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A new procedure to develop lumped kinetic models for heavy fuel oil combustion Yunqing Han, Ayman M. Elbaz, William L. Roberts, and Hong G Im Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01685 • Publication Date (Web): 20 Sep 2016 Downloaded from http://pubs.acs.org on September 22, 2016
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A new procedure to develop lumped kinetic models for heavy fuel oil combustion
Yunqing Han a, *, Ayman. M. Elbaz a, b, William L. Roberts a, Hong G. Im a Clean Combustion Research Center a
King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia
b
Mechanical Power Department, Engineering-Materia, Helwan University, Cairo, Egypt
Abstract A new procedure to develop accurate lumped kinetic models for complex fuels is proposed, and applied to the experimental data of the heavy fuel oil measured by thermogravimetry. The new procedure is based on the pseudo-components representing different reaction stages, which are determined by a systematic optimization process to ensure that the separation of different reaction stages with highest accuracy. The procedure is implemented and the model prediction was compared against that from a conventional method, yielding a significantly improved agreement with the experimental data. Keywords: low grade fuel, heavy fuel oil, combustion, TG, lumped kinetic model
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1. Introduction Toward clean and efficient utilization of petroleum resources, there is growing interest in better understanding of the combustion characteristics of low grade fuels, such as heavy fuel oil (HFO). HFO has a complex molecular composition with a wide variety of chemical kinetic and physical decomposition behavior. These characteristics are commonly measured by heating an HFO sample under controlled conditions. Thermogravimetry (TG) is used to determine the mass loss, and differential thermal analysis (DTA) or differential scanning calorimetry (DSC) is further used to identify the exothermic and endothermic nature. Based on the thermal analysis results, many important features have been characterized. It is in general understood that HFO undergoes three main stages during combustion: the low temperature oxidation (LTO), fuel deposition (FD), and high temperature oxidation (HTO).1-6 During LTO the residue provides fuel for FD, which subsequently produces fuel for HTO, during which the majority of exothermic heat is generated through the oxidation process. 7-9 Furthermore, the consecutive distillation during HFO oxidation process involves a large number of high molecular weight hydrocarbon species: the distillate components such as benzene, ligroin, kerosene, and gas oil; and the heavy residues such as paraffins, light/middle/heavy base oils, resins and asphaltenes.1,3,10 Therefore, the detailed kinetic modeling of HFO combustion based on first-principle approaches remains a significant challenge with large uncertainties in the predicted and measured chemical kinetic rates of various sub-processes. As an alternative to describe such complex physical and chemical processes, a lumped kinetic model is considered a rational compromise, in which several key reaction steps are determined by kinetic parameters in the Arrhenius form:
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dα = Ae − E/RT f α dt
( )
(1)
where α is the normalized degree of reaction progress, dα/dt is reaction rate, t is time, T is temperature, R is the gas constant. The kinetic parameters to be determined are: A, the preexponential factor; E, the activation energy, and f(α), referred to as the model function. The primary aim of any kinetic study is to determine the kinetic parameters of E, A, and f(α). A “model-fitting” approach adopts various forms of f(α) at different stages of the reaction progress, and the kinetic parameters are determined based on the regression analysis of the experimental data. Among a number of model-fitting approaches the Coats-Redfern method11 has been widely used to determine the integral of the model function using an asymptotic approximation. Recent studies12-15 employing this method to the three stages of the HFO oxidation processes determined the activation energies for the stages of LTO and HTO with a wide range of variation from 2.4 to 138 kJ/mol and 42.3 to 370 kJ/mol, respectively. Studies using other model-fitting approaches, such as the Arrhenius method,16-19 also failed to yield consistent results. The discrepancies in different model predictions have been attributed to the uncertainties in the experimental conditions, the sample properties, and different kinetic methods associated with specific parameters and assumptions,20 resulting in inaccuracies in the prediction of the HFO reaction rates throughout the entire process. In these studies, however, an important point to note is that various reaction stages of the HFO combustion, such as LTO, FD, and HTO, were distinguished in an ad hoc manner and the overall reaction degree was applied uniformly to the different stages, thereby limiting the level of quantitative predictions of distinct physical and chemical sub-processes. Furthermore, studies of HFO combustion often considered the model function power law in a linear function form,12,14 i.e. f(α) = (1−α)n with n = 1, and the validity of this assumed form in complex HFO reaction processes needs careful examination. 3 Environment ACS Paragon Plus
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Therefore, the present study proposes a modified framework of kinetic modeling that leads to improved accuracies in quantitative prediction of the measurement behavior. This is accomplished by a new procedure to separate the individual pseudo component reaction from nearby reactions, accompanied with different power-law relations for the model functions at different stages. In the following, experimental set-up and conditions are summarized and reference experimental results are presented. Subsequently, the adopted kinetic modeling theory based on the Coats-Redfern method is reviewed and the specific procedures for the newly proposed HFO reaction models are discussed. The results are compared against the experimental measurements as well as the predictions by the conventional Coats-Redfern method. Finally, key results and findings are summarized in the conclusions section. 2. Experiment 2.1 Materials and Procedure The HFO sample examined in this study was provided by Shoaiba power plant in Saudi Arabia on the coast of Red Sea, south of Jeddah. It is in the form of black liquid with a high viscosity of 712.9 cst at 313 K and a density of 974.6 kg/m3 at 298 K. An elemental analysis shows the mass fraction of carbon, hydrogen, sulfur, oxygen, and nitrogen to be 86.333%, 10.74%, 3.328%, 0.037% and 0.166%, respectively. The combustion experiment was carried out using a NETZSCH 209F1 thermogravimetric analyzer with TG/DTG modules, capable of a wide range of operating conditions from room temperature up to 1500K. The instruments were calibrated using calcium oxalate monohydrate as reference material. Experiments were performed with a sample weight of approximately 10 mg conditioned in Aluminum oxide crucibles. The experiments were conducted in triplicate to determine the repeatability. The samples were heated from room temperature up to 950K at a heating rate of 10K/min. The
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oxidizing atmosphere was air supplied through the furnace of the analyzers at a constant flow rate of 50 ml/min with nitrogen as a purge gas at 25 ml/min. The weight loss was recorded as a function of time or temperature by the control and data acquisition system. For more details about the experimental method can be found elsewhere. 21 2.2 Experimental Results Fig. 1 shows the normalized TG/DTG curves of HFO combustion. The extent of conversion,
α, is defined as:
α=
()
mini − m t mini − mf
(2)
where mini, m(t) and mf are the initial, in the process and final sample mass, respectively, such that α changes from 0 to 1 through the reaction progress. In the present study, mini = 10 mg, and mf = 0.8 mg. The curves show that reaction process starts at 350 K and burns out at 900 K. Similar to previous studies, the three main oxidation stages, LTO, FD and HTO, are also denoted as zone I, II, and III, respectively, based on the minimum points of the DTG curves (multiple peaks within zone II need further consideration and will be discussed later). The temperature intervals, peak temperature and the mass loss in each region are summarized in Table 1, of which the temperature for additional peaks within II are 640, 673, and 725 K, respectively. The first stage of LTO, ranging from 350 to 540 K with a maximum decomposition rate at 430 K, exhibits a relatively slow weight loss curve and a broad peak in its profiles of TG and DTG, respectively. This stage is dominated by the distillation and visbreaking processes, of which the side- or end-chains of heavy compounds are cleaved off to produce partially oxygenated compounds, such as alcohols, ketones, aldehydes, and hydroperoxides.4,22,23 The residue of the LTO stage is deposited as fuel for the second stage of FD, which is considered to form material for the last stage of HTO.1 The temperature
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range of FD is from 540 to 770 K, of which the TG curve exhibits a tortuous shape and the corresponding DTG curve shows three distinct peaks, indicating the occurrence of complex reaction processes. The third stage of HTO, ranging from 770 to 900 K, shows a distinct reaction curve during which the fuel is burned out and produces carbon oxides and water.2 The complex shape of the DTG curve with several distinct peaks shown in Fig. 1 indicates that it is insufficient to characterize the entire HFO combustion process as a single step reaction which can capture only one peak in the DTG curve. On the other hand, it is unrealistic to identify all components in the HFO mixture and determine their individual reactions correlated with temperature. Therefore, a practical and consistent strategy is to employ lumping strategies that use a smaller number of pseudo-components to represent the fuels and products at different stages, and the kinetic data are calculated to best approximate the complex consecutive reactions.24,25 In a classic approach,1 HFO was separated into saturate, aromatic, resin and asphaltene (SARA) fractions, and the TG/DTG experiment for those fractions were carried out. For modeling of the overall combustion process, the study attempted to model the kinetics of SARA fractions instead of HFO as an integrated fuel. The limitation of this approach is that the combustion of separate fractions could not sufficiently reflect the overall characteristics of HFO combustion due to a significant level of coupling effects between individual components of HFO, in terms of both chemical reactions and heat/mass transfer. In fact, the study showed that the mass loss range of the TG curves of SARA fractions was from 370 to 870K, while HFO was from 320 to 840K.1 Therefore, it is more appropriate to define the pseudo-components of the HFO combustion based on the combustion behavior rather than on chemical composition.24,26 The present experimental TG/DTG curves in Fig. 1 clearly show that the temperature range can be divided into multiple regions: a region of the distillation of liquid constituents (the distillates), and others of oxidative cracking of the heavy residue, which occur in the order of 6 Environment ACS Paragon Plus
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paraffins and light oils, middle base oils, heavy base oils, resins, and asphaltenes. For the LTO stage, the governing component is the distillates, paraffins and light oils. Asphaltenes are the most resistant part of HFO, such that the HTO stage consists mainly of the oxidation of asphaltenes.1,10 Since the three temperature intervals of major components before asphaltenes are middle base oil, heavy base oils and resins, it is reasonable to postulate that these components govern the reactions corresponding to the three distinct peaks in FD stage. This is also supported by the observation of the formation of resins which is the result of the reaction between aromatics and oxygen.1 The modified kinetic model proposed in this study incorporates such observations. In the present study, the pseudo-components are defined as the major components consecutively governing the reactions of the LTO, FD and HTO stages. In the following, the kinetic modeling will be attempted to reproduce the experimental results shown in Fig. 1 by first employing the conventional procedure based on the Coats-Redfern method as a reference, and subsequently using a newly proposed procedure. 3. Kinetic Modeling with Conventional Coats-Redfern Method First, the Coats-Redfern method to be employed in the present study is briefly described. For a linear heating program, β = dT/dt, Eq. (1) is written as:
dα 1 − E/RT = Ae f α dT β
( )
(3)
Integrating the above equation gives:
( )
α
1
0
f α
h α =∫
( )
dα =
A
β∫
T
0
e − E/RT dT
(4)
The right hand side of Eq. (4) has no exact solution. By expanding it into an asymptotic series
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and neglecting higher order terms yield,
2RT − RT/E e h α = ART 2 1− / βE E
( )
( )
(5)
For the general cases 2RT/E
