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Stoichiometric Equilibrium Modeling of Corn Cob Gasification and Validation using Experimental Analysis Kathapillai Arun, Swaminathan Sai Ganesh, and M. Venkata Ramanan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01634 • Publication Date (Web): 24 Aug 2016 Downloaded from http://pubs.acs.org on August 25, 2016
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Stoichiometric Equilibrium Modeling of Corn Cob Gasification and Validation using Experimental Analysis K. Arun1*, S. Sai Ganesh1 and M. Venkata Ramanan2 1
Department of Mechanical Engineering, St. Joseph’s College of Engineering, Chennai, India 2
Institute for Energy Studies, College of Engineering Guindy, Anna University, India *
Corresponding author:
[email protected] Abstract Prediction of gasifier performance is generally carried out using two techniques namely kinetic modeling and chemical equilibrium modeling. The later model was adopted in the current study as it is not governed by any of the gasifier design parameters. Chemical equilibrium modelling was deployed to compute and ascertain the influence of Equivalence Ratio (ER), Moisture Content (MC) and Reaction Temperature (RT) on corn cob gasification. The simulation studies reveal that for a gasification system; with increase in ER (for a constant MC and RT) the HHV of producer gas drops, with increase in MC (for a constant ER and RT) the H2 content increases but CO decreases and with increase in RT (for a constant ER and MC) both the H2 and CO content increases. An attempt was made towards validating the simulated results by subjecting corncob to gasification in a 25 kWth downdraft gasifier. A comparative analysis on the simulation results and experimental outcome revealed that the mole fraction of H2, CO, CO2 and CH4 predicted by the model was inferred to deviate from the experimental results by +36.62%, 26.56%, –13.08% and –22.31% respectively. HHV of the gas predicted by the model was observed to deviate from the experimental outcome by +27.51%. These deviations could be attributed to certain non-trivial assumptions made in simulation studies. However at ER greater than 0.3 the composition of gas and HHV predicted by the model and experimental values concur well. Keywords: Stoichiometric modeling, Gasification, Corn cob, Producer gas, HHV of gas
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1.
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Introduction
Corn:
Kingdom
–
Plantae,
Subkingdom –
Tracheobionta,
Superdivision –
Spermatophyta, Division – Magnoliophyta, Class – Liliopsida, Subclass – Commelinidae, Order – Cyperales, Family – Poaceae, Genus – Zea L, Species – Zea mays L,1, generally known as maize, is one of the most versatile emerging crops having wider adaptability under varied climatic conditions. India produced 23.5 million tons of corn in 2015 and is ranked as 8th based on its production.2 Cobs generated during processing of maize are presently dumped as waste and hence it was planned to utilize these for gasification. The parameters affecting the performance of gasifiers are: moisture content (MC), volatile matter, bulk density, particle size, surface area, equivalence ratio (ER), pressure, reaction temperature (RT), residence time, gasifying medium and its temperature.1-7 Among these parameters ER, MC and RT are the major critical parameters that influence gasification process.8,9 As experimental study on the effect of influencing parameters on gasification process is a laborious process,10 it was decided to develop a model to predict the performance of the gasification and to arrive at the optimal one. In general, the prediction of gasifier performance is carried out by using two techniques viz., mathematical (kinetic) modeling and chemical equilibrium modeling. In chemical equilibrium modeling two distinctive options are adopted: (i) based on equilibrium constant (referred as stoichiometric modeling) and (ii) based on minimizing the free energy (referred as non-stoichiometric modeling).8 Difficulties in generalizing the model, requirement of detailed reaction kinetics and confinement for a specific gasifier configuration/shape are the backlogs in the selection of kinetic models.3 Owing to the independency of gasifier design, thermo chemical equilibrium models are commonly used.3,11 Various
researchers
like
Venkata
Ramanan
et
al.,8,9
Altafini
et
al.,12
Jarungthammachote and Dutta,13 Ruggiero and Manfrida 14 and Zainal et al.15 have analysed
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thermodynamic equilibrium models for downdraft gasifiers, in consideration with fast reaction rates and very high residence time within the gasifier. The authors concluded that the predicted data from the models were in accordance with the experimental results. Thus it is inferred that the performance of a downdraft gasifier can be forecasted to a reasonably good level using chemical equilibrium modeling. Accordingly a stoichiometric equilibrium model was developed to predict the performance of corn cob gasification under varying three performance influencing parameters namely ER, MC and RT. 2.
Equilibrium Modeling The assumptions made in the developed equilibrium model are: Ideal gas laws are
valid; all reactions are at thermodynamic equilibrium; gases are in equilibrium during flow through the char bed; pressure in the char bed is atmospheric and constant; reactions proceed adiabatically; nitrogen present in both fuel and air is inert; ash is inert and is not involved in any of the reactions, either as a chemical species or as a catalyst; no radial temperature gradients/concentrations exist; no gas accumulation in char bed; nil resistance for conduction of heat and diffusion of mass inside the char particles; no tar in the gasification zone; carbon conversion efficiency is 100 %; producer gas comprises only CO2, CO, H2, CH4, N2 and H2O. The ultimate analysis – as listed in table 1 - revealed the composition of corn cob as C - 44.70%, H - 6.30%, O - 45.20%, N - 1.2%, S - 0.09%. Hence the typical chemical formula of corn cob, based on a single atom of carbon, is observed to be CH1.69O0.76. Equilibrium modeling was carried out, based on this formula, for predicting its gasification characteristics. Based on the assumptions, the global gasification reaction of corn cob with air could be written as: CH1.69O0.76+wH2O+m (O2+ 3.76 N2)=X1H2+X2CO+X3CO2+X4H2O+X5CH4+3.76 m N2
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(1)
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Where X1, X2, X3, X4 & X5 represent the coefficients of product constituents, ‘w’ is the amount of water per kmol of corn cob and ‘m’ corresponds to the molar quantity of air used upon the corn cob gasification. The equation (1) represents an overall reaction but a number of competing intermediate reactions take place during the process,9,16 predominant of which are: Oxidation
C + O2 = CO2
(- 393.8 kJ/mol)
(2)
Steam gasification
C + H2O = CO + H2
(+ 131.4 kJ/mol)
(3)
Boudouard reaction
C + CO2 = 2CO
(+ 172.6 kJ/mol)
(4)
Methanation reaction
C + 2H2 = CH4
(- 74.9 kJ/mol)
(5)
Water gas shift reaction
CO+ H2O = CO2+ H2
(- 41.2 kJ/mol)
(6)
Among these, only four reactions are independent viz., oxidation, steam gasification, boudouard and methanation reaction. Von Fredersdorff and Elliot17 stated that oxidation reaction is assumed as very fast completes quickly while the other three reactions are in equilibrium. Since, water gas shift reaction can be regarded as the subtraction of the steam gasification and boudouard reactions, it can also be considered to be in equilibrium. In the global reaction (Equation 1), there are six unknowns X1, X2, X3, X4, X5 and m, representing molar composition of five unknown species in producer gas and oxygen content of the reaction. Hence, to predict the constituents of producer gas, a set of six equations are required, which are formulated by balancing different constituents involved in equation (1). Carbon balancing
1 – X2 – X3 – X5 = 0
Hydrogen balancing
2 w + 1.69 – 2 X1 – 2 X4 – 4 X5 = 0 (8)
Oxygen balancing
w + 0.76 + 2 m – X2 – 2 X3 – X4 = 0 (9)
Equilibrium constant from methanation reaction
K X − X = 0
Equilibrium constant from water gas shift reaction K X X = X X . ∗
w = ()
Amount of water per kmol of corn cob 4
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(7)
(10) (11) (12)
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Heat balance of reactants and products of the global reaction is presented below: H + w H !
" (#$%)
+ H !
" (&'()
) + m H + 3.76 m H . "
= X H ! + X H + X H + X H ! "
"
" (&'()
"
+ X H !
+ (T − T )1X (C3 )!" + X (C3 ) + X (C3 )" + X (C3 )!" (&'()
/
+ X (C3 )!/ + 3.76 m (C3 )." 4
(13) Heating value of corn cob H was determined experimentally by a bomb calorimeter. Heat of formation ∆H of gases could be sourced from JANAF thermo
chemical table framed by Stull and Prophet18 given in table 2. T1 and T2 are ambient and gasification temperature at reduction zone respectively. CP is the specific heat of substance. Average specific heat over temperature range T1 and T2 suggested by Robert and Don19 is:
(C3 )'&6 = R A + B T'&6 + 4T'&6 − T T + <
;
=
