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Process Modeling of a Biomass Torrefaction Plant Yousef Haseli Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03956 • Publication Date (Web): 08 Mar 2018 Downloaded from http://pubs.acs.org on March 9, 2018
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Energy & Fuels
Process Modeling of a Biomass Torrefaction Plant
Y. Haseli School of Engineering and Technology, Central Michigan University, Mt Pleasant, 48849 MI, United States
Abstract A process model is developed to simulate the performance of a biomass torrefaction unit, which consists of a dryer, a torrefaction reactor, a combustor, and two heat exchangers. The model is capable of predicting the composition of volatiles and torrefied biomass, mass and energy yields, thermal efficiency, process heat requirement, and CO2 emissions. Useful correlations are presented for the heating value, molecular weight, and specific heat of the volatiles. A comparison of the model prediction with the experimental data reported in the literature showed a very good agreement. The effect of moisture content, torrefaction temperature, residence time and adiabatic flame temperature on key process parameters are examined. The thermal efficiency is found to be 88% at a moisture content of 50% (dry basis), which increases to 94% as the moisture content drops to 20%. The results show that the carbon dioxide produced in the process is notably affected by the torrefaction temperature and the moisture content. A higher moisture content or torrefaction temperature may lead to a higher CO2 emission. Furthermore, the conditions at autothermal operation are identified and discussed.
Keywords: Biomass torrefaction; Process modeling; Thermal efficiency; CO2 emission
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1. Introduction Interest in biomass energy has been growing fast since biomass is known to be a source of renewable energy available in most parts of the globe. The energy from biomass harvested through thermochemical processes include combustion, gasification and pyrolysis. The energy content of biomass is released during combustion which can be used as the source of thermal energy in utility and power sectors. Gasification and pyrolysis are processes which convert biomass into useful byproducts such as syngas (gasification) and bio-oil (pyrolysis). Major shortcomings of biomass that limit its wider utilization for power generation are low bulk density, high moisture content, low heating value, high energy requirement for grinding, and low combustion efficiency1-3. In order to improve the fuel properties of biomass, it needs to be pretreated. Torrefaction technology is a promising method to preprocess low quality biomass into high energy density feedstock. Torrefaction is a mild pyrolysis which takes place at temperatures between 200-300 ℃ in a non-oxidative environment. Biomass torrefaction removes the moisture content, increases the heating value, and lowers the energy requirement of grinding. The energy density of torrefied biomass is close to that of coal. So, it can be used to for syngas production with a higher gasification efficiency compared to raw biomass gasification4, or be co-combusted with coal in the furnace of an existing coal-fired power plant without major infrastructure changes5, 6. The higher energy density of torrefied biomass as compared to sawdust and conventional wood pellets would reduce transport costs and facilitate higher rates of co-firing with coal at power plants. The findings of a recent study7 suggest that the delivered cost of torrefied pellets (produced in North America) to Northwest Europe is 9% lower than that of the regular pellets due to saving in transportation and end-use costs. The grindability is also improved by torrefaction, which enables more efficient co-firing in existing pulverized coal power plants8, 9. These properties give the torrefied product advantages over raw biomass for transportation, storage, milling and feeding10. A literature survey indicates that a majority of published articles on this subject are experimental research with a particular focus on understanding the kinetic mechanism of torrefaction and fuel properties of the torrefied product11-20. The modeling studies on biomass torrefaction available in the literature allow prediction of the weight loss, energy yield, volatiles and solid yields, and 2 ACS Paragon Plus Environment
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Energy & Fuels
composition of volatiles evolved during torrefaction21-23. A few studies have been reported with the aim of process modeling of biomass torrefaction at system level, in that a torrefaction unit or process is simulated using an in house or commercial software2, integrated torrefaction and a CHP plant have also been documented
24-26
2, 27
. Preliminary results on
.
Joshi et al.24 studied a biomass torrefaction unit which uses pressurized superheated steam as the source of heat to the process through two heat exchangers. They used CycleTempo to simulate the torrefaction process by incorporating the externally programmed unit operation models of drying and torrefaction. Park et al.25 conducted process simulation of a torrefaction unit using Aspen Plus, and they developed a one-dimensional reactor model in Matlab which was linked to the Aspen Plus model. The reactor model takes the temperature and mass flowrate of the torrefying gas as the input parameters, and then generates temperature profiles of the solid and gas within the reactor. Their research was primarily focused on understanding the effect of temperature and flowrate of the torrefying gas on process parameters such as the energy and solid yields and heating values of the torrefaction products. Sermyagina et al.2 and Peduzzi et al.26 have individually presented a simplified semi-empirical approach to model the torrefaction process. Sermyagina et al.2 used correlations to determine the mass loss and heating value of the volatiles as a function of the torrefaction temperature. A constant value of -500 kJ/kg was used in the calculations and the effect of sensible heat was neglected. The model was employed to study various scenarios for integration of torrefaction and cogeneration plants. The model of Peduzzi et al.26 is based on the correlations obtained using the experimental data16,
28
which gives the C-H-O content of the torrefied beech as a function of
weight loss. The composition of the volatiles was then obtained by elemental difference between the biomass and torrefied biomass. The model was used to determine the heat requirement of the torrefaction of an unspecified biomass with a moisture content of 35% (wet basis) at a reaction temperature of 523 K which would yield a mass loss of 20%. They found a thermal efficiency of 90% (82%) with (without) combusting the volatiles (assumed to be a mixture of H2O, CO2 and C2H4O2). The previously cited works provide useful insight into torrefaction process, but they lack proper experimental validation. Furthermore, the few process models available in the literature were usually applied to certain torrefaction conditions, and a rigorous analysis of performance 3 ACS Paragon Plus Environment
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parameters such as thermal efficiency, utility fuel consumption, heat requirement, and emissions over a wide range of torrefaction temperature, residence time, and moisture content has not been presented yet. The literature lacks a comprehensive integrated model to describe the operation of a biomass torrefaction plant. Such a model requires accurate prediction of the properties of biomass, volatiles and torrefied biomass; e.g., LHV, specific heat, enthalpy of reaction, at a broad range of operating conditions. The objective of the present article is to develop a detailed process model that integrates the thermodynamic models of the components of a torrefaction unit shown in Fig. 1. Using the experimental data reported in past studies, simple correlations are derived to predict the composition of torrefied biomass, LHV, molecular weight and specific heat of volatiles at a given reaction temperature and residence time. The model is employed to examine the effect of key parameters on total heat requirement and thermal efficiency of the process. The operational conditions at which the process becomes autothermal, as well as the carbon dioxide emissions from the system are studied.
2. Modeling Approach The torrefaction plant to be modeled is schematically shown in Fig. 1, which is originally developed by the Energy Research Center of the Netherlands (ECN). It consists of a dryer, a torrefaction reactor, a combustor, a heat exchanger to preheat the torrefaction gas, and another heat exchanger to cool the torrefied biomass. Raw biomass is admitted to the dryer to remove the moisture content of the biomass before it is sent to the torrefaction reactor. The dried biomass undergoes a mild pyrolysis at a temperature up to 573 K. A portion of the volatiles leaving the reactor is recycled using a blower (not shown in Fig. 1) back to the reactor, which is used as the torrefaction gas. The rest of the volatiles is sent to a combustor where they burn in air. The torrefied biomass leaving the torrefaction reactor at a high temperature is cooled in a heat exchanger. The hot combustion products provide the thermal energy requirement for the drying and torrefaction processes as shown in Fig. 1. The thermodynamic model of each component of the torrefaction unit shown in Fig. 1 will be described in the following sub-sections.
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2.1 Dryer Biomass naturally contains a significant portion of moisture which consists of bound and liquid moisture content. = +
(1)
To determine the amount of bound and liquid moisture, one needs to find the fiber saturation point (FSP) using Eq. (2). = 0.598 − 0.001
(2)
where T is in Kelvin. For a moisture content higher than FSP, = and = − . However, if the moisture content is less than FSP, the entire moisture content is the bound water29. The amount of thermal energy required to remove the moisture content of biomass is determined by = ℎ + ℎ + ℎ
(3)
where ℎ is the enthalpy of evaporation, and ℎ denotes the enthalpy of sorption, which can be obtained by a correlation proposed by Stanish et. al30 and cited by Peduzzi et al.31. ℎ = 0.4ℎ "1 −
#$%& + '()
*
(4)
The total heat required for the drying process is found by accounting for both the sensible heat and evaporation heat2, 32. , - = ., + .,0 1+ − 2 3 + ., 122 − + 3 +
(5)
where is the mass flowrate of dry biomass, ., is the specific heat of dry biomass, .,0 and
., are the specific heats of water and steam, respectively.
A correlation recently proposed by Dupont et al.33, similar to the correlation reported by Ragland and Aerts34 and Bryden and Hagge35, is used to estimate the specific heat of the dry biomass. 8
., = 111 + 3.783 [ ] 9 .:
(6)
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The heat requirement of the drying process is provided by combusting the flue gases. Hence, applying the conservation of mass and energy to the dryer in Fig. 1, we have 22 = 2< +
(7)
2< ℎ2< = 22 ℎ22 + , - + ,,
(8)
where ,, denotes the heat losses from the dryer. 2.2 Torrefaction reactor The dry biomass with a known elemental composition enters the torrefaction reactor, where it undergoes a mild pyrolysis process at a given temperature and residence time. The dry biomass then decomposes into volatiles and torrefied biomass. E FEGH E IJ & EJ
=>? @ABCDD LMMMMMMMMMMMMMMMN OBPCQAPRD + B>>RSART @ABCDD where Tt is the torrefaction temperature and tt denotes the residence time. As shown in Fig. 1, a portion of volatiles is recycled and used as the torrefying gas. The conservation of energy applied to the torrefaction reactor yields + ℎ+ + U ℎU = V ℎV + W ℎW + E,
(9)
where E, is the rate of heat losses from the torrefaction reactor, and + =
(10)
V = XE
(11)
W = X + U
(12)
Note that Yt and Yv denote the torrefied biomass and volatiles yields, respectively, per unit mass of the dry biomass. Also, in Eq. (9), h is the total enthalpy at a given temperature T. Hence, I
ℎ = ℎ + YI . T
(13)
Z
where ℎ is the enthalpy of formation at standard temperature < = 298.15 \. It is obvious that Eq. (9) implicitly accounts for the heat of torrefaction reaction since all enthalpies in Eq. (9) include formation enthalpy of raw biomass, torrefied solid and volatiles.
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To be able to determine the mass yields of the volatiles and torrefied biomass, we need to adopt a kinetic scheme. The well-known two-stage first-order reaction mechanism proposed by Di Blasi and Lanzetta36 adopted in several past studies17, 21, 26 to model the kinetics of torrefaction is used in this work to describe the biomass decomposition.
Volatiles 1
kV1
Biomass
k1
Volatiles 2
kV2 k2
Intermediate Solid
Char
Applying the conservation of mass to the solid species gives ,]%
= −1^_2 + ^2 3
(14)
,]`a
= ^2 − ^_b + ^+ G
(15)
,]c
= ^+
(16)
,E
,E
,E
Integrating Eqs. (14)-(16) subject to the boundary condition Q = 0, = ,
