An Aspen Plus Model of Biomass Tor Ref Action | Biomass | Gases

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An Aspen Plus Model of Biomass Torrefaction University Turbine Systems Research (UTSR) Fellowship 2009 Ryan Dudgeon University of Iowa Electric Power Research Institute Charlotte, NC Introduction Biomass offers much potential as a renewable fuel for displacing coal in large- scale power plants. Given that the carbon contained in biomass is taken directly from the atmosphere, the fuel is largely considered to be carbon-neutral. Biomass also contains less sulfur, nitrogen,
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   An Aspen Plus Model of Biomass Torrefaction University Turbine Systems Research (UTSR) Fellowship 2009Ryan DudgeonUniversity of IowaElectric Power Research InstituteCharlotte, NC IntroductionBiomass offers much potential as a renewable fuel for displacing coal in large-scale power plants. Given that the carbon contained in biomass is taken directly from theatmosphere, the fuel is largely considered to be carbon-neutral. Biomass also containsless sulfur, nitrogen, ash, and heavy metals than coal. However, there are currentlyseveral problems with biomass that prohibit it from being used on a larger scale forpower production. It has a much lower heating value compared to coal and suffers fromlogistical issues related to transportation, handling, and storage. Because of highmoisture contents and low energy densities, the cost of transportation to the plant is high.In addition, the fibrous nature of biomass often causes handling problems, requiring moreenergy to be spent on grinding the material. Open-air storage can also be a hindrance asthe hydrophilic nature of biomass can cause it to absorb more moisture over time.Torrefaction has the potential to solve these problems by improving the properties of thefuel. It produces a higher quality product with increased heating value, increased energydensity, and improved grindability properties.Torrefaction is a mild pyrolysis process occurring at low temperatures in therange of 250  –  300°C and in the absence of oxygen. When biomass is torrefied, a portionof the volatile matter is driven off in the form of light gases and other condensableorganic compounds. The resulting solid material contains virtually no moisture, less  2volatile matter, and an increased fixed carbon content. A typical torrefaction processmay cause 30% of the mass to be lost in the form of volatile species, but a higherpercentage of the energy, typically 80-90%, is retained in the solid product, resulting inenergy densification of the biomass. The end product is often called bio-char or bio-coal.Torrefied biomass has much improved grindability properties and heating values muchmore comparable to coal. In addition, torrefaction results in a solid product that ishydrophobic in nature, making it easier to store for longer periods without absorbingsignificant amounts of moisture.Three important definitions related to the torrefaction process are the solid yield,energy yield, and reaction time. The solid yield is expressed on a dry-ash-free basis sincethis is the organic, reactive portion of the material. The solid yield is defined on a massbasis as shown below. daf  feed torr S mm      .    The energy yield is also reported on a dry-ash-free basis and can be based on the lowerheating value (LHV) or higher heating value (HHV). For this work the higher heatingvalue was used. daf  feed torr S E   HHV  HHV       .     The purpose of this project was to develop a model in Aspen Plus for simulatingthe torrefaction reactor and other unit operations associated with the torrefaction process.The purpose of the model was to determine optimal torrefaction and drying conditions formaximizing product output, product higher heating value (HHV), and process efficiency.Another key target of the model was determining conditions for autothermal operation tominimize utility fuel consumption. Another objective of the model was to serve as apredictive tool that could be compared to pilot-scale torrefaction testing being done witha local torrefaction company. A final goal of this project was to develop a first draft of specifications of torrefied biomass pellets based on literature review, existingspecifications of wood chips, and collaboration with the torrefaction company.  3Torrefaction Model DevelopmentModeling was performed with Aspen Plus [2]. The torrefaction reactor ismodeled based on the experimental data of [3]. They reported mass yields of solid,liquid, and gaseous products for torrefaction temperatures from 230°C to 280°C andreaction times from 1 hr to 3 hr. Data was reported for five biomass feedstocks includingbirch, salix, miscanthus, straw chips, and wood chips. For each feedstock, temperature,and reaction time, the ultimate analysis of the solid torrefied product was reported as wellas the volumetric concentrations of CH 4 , C 2 hydrocarbons, CO, and CO 2 in thetorrefaction gas. For each feedstock, a set of equations were created based on multi-variable linear regression of the experimental data to predict solid, liquid, and gaseousyields, solid product composition, and gaseous product composition as a function of torrefaction temperature and reaction time.Based on a user-specified torrefaction temperature and reaction time, theregression equations determine the mass flows to be dedicated to solid, liquid, andgaseous products based on the respective calculated yields. Regression equations alsodefine the composition of the solid torrefied product in terms of the ultimate analysis.The ultimate analysis and solid yield are used to calculate the amounts of carbon,hydrogen, oxygen, and nitrogen consumed for the torrefied solid product. Regressionequations also define the composition of the product gas. As was done with the solidyield, the predicted gaseous composition is used to determine the amounts of carbon,hydrogen, and oxygen consumed for the torrefaction gases. The remaining amounts of carbon, hydrogen, and oxygen not used for solid and gaseous products were thusdesignated as constituents of the liquid products. These components were reacted in twoRStoich blocks to predict condensable organic species.The torrefied solid product, being represented by a non-conventional component,must be given material properties. The model automatically fills the ultimate, proximate,and sulfur analyses of the torrefied biomass. However, the heating value must bepredicted because Aspen Plus uses it to calculate the enthalpy of the stream. Testsimulations showed that relatively small variations in the heating value for the torrefiedbiomass affected the torrefaction process immensely in the heat demand required fortorrefaction. The correlation chosen for the model was taken form [4]. It uses the  4ultimate analysis to predict the higher heating value (HHV) on a dry ash-free basis asdefined below.   9.58767.133.3019.134 /   O H C lb Btu HHV  daf   Figure 1: Aspen Plus model of torrefaction reactor.Simulation Procedure and Process DescriptionThe following torrefaction study considers a plant with a dryer, directly-heatedtorrefaction reactor, combustor, and heat exchanger. For the woody biomass (wood chipsand salix), the moisture content of the raw material is assumed to be a typically highvalue of 35%. For miscanthus, the wet feed was assumed to have a high moisture contentof 50%. If required, a portion of the raw biomass is diverted to the combustor withoutbeing dried, and the rest enters the dryer. The dryer is maintained at a temperature highenough to guarantee the drying medium does not become saturated, but low enough thatmild volatilization of the biomass is minimal. A dryer temperature of 105°C wastherefore used. The wet biomass was dried to 15% moisture, a value used as a base casescenario  –  the effects of moisture on the torrefaction process are discussed later.Combustion of the wet biomass and torrefaction gases was achieved using 150% of thetheoretical amount of air required for complete combustion. This value was chosen toensure that the flame temperature was kept below about 2500°F. Ash was separated fromthe combustion products before the flue gas was directed to the heat exchanger. Themajority of the torrefaction gases were circulated back to the torrefaction reactor fordirect heat exchange. These gases were heated to 50°C above the torrefaction
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