Stoichiometric Approach to the Analysis of Coal Gasification Process

Coal is a solid fuel and less convenient for storage and transportation than petroleum and natural gas. In addition, it usually holds undesirable compounds containing S, N, and so on. Gasification converts coal into H2, CO, and CH4 by the reaction with gasifying agents such as O2 and H2O. There are three types of commercialized processes, 1) fixed bed gasifier: lump coal is gasified in a shaft reactor at 900~1000°C, 2) fluidized bed gasifier: crashed coal is gasified in a fluidized reactor at around 900°C, and 3) entrained bed gasifier: pulverized coal is gasified by burner system at 1350~1600°C. In order to improve the performance of 500~2000t/d plants, operating conditions should be appropriately determined and controlled based on the understanding of chemical reaction process that occurs in gasifier. It has been believed that coal supplied to gasifier is decomposed thermally to produce gases such as H2, CO, CO2, H2O, and CH4, tar, and char. Tar and char react with O2 and H2O supplied to form H2, CO, CO2 and CH4. However, it is quite difficult to understand gasification mechanism only based on the kinetics of above reactions.


Estimation of reaction formula
We express the gasification reaction by formula (1), where CH m O n and CH m' O n' are coal and tar respectively.
(5) When the concentrations of H 2 , CO, CO 2 , CH 4 , C 2 H 4 , and C 2 H 6 (dry and N 2 free) are represented by p, q, r, s, t, and u respectively, the mole number of each gases is described as follows. (6) A sampling gas for analysis has usually been drawn out from the main stream of product gas, passed into cooler to remove condensable H 2 O and tar, and then measured its volume by gas meter and analyzed its composition by various kinds of analyzer.Yield of tar has been evaluated as its moles per unit volume of gas produced.The molar yields of tar is presented by v, in formula (1) is described as equation ( 12) that is written in the same form as that of equation ( 6) ~ ( 11).( 12) Consequently, we obtain ten solutions, ~ and Σ, mathematically because we could prepare eleven equations from (2) to (12).Equation ( 2) is rewritten as (13) by employing (7) ~ ( 12).

Feature of this method
Since every equation used to evaluate ~ has been derived from (2), (3), and (4) without any arbitrary assumption and approximation, it may be applicable to any practical process.Since the left side of (1) is the reactant of gasification, and the right side is product, although (1) is written in the simplest form, it expresses exactly the material balance of gasification.Therefore, we can readily estimate the molar amounts of H 2 , CO, and CH 4 from , , and to judge whether the molar ratio of O 2 or H 2 O to coal is appropriate or not from or .In addition, each value of ~ is estimated simultaneously by arithmetic calculations using the values of concentration of each gas and H/C and O/C ratios of coal and tar.Formula (1) seems to be also obtained from the flow rates of coal, gasifying agents, and each gas produced.The carbon conversion for practical process has been usually found to be less than 100%, however, the accuracy of industrial instruments to control each flow rates of raw materials (coal, O 2 , and H 2 O) or to measure that of products (gas, tar, drain, ash, and residual char) are insufficient to get a formula satisfying the law of conservation of mass.

Analysis of reaction formula obtained
We consider that a reference standard should be necessary to elucidate the reaction process concealed in formula (1).We tried to prepare the reaction formula expressed in mathematical form by a theoretical approach for this purpose.The quantitative details of formula (1) obtained from composition of gas becomes apparent by the comparison with a theoretical formula which is derived from (24) by mathematical means. (24)

Hypotheses for the chemical process
We assumed that the reaction process of gasification has been divided for convenience into two categories, namely partial oxidation and secondary reaction.Coal is converted into H 2 , CO, CO 2 , and H 2 O by the reaction with O 2 and H 2 O.Most solid-gas reactions except for hydrogenation described later are classified in it.In secondary reaction step, the gas produced by partial oxidation changes its composition by shift reaction and formation of organic compounds.We transform standard reaction formula (24) mathematically according to both steps mentioned above and derived a formula having the same form of formula (1).

Partial oxidation step
The amount of O 2 in (24), i.e. 0.5(1-n), is regarded as a standard amount of O 2 for gasification.in (1) is written as bellow. (25) In the case of O ex >0, it can be assumed that coal is firstly gasified with 0.5(1-n)O 2 and converted into product gas, i.e., 0.5mH 2 + CO.Then 2O ex mole of the product gas is burned with O ex mole of O 2 , 2O ex mole of H 2 O and CO 2 in total was generated.When moles of H 2 burned is taken as variable x, that of CO is presented as (2O ex -x) and reaction formula after partial oxidation step was written as follows; (26) In the case of O ex < 0, sinceαis not enough to complete (24), -2O ex of residual carbon is produced intermediately.The reaction is expressed by following formula.
-2O ex C in above formula should be gasified with -2O ex H 2 O as follows; The final formula after partial oxidation step is estimated as follows; (27) On the other hand, we can understand the partial oxidation step in further detail from a broader standpoint of view.Since it is stipulated that organic constituents are not produced in the partial oxidation, formula can be expressed as follows.
The elemental balance of this formula is given below. (C) Stoichiometry and Materials Science -When Numbers Matter 420 The calculation, 2(C)+0.5(H)-(O)was performed.
As theoretical moles of O 2 for complete combustion of coal is defined by ( = 1+0.25m-0.5n), the equation that shows the relationship between the sum of H 2 and CO produced and amount of O 2 reacted is obtained.
When the both side is divided with and let / be replaced by χ, (28) is obtained.χ is called oxygen ratio and generally used in the analysis of combustion. ( Since is moles of H 2 O decomposed, -means that produced.Therefofe, ( -) in equation ( 29) indicates the sum of CO 2 and H 2 O produced in the case of χ > 0.5(1-n )/ .
(28) is represented by black solid line with slope -2, ( 29) is done by red solid one with slope 2, and ( 30) is done by blue dotted lines with slope -2.Since and χ are the parameters that have been commonly used in the theoretical analysis of combustion, Fig. 1 allows us to investigate the partial oxidation step from the macroscopic point of view that covers not only gasification but also combustion.The partial oxidation step can be realized as general comprehension of oxidation process of coal based on Fig. 1.
Partial oxidation step can be further subdivided into various elemental reaction such as pyrolysis, combustion of C and H in coal, water-gas reaction, and Boudouard reaction.
Taking all these into account, however, the reaction model may complicate analysis of gasification too much and may be far from practical application.

Shift reaction
In the case of shift reaction, we can easily express the variation of yields of CO, H 2 O, H 2 , and CO 2 numerically based on its reaction formula.
In the case of O ex >0, let y be moles of CO caused shift reaction, formula ( 26) is modified as follows; (31) In the case of O ex <0, formula ( 28) is modified as follows; (32)

Formation of organic compounds
Organic constituent such as CH 4 , C 2 H 4 , and C 2 H 6 are produced by the synthetic reaction as well as pyrolysis of coal and hydrogenation reaction.There is no question that the synthetic process is classified as secondary reaction step.However, the idea that pyrolysis and hydrogenation are also classified in the same step appears doubtful.
The formulae of pyrolysis, hydrogenation, and synthetic reaction are descrived as follows; Pyrolysis: Hydrogenation reaction: Synthetic reaction: Where C l' H m' O n' is organic compound and l'H 2 O is moisture in an atmosphere.When m'H in the left side of (33) cause 0.5m'H 2 , it changes into (34).When water-gas reaction is occurred between l'C and l'H 2 O in the left side of (34), it turns into formula (35).In other words, it means that the effect of the formation of organic compound on the yields of other inorganic components can be appreciated mathematically according to formula (35).Since O atom in the product gas is very reactive, however, it readily reacts with H 2 .Therefore, formula ( 35) is finally altered as follows; The difference in formation processes of organic compounds in (1) is shown as follows; In the case of O ex >0, when z mole of CH 4 is formed, formula (31) is modified. (49) When w mole of C 2 H 4 is formed, ( 49) is transformed into (50) according to (42). (50) When u mole of C 2 H 6 is formed, ( 50) is modified to (51) according to (45). (51) When v mole of CH m' O n' is formed, ( 51) is transformed into (52) according to (48). ( z, w, u, and v are found to be equal to , , , and respectively.Following equation is obtained relating to the value of x and y. (57) Since the value of x, i.e. quantity of H 2 burned with O ex O 2 , cannot be determined only from the results of ultimate analysis and gas analysis, we assume reasonable value of x based on the rate of combustion of H 2 and CO.When gasifying temperature is too low to maintain the combustion rate of CO, only H 2 seems to be virtually burned.In this case, x become 2O ex and equation ( 58) is valid. (58) When ( 58) is substituted in ( 57), ( 59) is obtained. ( In usual gasification, the product gas in ( 24) is burned with O ex O 2 as it is, x is calculated by 2O ex {0.5m／(1+0.5m)}and equation ( 60) is obtained. (60) In the case of O ex <0, we can find the relation, y = ε, from Table 1.Since the quantity of O 2 is not enough to satisfy the standard reaction formula, CO 2 cannot be produced by the combustion of CO.
The quantity of H 2 or CO 2 formed by shift reaction has never been evaluated.Our approach can account for the contribution of shift reaction to the composition of gas based on the rational assumption of the quantity of H 2 burned.

Application to underground coal gasification
The principle of underground coal gasification (UCG) is shown schematically in Fig. 2.
UCG is a process that gasifies coal in seam with O 2 or air injected through a borehole drilled from the surface.The gas produced is withdrawn to the surface through another borehole.
In recent years, UCG technique is advanced drastically by application of horizontal digging technique developed in oil excavation, and several commercial processes have been scheduled.
It is difficult to insert various sensors to measure temperature, pressure, flow rate of gas from ground surface into reacting spots.Chemical phenomenon occurred in UCG has not been well understood compared with surface gasification processes because of the lack of information described above.Therefore, the reaction formula, heat of gasification, and adiabatic temperature of UCG should be helpful to understand the reaction process.We applied our method to the data of gas composition and ultimate analysis of coal obtained by UCG tests carried out at five China mines, and investigated the feature of each chemical processes.
Heat of reaction of gasification, h r (kcal/mol-coal) is calculated by (61).

Estimation of adiabatic temperature
In the case of UCG, it is presumed that h r generated is partly transferred to the wall made of coal or char.As the heat conducted to the wall is utilized in water gas reaction, pyrolysis, drying, and preheating of coal effectively.Consequently, the reactor of UCG can be thought as adiabatic one, and most of h r turns to the sensible heat of gas in the reactor.
Gas in the reactor consists of the products presented in right side of formula (1'), N 2 , and H 2 O.The total moles of product gas, i.e.Σ in (1'), is expressed by Σ = 1/(q + r + s) as shown in equation ( 13), the yield of N 2 per 1 mol of coal gasified is defined by .
The amount of residual H 2 O remained in the reactor is expressed by .In the case of UCG, it is difficult to estimate precisely based on the result of gas analysis.We estimate based on the equilibrium relationship of shift reaction.
The relationship between equilibrium constant of shift reaction and temperature is shown in Table 2.
Table 2. Equilibrium constant of shift reaction Heat of product gas, Q sh is calculated by the integration of thermal capacity of each component gas, which is given by following eq., Cp = a+bT+cT 2 +dT 3 (kcal/kg-mol-deg), from 298K to gasification temperature T. The values of a ~ d of each gas are shown in Table 3.

Analysis of chemical process
We estimated reaction formula of gasification, heat of reaction, and gasification temperature of UCG carried out at Fuxin mine, Xinghe mine, Liuzhuang mine, Ezhuang mine, and Xiyang mine in China.The results of ultimate analysis and molecular formulae estimated for five coals were shown in Table 4 with their heating values.
Table 4. Ultimate analysis, molecular formula and heating values of coals As an example of analysis of chemical process of UCG, results for Ezhuang mine are summarized below.The daily variation of the concentration of each gas components is shown in Fig. 3.

Fig. 3. The daily variation of the concentration of each gas
The change in the each coefficient of reaction formula (1') with elapse of time was shown in Fig. 4. Value of may be applicable to estimate the amount of coal gasified.Therefore, total volume of the cave formed in coal seam can be estimated by the integrated value of .
Value of is important information to understand the reaction mechanism quantitatively.
The linear relationship was found in the plots of the values of other coefficients against as shown in Fig. 5 and 6.
Figs. 5 and 6 show that yields of combustible gas such as H 2 , CO, and CH 4 decreased in proportion to increase of oxygen reacted.was responsible for the formation of every component gas in the case of UCG carried out in Ezhuang mine.The change in h r and T ad with elapse of time was shown in Fig. 7. Fig. 7. Change in h r and T ad with elapse of time Fig. 8 indicates the plot of h r vs. , and T ad vs. .A plot of h r vs. gave a straight line, but that of T ad vs. was widely scattered.The distribution of plots may be attributed to the uncertainty in the estimation of residual H 2 O in product gas, which was calculated from equilibrium of shift reaction.It is presumed that the gas composition in UCG reactor has not been attained to chemical equilibrium.The approach to improve the reliability of estimation for T ad without depending upon the equilibrium theory has not been reported.Our attempt might be the first to predict the temperature of UCG reactor.Fig. 8. indicates the plots of h r vs. , and T ad vs. .

Comparison of reaction process in five coal mines
We compared the result of analysis of the data of UCG carried out at five coal mines.
Average compositions of gases produced by UCG processes are listed in Table 5.Table 5.Average compositions of gases produced by UCG The mean value of each coefficient of reaction formula of UCG is shown in Table 6.
Table 6.The average coefficients of reaction formula of gasification The result of analysis of the data obtained by fixed bed gasifier which is a conventional surface process is summarized in Table 7 as a reference.
Table 7.The coefficients of reaction formula of surface fixed bed gasification Value of each coefficient in Table 7 distributed in narrower range compared with Table 6.
Comparing to the fixed bed process, the value of of four mines except for Xiyang were around the same as well as those of fixed bed.The relatively small of Xiyang may be due to its different chemical composition and heating value as shown in Table 4.
The partial oxidation process is considered to be practically governed by the value of O ex .
On the other hand, the secondary reaction proceeded more actively compared to surface fixed bed gasification.
The change in h r with elapse of time for five mines is shown in Fig. 9.The relationship between h r and is shown in Fig. 10.As plots in the region of h r > 0 stays the same straight line characteristic for each coal mine, it is indicated that h r > 0 has not accidentally obtained.Positive value of h r is assumed to be attributed to excessive water gas reaction promoted by the heat accumulated at the wall of gasifier.The periodic fluctuation of h r may indicate an essential feature of actual UCG reactions.Since the plots scattered in the range indicated by ellipse and the linearity was hardly found, it is considered that shift reaction was not affected by partial oxidation.
The plot of vs. is shown in Fig. 13.
A relatively linear relationship is found to exist between and showing decrease in along with increase in .This is presumably a result of thermal effect on the stability of CH 4 www.intechopen.comdetermined by the chemical equilibrium.Average o f X i y a n g m i n e i s f o u n d t o b e 0.281mol/mol, which is much larger than results of other mines as shown in Table 6.As carbon content of Xiyang coal is 92.34%, it should be classified as anthracite.It is generally accepted that most of H atoms in molecular structure of anthracite are combined with C atoms directly at the rim of aromatic rings, and very limited numbers of methyl group or hydrocarbon chains exist.This means that CH 4 is produced by hydrogenation or synthesis reaction between H 2 and CO, not by pyrolysis.It is well known that hydrogenation occurs only with high pressure H 2 .Therefore, it is considered that CH 4 is mainly produced by synthetic route in the case of this coal.Amounts of H 2 and CO have been used as indicator to estimate chemical process of surface gasification.For example, the yields of H 2 and CO are expected to increase with decrease in the amount of O 2 reacted.Such tendency, however, was not found in the plots of vs. and vs. presumably due to the appreciable progress of secondary reaction.In the case of UCG, we need to evaluate the amounts of H 2 , CO, and CO 2 after estimation of the effect of secondary reaction.

Conclusion
It is generally accepted that gasification consists of more than five chemical processes such as pyrolysis, partial oxidation of char, further decomposition of tar, secondary reactions, and combustion of char or gas.It is obviously difficult to simulate actual coal gasification precisely by applying reliable scientific analysis of fundamental experiment.Since coal gasification is a very complicated both from experimental and theoretical points of view, its chemical process cannot be completely understood merely by the accumulation of kinetic data.Our method described here may be the first one that can scientifically elucidate reaction process based on the stoichiometry using gas composition obtained at a practical gasification plant.Since this method is constructed based on stoichiometry of the reaction formula without any arbitrary assumption and approximation, it is applicable to any gasification process regardless of the type of gasifier or a rank of coal used.The feature of this study is to elucidate gasification mathematically based on material balance of coal gasification reactions which was traditionally used to calculate carbon conversion and cold gas efficiency.The mathematical reaction formula derived in this study offered a novel point of view to estimate practical reactions that occur in a gasifier more precisely and it should help to attain optimum operation condition in practical gasification plant.
As a good example of the application of our method, results of the analysis of UCG is introduced.Actual operation data of UCG carried out at five mines in China was investigated by our method.We have consequently succeeded to obtain the reaction formula of gasification, progress of shift reaction, reaction heat of gasification, and adiabatic gasification temperature.These results allow us to understand the partial oxidation step and secondary reaction step of UCG.

Fig. 4 .Fig
Fig. 4. Change in the coefficients with the elapse of time

Fig. 10 .
Fig. 10.Relationship between and h r Relationship between and was shown in Fig. 11.

Fig. 11 .
Fig. 11.Relationship between and Plot of y vs. for each mine is shown in Fig. 12.

Fig
Fig. 14.Relationship between and

Table 3 .
The values of a ~ d to estimated Cp of gas The quantity of H 2 O reacted decreased with in the same way.On the other hand, the yield of CO 2 increased with .It was thus proved that The change in h r with elapse of time Although the rate of air supply is nevertheless kept constant during every tests, h r repeated increase and decrease.It occasionally rose into positive value indicating that the endothermic reaction process proceeded.
The plot of vs. gave a straight line in all coal mines.O ex in UCG is almost zero or negative as shown in Table9.In the case of O ex < 0, -2O ex H 2 O was consumed in water gas to compensate the lack of O 2 .Therefore, the amount of water reacted basically increased in proportion to the reduction of oxygen reacted.Besides, it is considered that progress of shift reaction and formation of CH 4 influenced on the slope of each straight line. reaction