannot be recovered by any other means than condensation. If the dryer is integrated with a plant using steam in processes directly involving the biomass, the moisture evaporate d can be then re – used without condensing and the potential for heat recovery is significantly increased. The integration of the steam belt dryer within a biomass gasification plant , using steam as the gasification agent , has been investigated further in a case study, since it introduces additional advantages and can be beneficial for the whole process. In indirect gasification, p re – heating of the stream s entering the gasifier (fuel and gasification/fluidization steam) is beneficial for the heat balance of t he gasifier – combustor system. Reducing the heat demand of the gasifier can increase the yield of product gas since less fuel needs to be burnt in the combustor. Shifting of fuel drying and pre – heating outside of the gasifier also enables low – temperature h eat to be used instead of high – temperature heat from fuel combustion. The temperature in the last part of the dryer can be raised to pre – heat
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9 the fuel to 105ºC 140ºC. However, the highest temperature in the biomass bed should not be higher than 200°C [11 ] in order to avoid significant devolatilization of the biomass. Furthermore , the moisture evaporated along the dryer is recycled to the gasifier and used as a gasification agent, reintroducing the moisture in to the mass balance of the system and reducing the steam consumption. In a steam dryer , the moisture content can be lowered from 20% w.b. to around 2 % w.b. , leading to a ratio between the removed moisture and the dry biomass of around 0.2 3 . The steam – to – fuel (dry ash free) ratio for gasification and fluidization in a bubbling bed reactor is in the range of 0.5 1 [12] . Therefore , a significant part of the gasification steam can be substituted using the moisture, which contains some fraction of volatiles components as well. Indirect gasification techn ology has the advantage of producing nitrogen – free gas by using steam as gasification agent . At current state of the art, the biomass is purged in the gasifier by using carbon dioxide, if available, or flue gas, allowing a small fraction of nitrogen in the product. Both carbon dioxide and flue gas introduce contaminations in to the product gas and these have to be removed later in the fuel synthesis process, which is expensive. T herefore , for this type of gasifier , the optimal choice for bio mass pre – heating and purging is steam. By c ombining the d r yer and feeding system, it is possible to achieve efficient drying in an inert atmosphere, pre – heat the biomass and part of the gasification steam, and purge the fuel without contamination. 3 Modeling Macroscopi c modeling of the dryer based only on global heat and mass balance cannot be accurate, because the correct combination of the steam flow and the steam temperature cannot be found a priori , without considering the fluid dynamics in the dryer. A multi – scale modeling approach is used instead to evaluate the viability of the proposed dryer design. On
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10 the macroscale, the entire dryer is simulated using macroscopic heat and mass balances. Information about the drying process for a two – dimensional cut of the steam flow through the packed bed of wood particles is obtained from computational fluid dynamics (CFD) simulations. In these CFD simulations, the evolution of the drying front inside an individual particle is modeled using a particle submodel. The results from the CFD simulations are used in the macroscopic description of the dryer to enhance the numerical predictions of the capacity of the dryer. The minimum temperature of the steam leaving the belt must be limited to avoid steam condensation. Here, the minimu m steam temperature allowed along the dryer is 105°C. Furthermore, the highest temperature inside the biomass particle should be monitored to control the release of volatiles. To maintain the steam and the biomass temperatures within an appropriate range, the biomass bed height and the steam injection temperature are varied along the dryer. The dryer is divided in two sections. In the first section the biomass is heated to around 100°C, with minimal moisture evaporation. To prevent steam condensation in th e first section, the steam injection temperature is higher than in the rest of the dryer and steam flow per kg of biomass is increased by lowering the height of the biomass on the belt. Most of the moisture is evaporated in the second section of the dryer, where the bed height and the steam injection temperature are adjusted to limit the temperature of the dry biomass to reduce the VOC emissions. This approach is applied both in the macroscopic model and in the CFD simulation. Steam diffusion along the belt is neglected and the steam mass balance is calculated independently in the two sections. 3.1 Thermodynamics of moisture desorption Moisture in wood exists in two basic forms: liquid free water in the wood cavities and bound water sorbed within the wood c ell walls. In studies of wood drying, it is necessary to take into
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