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Ind. Eng. Chem. Res. 2010, 49, 9230–9237
Theoretical Evaluation of Feedstock Gasification Using H2/C Ratio and ROC as Main Input Variables Michael Stemmler and Michael Mu¨ller* Institute of Energy Research, IEF-2: Microstructure and Properties of Materials, Ju¨lich Forschungszentrum, Leo-Brandt-Strasse 1, 52425 Ju¨lich, Germany
Chemical equilibrium models for simulation of thermochemical processes such as feedstock gasification developed so far either focus on the determination of several producer gas compositions or include a parametric study of feedstock depending parameters such as relative fuel/air ratio (Frg) and moisture content of biomass (h). This article presents a thermochemical process model based on the H2/C ratio and relative oxygen content (ROC). Therefore, all oxygen-, hydrogen-, and carbon-containing components inside a gasifier are considered, and the model enables the prediction of feedstock behavior as well as an optimization of the feedstock gasification. Furthermore, producer gas compositions are determinable in a unique way as well as the achievable producer gas compositions deriving from several feedstocks combined with several gasifying agents (O2 and H2O). The calculated results show that the area of achievable producer gas compositions (AAPGC) derived from hard coal clearly exceeds the AAPGC derived from biomass. Whereas the AAPGC derived from lignite only exceeds the AAPGC derived from biomass for low H2/C ratios. The LHV of the producer gases decreases with increasing the H2/C ratio and ROC values. However, the resulting gas flow for the steam gasification is clearly higher compared to that of the oxygen gasification. 1. Introduction Feedstock gasification is a well-known process which enables a flexible, efficient, and renewable (biomass gasification) power generation via the process of integrated gasification combined cycles. As most of the feedstocks used today are very inhomogeneous, the prediction of producer gas compositions seems complex. However, chemical equilibrium models using the Gibbs free energy minimizing method already agree with producer gas compositions measured in several gasification plants even if these only focus on the six major components CO, CO2, CH4, H2O, H2, and N2 and treat the gasifier as “black-box”.1-4 Therefore, simple chemical equilibrium models are commonly used tools to predict producer gas compositions of thermochemical processes such as feedstock gasification. As the chemical equilibrium models developed so far mostly focus on the determination of several producer gas compositions derived from specific feedstocks, no evaluation of main input variables was performed. Further works conducting parametric studies show the influence of several main input variables. Whereas some works only investigate the influence of induced steam,5-8 other works only investigate the influence of induced oxygen,9-14 and some works investigate both parameters.15-27 All of these works report a tremendous influence of the gasifying agent (mostly air) and the feedstock moisture content on the gasifier performance as well as on the producer gas composition. However, the large number of different parameters used for the parameter studies is remarkable. The used parameters corresponding to the gasifying agent are always specific values, but relate to different variables. So, the parameters such as air/ feed ratio, air/fuel ratio, Frg (relative fuel to air ratio), ROC (oxygen to coal ratio), Ra (air ratio) are all defined as molar or mass ratio of the induced air or oxygen and feedstock streams. However, the frequently used ER (equivalence ratio) is defined * To whom correspondence should be addressed. E-mail: mic.mueller@ fz-juelich.de. Tel.: +49-2461-616812. Fax: +49-2461-613966.
in different ways. Jand et al.28 and Sharma23 define ER as the air to fuel ratio, but Ramanan et al.24 and Mahishi et al.20 define ER as the ratio of actual air concentration and stoichiometric air concentration. Furthermore, parameters such as λ value, Φ (equivalence ratio), and R (equivalence ratio) were used, which are related to the stoichiometric gas composition. The used parameters corresponding to the moisture content such as steam input, moisture/feed, RSC (steam to coal ratio), MC (moisture content), SBR (steam to biomass ratio), h (relative moisture content of biomass), steam/feed and Rh (moisture ratio) are defined as molar or mass ratio of the induced steam and feedstock streams. On the basis of these parameters, achievable producer gas compositions derived from different feedstocks are separately presentable in h-Frg and SBR-ER diagrams.28-30 Further works use molar ratios such as rO/C (molar oxygen to carbon ratio), O/C (oxygen to carbon ratio) and St/C (steam to carbon ratio).31,32 These parameters consider the total molar oxygen (gasifying agent and coal) and steam amount divided by the molar carbon content. Just by using molar parameters the critical carbon content of producer gas which leads to graphite building is presentable in a C-H-O-triangular diagram as well as several feedstocks being comparable in the same diagram.15,33-35 This work aims on a parametric study using parameters which are not connected to any feedstock. Hence, the achievable producer gas compositions derived from several feedstocks would be comparable in a direct way. The resulting theoretical evaluation of feedstock gasification is based on chemical equilibrium models implemented in SimuSage (GTT-Technologies).36 2. Methodology and Model 2.1. Variables for the Theoretical Evaluation of Feedstock Gasification. To perform a theoretical evaluation of feedstock gasification, suitable variables have to be found. Since the compositions of nearly all commonly used gasification feedstocks such as hard coal (hc), lignite coal (li), and biomass (bio)
10.1021/ie100726b 2010 American Chemical Society Published on Web 08/19/2010
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Table 1. Compositions of Feedstocks Common for Gasification (wt %)
C H N S O
hc_low
hc_high
li_low
li_high
bio_low
bio_high
85.4 6.18 1.7 0.69 1.43
74.8 4.52 1.25 1.25 15.3
65.8 4.98 0.84 0.51 26.6
55.5 4.86 0.577 0.28 37.6
50.9 6.51 0.32 0.024 38.4
42.2 5.4 0.5 0.012 47.7
vary in a wide range, feedstocks with a high and low oxygen content are shown in Table 1 for each category. Therefore, the suitable variables should exactly take all gasification inlet streams into account. As feedstock gasification is a thermochemical process with under-stoichiometric oxygen content, oxygen has a big influence on the producer gas composition. The commonly used parameters for oxygen indication such as air/feed ratio, air/fuel ratio, Frg, ROC, Φ, ER, Ra, m, moisture/feed, RSC, MC, steam/coal, SBR, h, steam/feed, and Rh are molar or mass ratios of induced gasifying agent and feedstock streams. Therefore, the oxygen content of the feedstock is not included in these parameters and the resulting oxygen content inside the gasifier is not indicated in a unique way as the oxygen content of gasification feedstocks ranges from 1.43 wt % for hc_low to 47.7 wt % for bio_high. Furthermore, h-Frg- and SBR-ER-diagrams are not suitable as multi feedstock diagrams. Other commonly used parameters related to the stoichiometric gas composition such as λ value, Φ, and R seem to be feedstock independent, as these consider the added amount of oxygen resulting from the gasifying agent (eq 1). Following, λ is used as representative for stoichiometric parameters focused on a gasifying agent. λ)Φ)R)
O2_gasi O2_stoich
(1)
To verify the feedstock independence of stoichiometric parameters, the courses of λ for different feedstocks such as hc_low, li_low and bio_low are shown in Figure 1 for varying gasifier oxygen contents. The gasifier oxygen content is defined as the molar ratio of the total oxygen content O2_gasi, coming from the gasifying agent and the feedstock, to the stoichiometric oxygen content O2_ stoich. The courses of λ show that the stoichiometric parameters are not suitable for a unique description of the total gasifier oxygen content if feedstocks with different oxygen contents are used. Therefore, the usage of parameters which ignore the oxygen content of feedstocks always leads to a feedstock dependent
Figure 1. Distribution of λ for hc_low, li_low, and bio_low.
Figure 2. Distribution of λ and ROC for hc_low, li_low, and bio_low.
parametric study, even though these are related to the stoichiometric oxygen content. To take the whole oxygen content inside the gasifier into account, the relative oxygen content (ROC) is defined as ROC )
O2_gasi + O2_feed O2_stoich
(2)
As it is obvious by these definitions, ROC and λ both amount to 1 in the case where all oxidizable species inside the gasifier are oxidized. However, the decisive difference between ROC and λ is the inclusion of the feedstock oxygen content. The consequence of this difference is shown by the courses of ROC and λ for hc_low, li_low, and bio_low in Figure 2. Because λ refers to the stoichiometric producer gas composition, it nearly corresponds to ROC for oxygen-poor feedstocks such as hc_low (Figure 2). For feedstocks rich in oxygen such as lignite and biomass, the oxygen input via feedstock is considerable. This results in feedstock-dependent courses of λ, as already mentioned. The course of ROC clearly differs from the courses of λ as it corresponds to the total oxygen content inside the gasifier, therefore, ROC is suitable as a feedstock independent parameter and indicates the oxygen content inside a gasifier in a unique way. However, the indication of the producer gas oxidation grade by ROC is not sufficient for the determination of producer gas composition. Because of the lack of knowledge concerning the ratio of oxidizable species inside the gasifier, the calculation of producer gas composition is not feasible. With regard to the feedstock compositions shown in Table 1, hydrogen and carbon amount to over 99% of all oxidizable species. A further discharge of oxidizable species only occurs if water is used as gasifying agent. Therefore, a steam induction affects the gasifier performance in a different way as an oxygen induction. Nevertheless, hydrogen and carbon cover at least more than 99% of all oxidizable species in the gasifier inlet streams. So, the strike of all other oxidizable species, for example, sulfur, is an admissible simplification. Hence, the calculation of a producer gas composition by using a combination of ROC and the total hydrogen (H2_total) amount as well as total carbon amount (Ctotal) of all inlet streams as main input parameters should be feasible, but still has to be shown. 2.2. Suitability of ROC and H2/C Ratio as Input Variables. To validate the suitability of ROC and H2/C as input parameters for the thermochemical model, the calculation of the resulting hydrogen content in the producer gas xH2 will be shown exclusively in dependence on ROC, H2_total, Ctotal, and
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Table 2. Reactions Equations for Feedstock Gasification C(s) + CO2 T 2CO
(R1)
Boudouard equilibrium
C(s) + 2H2 T CH4
(R2)
methanation
C(s) + H2O T CO + H2
(R3)
reforming
CH4 + H2O T CO + 3H2
(R4)
steam methane reforming
CO + H2O T CO2 + H2
(R5)
water gas shift (syngas equilibrium)
boundary conditions. First, the molar balances of carbon and hydrogen inside the gasifier are presented in eq 3 and eq 4: Ctotal ) CO + CO2 + Cres
(3)
H2_total ) H2 + H2O + H2_res
(4)
However, if the amount of other oxidizable species is significant, they can easily be taken into account as well. As the occurrence of compounds such as tars and light hydrocarbons, for example, methane, are not included in chemical equilibrium calculations since they amount ,1 vol %, the so-called res variables (Cres and H2_res) are included into both balances. Therefore, the model is adjustable to disequilibrium reactions in the case where the res variables are known. Furthermore, ROC is determinable by the inlet streams as well as the hydrogen and carbon content of the producer gas (eq 5).
ROC )
O2_gasi + O2_feed +
O2_total ) H2_total Ctotal + 2 CO + H2O CO2 + 2 (5) H2_res + H2O + H2 + CO2 + CO + 2 O2_stoich
Cres
H2Ogasi 2
)
By substituting H2O and CO2 in eq 5, using the balances eq 3 and eq 4, the H2 substance amount is already determinable in dependence on the CO substance amount (eq 6). H2 ) (1 - ROC)(2Ctotal + H2_total) - 2Cres - H2_res - CO (6) To determine the CO substance amount, the water gas shift reaction and its chemical equilibrium is taken into account (Table 2, eq 7 and eq 8). CO(g) + H2O(g) T CO2(g) + H2(g) KWGS )
CO2 · H2 ) SER CO · H2O
(7) (8)
Since a water gas shift is not included in this model in the following, the KWGS will be substituted by the syngas equilibria ratio (SER). By substituting the H2O and CO2 in eq 8, the substance amount of CO is exclusively depending on the H2: CO )
(H2_total
(Ctotal - Cres) · H2 - H2_res)SER + (1 - SER) · H2
(9)
Figure 3. Course of SER in the temperature range of 600-1000 °C.
The combination of eq 6 and eq 9 results in a quadratic equation: 0 ) H22 + pH2 + q
(10)
with p)
Ctotal - Cres + (H2_total - H2_res) · SER 1 - SER (1 - ROC)(2Ctotal + H2_total) - 2Cres - H2_res q)-
SER · H2_total(1 - ROC)(2Ctotal + H2_total) 1 - SER
By solving the quadratic equation and dividing the substance amount of H2 by the total gas amount of gaseous substances Ngas, the hydrogen content in the producer gas xH2 is determinable. However, potential producer gas dilution by inert gas phases such as N2 has to be taken into account via a dilutor variable D: xH2 )
H2 H2 ) Ngas Ctotal - Cres + H2_total - H2_res + D
(11) A closer verification of eq 10 shows that the calculation of xH2 depends on the boundary variables such as ROC, Ctotal, and H2_total and SER. Because the value of SER is essential for the calculation of xH2 its course is shown in Figure 3. By its definition SER exclusively depends on the temperature T and is restricted to values >0. The solution of the quadratic equation, eq 10, shows that for 1 < SER the subtraction and for 0 e SER < 1 the addition result in physically meaningful values:
xH2(SER) )
{
[ [
( 2p ) - q]:0 < SER < 1 p - ( 2p ) - q]:1 < SER 2
1 p - + Ngas 2
2
1 Ngas
2
(12)
In the case where the SER amounts to 1, the calculation of xH2 is critical, because the denominator amounts to zero. As the left and right-hand limit have the same amount, xH2 is also determinable for 827 °C (SER ) 1). Hence, the calculation of the producer gas composition, which exclusively depends on ROC, H2_total, Ctotal, and the boundary conditions is feasible in a unique way. Furthermore, the calculation of the producer gas composition is that simplified,
Ind. Eng. Chem. Res., Vol. 49, No. 19, 2010
Figure 4. Flowcharts of parametric study equilibrium model. Table 3. Simplifying Assumptions Made to the Model sufficient residence time to attain chemical equilibrium complete conversion (all C and H2 of feedstock are transformed to hydrocarbons with two C maximum, CO, CO2, H2, and H2O) the producer gas is not diluted by inert gas phases such as N2 (D ) 0) ideal well-stirred chemical reaction the fuel is uniformly distributed in the gasifier the gasifier operates under steady-state conditions the feedstocks are perfectly mixed all producer gas compounds lower than 0.01 vol % are stricken heat losses are neglected
that the chemical model is substitutable by the course of SER in the case of no graphite precipitates (Table 2). Therefore, the parametrizations of the total oxygen amount and the ratios of all oxidizable species inside the gasifier are essential for a theoretical evaluation of feedstock gasification. Following, the H2/C ratio and ROC are used as main input parameters for feedstock gasification. 2.3. Chemical Equilibrium Models. As already mentioned the calculation of producer gas composition is feasible without a chemical equilibrium model in the case of no graphite precipitates. Therefore, the flowchart of the used chemical equilibrium model is simple, as shown in Figure 4. The chemical equilibrium model was implemented in SimuSage (GTT-Technologies)36 and based on equilibrium reactions in gaseous, liquid, and solid phase in a zero dimensional (black-box) scheme. Therefore, simplifying assumptions were made to the implementation of the model, as shown in Table 3. These simplifying assumptions have already been confirmed by the majority of authors mentioned in this work. The equilibrium calculation starts with the specification of the boundary conditions and the inlet streams. For the parameter study the C and O amounts are calculated from given ROC, H2/C ratio, and H2. Following, the chemical equilibrium is calculated via the Gibbs free energy minimizing method using
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data from FACT53-database for the given boundary conditions such as pressure P and temperature T. The used data include 699 species of the following elements C, H, N, S, O, Cl, Al, Ca, Fe, K, Mg, Na, P, Si, Ti, Ce, Cu, Zn, Co, Cr, Mo, Ni, Ar Ba, Sr. All hydrocarbons with more than two C are not included in the used database. This study aims, for now, on the theoretical evaluation of feedstock gasification with complete conversion (all C and H2 of feedstock are transformed to CO, CO2, CH4, H2, and H2O). Therefore, all res variables (Cres and H2_res) are set to zero, and the equilibrium calculation is followed by a check for solid C (graphite). The conditions under which carbon could deposit is desirable to know because the prudent engineer would not design for such conditions.37 In the case of graphite precipitation the calculations are aborted and the producer gas composition is given as a result combined with the notation of solid graphite. In case graphite does not precipitate, one predefined producer gas content xi is compared to a target value xi_t via iterator. If the value of xi is not equal to the one of xi_t the iterator adjusts the value of ROC until the equality is attained. 2.4. Validation of the Model. To validate the model used in the present work the predicted producer gas compositions were compared to experimental data. As this model focuses on the complete conversion of feedstocks with the addition of oxygen and/or water, the number of comparable data is very few. To prevent the producer gas composition from any dilution and the occurrence of fixed carbon or tars, the needed data should originate from high temperature feedstock gasification without air addition. Therefore, the present model has been compared to experimental data of a Texaco gasifier and predicted producer gas compositions presented in Watkinson et al.,1 as shown in Table 4. Although, the presented model is simpler than the one used by Watkinson et al.1 the predicted gas compositions show good accuracy with the experimentally obtained data. Furthermore, previous works already showed that the FactSage package is a useful tool for predicting producer gas compositions.38,39 2.5. Enthalpy Balances. For enthalpy calculations the derived heat of feedstock formation is essential. The common way to calculate the lower heating value (LHV) is the use of empirical equations, which are based on ultimate feedstock analysis. In this study the influence of the used combination of feedstock and gasifying agent or rather different producer gas compositions on the resulting LHV is of interest. Therefore, the LHV is calculated via the resulting producer gas composition combined with the standard formation enthalpies.40 Hence, the resulting LHV are given in kJ/m3 (STP). 3. Theoretical Evaluation of Feedstock Gasification For the theoretical evaluation of feedstock gasification a parameter study with independent parameters is essential.
Table 4. Comparison of Predicted Results with Experimental and Predicted Data from Watkinson1 (Wat.) fluid coke 1178 °C gas comp. [vol %]
exptl
Wat.
CO H2 CO2 H2O CH4 H2S N2
47.1 24.3 13.2 12.7 0.09 2.2 0.4
48.5 23.0 11.9 14.0 0.06 2.06 0.3
Illinois no. 6 1316 °C present model 49.1 25.0 11.7 14.0
