Puigjaner L. (ed.) Syngas from Waste

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this purpose. The temperature is reduced till 850C, which is the limit of the heat

exchangers material [6]. The syngas goes out of the gasiﬁer at around 1,4508C, and

the quench gas is at 235C. Second, the WHB proﬁts the gas heat in a high

pressure (HP) heat exchanger, cooling the gas down till 370C and generating HP

steam. Then, the gas moves to an intermediate pressure (IP) heat exchanger that

cools the gas till 235C, generating in this way IP steam. After this step, the

quench gas is recycled to be mixed with the syngas at the outlet of the gasiﬁer. The

quench gas ﬂowrate is determined by a FORTRAN block (a design speciﬁcation

block) that adjusts the temperate to 850C.

The gasiﬁer equilibrium approach as modelled in Aspen Plus



can be seen in

Fig. 5. The gasiﬁcation process itself (the equilibrium approach) only takes place

in the RGibbs reactor. As the feedstock stream is introduced as a non-conventional

component, before being introduced into the Gibbs reactor, it should be divided

into its elements, because there is no Gibbs’s energy information for a non-con-

ventional component. Therefore, before the gasiﬁer equilibrium, a yield reactor is

used to decompose the raw material into C

graphite

,S,Cl

O and

ashes according to its ultimate analysis and moisture content from the proximate

analysis. As the ultimate and proximate analysis of the feedstock dust are of

concern, previously to the RYield, a separator component is introduced to separate

the feedstock dust from the N

and the limestone introduced in the feedstock

system, which is latter introduced into the Gibbs reactor. The T

gasif

is estimated by

introducing into the Gibbs reactor the heat of reaction associated with the feed-

stock stream decomposition and the heat released into a component separator that

aims at purifying the raw syngas produced from solids (ashes and limestone,

named Slag in Fig. 5), named as Q Reaction and Q Slag in the same ﬁgure.

According to the gasiﬁer Gibbs’s model, all the inlet carbon is converted, thus, for

practical purposes, all the char is converted.

Oxygen, steam and nitrogen are also introduced into the Gibbs’s reactor.

Oxygen and steam ﬂowrates are estimated by speciﬁc ratios that depend on raw

material composition according to their ultimate analysis characteristics. The

oxygen ratio is deﬁned as the relation between the oxygen ﬂowrate divided by the

Feed

dust

GASIFIER

-RGibbs

Reaction

Solids

Others

graphite

, H

, S, Cl

Slag

IP steam

FEED DECOMP

-RYield

FEED SEP –

Component

separator

SLAG SEP –

Component

separator

Syngas

Fig. 5 Gasiﬁer modelling approach

78 M. Pérez-Fortes and A. D. Bojarski

stoichiometric oxygen needed to completely oxidize the raw material mixture after

the decomposition into its components on a mole basis (calculated through the

property set COMB-O2 [31]). This ratio is commonly known as equivalence ratio

(ER). The steam ratio is deﬁned on a mass basis and has as denominator the mass

ﬂow of C

graphite

. The ER value for the base case is 0.42 and the value for the steam

ratio is 0.16. They have been estimated by adjusting the gasiﬁcation temperature to

1,4508C, and according to ELCOGAS input conditions, respectively.

4.1 Calibration and Validation

For the gasiﬁer model calibration, as well as for calibrating the entire super-

structure, real ELCOGAS power plant working conditions have been considered.

The objective is to reach the closest model outputs to real data, maintaining certain

model independency from the ELCOGAS conditions, and thus maintaining the

sensitivity to raw material and operating conditions variations. This problem is

further discussed in chapter ‘‘Industrial Data Collection’’ under the data recon-

ciliation section. Different model variables are suitable for calibration, such as raw

material, proportions and proximate and ultimate analyses, HHV, grindability and

solid’s particle size distribution (PSD) that can be adapted to other speciﬁc case

studies.

In this model’s feedstock dryer, the required natural gas and air are settled as

constant values depending on the gasiﬁer load, and as 1.2 times of the necessary

stoichiometric oxygen, respectively. Global mass ﬂowrate, gasiﬁer load and

limestone proportion can be changed freely in a calculator block. The ER and the

steam ratio can be adapted to different speciﬁc feedstock requirements. Throughout

all the cases proposed and studied in this book, the gasiﬁer operating temperature is

maintained constant (see chapter ‘‘Selection of Best Designs for Speciﬁc

Applications’’).

Validation is done through syngas composition comparison with the actual

composition obtained by ELCOGAS, and the Gibbs’s equilibrium approach. The

required information is gathered through sensitivity analysis and compared to

bibliographic and plant-operating data.

First, the entrained bed gasiﬁer being modelled produces a syngas that consists of

21% of H

, 60% of CO, 4% of CO

, 3.5% of H

O, 1% of H

S, 10% of N

and the rest

of COS, on a mole basis at the above-mentioned conditions. The model syngas ﬂow

counts on a 20% of H

, 58% of CO, 4.5% of CO

,5%ofH

O, 1% of H

S, 11% of N

and the rest of COS, on a mole basis. It can be seen that compositions are close to

each other and that the equilibrium model closely predicts the plant composition.

Concerning the residence time, it can be said that according to the syngas compo-

sition, for this type of gasiﬁer and these speciﬁc conditions of temperature and

pressure, 7.7 s allows for a syngas close to the equilibrium composition.

Second, the results of the work of Fermoso et al. [13] and Hernández et al. [14]

are used to validate the Gibbs’s equilibrium approach. The ﬁrst work is used to

Modelling Syngas Generation 79

compare the gasiﬁer behaviour as input parameters variations, whereas the second

one is used to evaluate the effect of biomass addition to the base case blend.

Validation with Fermoso et al. [14] implies the adaptation of the gasiﬁer model

conditions to their working conditions, i.e., the gasiﬁcation of coal is considered

and the steam and oxygen ﬂowrates have also been adapted, as well as the pos-

sibility of reactor heating by dropping the adiabatic assumption. Simulation results

as the gasiﬁcation temperature, the oxygen ratio and the steam ratio vary are

represented in Fig. 6. Analogous trends to the ones reported in Fermoso et al. [14]

are found. Nevertheless, absolute composition ratios for the syngas composition

are approximate, but this was expected according to Fermoso et al. [14] because

temperatures are not high enough for the equilibrium approach to be used. Related

with the gasiﬁer temperature effect, H

and CO formation is favoured when the

temperature increases, whereas CH

remains quite constant and CO

concentration

decreases. The former syngas behaviour might be the result of ﬁrst, a temperature

increase that leads to K

decrease in all exothermic reactions, diminishing the

composition of CO

, and second, endothermic reactions are enhanced, increasing

the composition of H

and CO. Oxygen ratio increase at constant temperature

leads to the decrease in H

and CO formation and increase in CO

formation (as

the oxidation reactions are shifted towards products). In the same way, as the

steam ratio increases, the H

formation is enhanced (according to Eq. 22) as well

as the CO

one, whereas occur the contrary to the formation of CO. The most

affected equation is then Eq. 23.

Fig. 6 Effect of T

gasif

(C), O

O/C

in the syngas composition (mole/mass basis); daf

dry and ash-free basis. Filled symbols represent the data from [14]

80 M. Pérez-Fortes and A. D. Bojarski

Figure 6 shows that at lower reactor temperatures, gasiﬁcation reactions occur at a

too slow velocity (because endothermic reactions should be motivated) with little

practical value, given that for syngas applications CO and H

production is preferred.

Nevertheless, for power production purposes, CH

is the one that offers a higher

caloriﬁc value. Regarding the latter, high values of methane are related to low gasi-

ﬁcation temperatures, and reforming reactions (Eqs. 26 and 27) take place at relatively

low temperatures as seen in Fig. 6 with CH

the most stable component along the

performed analysis, but showing higher values at lower ratios. Overall, at lower

temperatures of gasiﬁcation, the LHV of the gas is higher but, for syngas applications,

it is not the optimal composition. Even more, higher char conversions are linked with

higher temperatures. At too high temperatures, gasiﬁcation products start to combust.

According to the work of Hernández et al. [13], more sensitivity analyses have

been performed with the aim of evaluating the behaviour of biomass blends, with

regard to representative syngas composition ratios, H

/CO, CH

/CO

and

CO/CO

, absolute composition in mole basis, and the CGE (Eq. 31). In this case, the

ELCOGAS PRENFLO gasiﬁer conditions have been maintained (thus, the gasiﬁ-

cation temperature is around 1,400C). See in Fig. 7 the obtained plots. It is worth

noting that our results show different tendencies with the temperature increase in

syngas composition and CGE, except for H

composition. The main reason for the

discrepancies should be given to the fact that co-gasiﬁcation synergies are not

0,5

1,5

2,5

3,5

4,5

Raw gas composition

(ratio)

Biomass content (%wt)

H2/CO CH4/H2 H2/CO2

/CO

0% 20% 40% 60% 80% 100%

Raw gas composition

(ratio)

Biomass content (%wt)

CO/CO2

CO/CO

Raw gas composition

(%vol

dry

)

Biomass content (%wt)

%H2 %CO %CO2 %CH4

0% 20% 40% 60% 80% 100%

Biomass content (%wt)

CGE

Fig. 7 Effect of biomass addition in syngas composition and CGE. Filled symbols represent the

data from [13]

Modelling Syngas Generation 81

quantiﬁed. Besides, regarding the values of our model, not only the tendencies but

they are also higher than the values reported in Hernández et al. [13]. All in all, the

addition of biomass enhances the production of H

, but leads to lower CGE (that

means that the LHV contained in the raw gas is smaller). Regarding the syngas

composition, the increase of biomass proportion implies less CO and more CO

This calibration and validation procedure shows that an equilibrium reactor is a

suitable approximation for the PRENFLO gasiﬁer behaviour under the considered

working conditions. It is also a good approximation for the biomass gasiﬁcation at

high temperature, even though tars are assumed to be zero. Nevertheless, the co-

gasiﬁcation scenarios are not well described, because the synergetic effect between

the species is not modelled. Research on this matter is needed to try to characterise

these effects.

4.2 Results

We have considered the performance of the gasiﬁer for the base case feedstock

composition carrying out different sensitivity analyses (SA) that take into account

variations in the gasiﬁer temperature and pressure, as well as in the ER and steam

ratio. Also feedstock variations have been considered to analyse the reactor

equilibrium behaviour. General output parameters that have been plotted for all the

SA are the syngas composition and the cold gas efﬁciency (CGE) deﬁned as per

Eq. 31. As it is seen from the equation, if the feed does not vary, the CGE varies

according to the LHV

obtained.

CGE ¼

 LHV

feed

 LHV

feed

ð31Þ

LHV

fuel

 LHV

ð32Þ

The CGE is calculated as the ratio between the chemical energy contained in

the raw gas (rg), as the syngas is called as it comes out from the gasiﬁer, and the

total chemical energy contained in the fuel, both of them measured through their

LHV. In the case of fuel blends, the LHV

fuel

is calculated as the contribution of

each ith fuel to the ﬁnal blend, on a mass basis (w

) (Eq. 32). The estimation of

LHV

is performed in Aspen Plus



using the QVALNET property set.

The considered ﬂowsheet contemplates the feeding system with the gasiﬁer

itself as modelled in our approach calibrated with ELCOGAS data. First (case

SA 1), variation of T

gasif

effects is shown in Fig. 8. As stated in Highman and

van der Burgt [2], the gasiﬁcation temperature range is between 800 and

1,800C. It is seen from the ﬁrst graph that syngas composition varies consid-

erably till around 1,000C, and then, the proportions of H

, CO, CH

and CO

are noticeably constant. Nonetheless, CO

and CO show certain variability, the

ﬁrst one decreasing and the second one increasing. As seen in the plot of the H

CO value, variation of T

gasif

leads to a variation of the two main syngas

82 M. Pérez-Fortes and A. D. Bojarski

components that favour the decreasing of H

proportion and the increasing of the

CO, as stated before; thus, the ration decreases. Because from around 1,000C

again, the CGE shows a positive trend (from 68 to 74%), even if it is fairly

constant. It means that as the LHV

feed

is constant, the LHV

is the one that has

the positive trend. It is directly seen too the inﬂuence of the higher caloriﬁc

value of the CO than that of the H

SA 2 corresponds to P

gasif

variation. It leads to a T

gasif

variation of around 2C

(from 1,489C at the lowest pressure considered, 1 bar; till 1,487C at the highest

pressure taken into account, 55 bar), not plotted in Fig. 9. The range of values

considered is based on literature gasiﬁcation plants (independently of the type of

reactor) working pressure, which at its turn and as commented in previous sections

is normally based on requirements of the process downstream the gasiﬁer.

The effect of the pressure is small if compared with the effect of the temper-

ature; it is possible to distinguish a trend of the values, but changes in composition

and in H

/CO and CGE are negligible. According to Sect. 3.3, only Eq. 24 has a

zero in mole increment. However, it is possible to say that in the pressure range

established the pressure effect is not noticeable, even for the reactions whose mole

increment is different from zero.

SA 3 corresponds to ER variation (see Fig. 10). The ER range has been deﬁned

according to the base value determined for the base case. This scenario has an ER

Raw gas composition

(%vol)

dry

gasif

(˚C)

H2 CO CH4 CO2

800

1000 1200 1400 1600 1800

800 1000 1200 1400 1600 1800

gasif

(

H2/CO CGE

Fig. 8 SA 1: Effects of T

gasif

variation

Raw gas composition

(%vol

dry

)

gasif

(bar)

H2 CO CH4 CO2

11121314151

gasif

(bar)

H2/CO CGE

Fig. 9 SA 2: Effects of P

gasif

variation

Modelling Syngas Generation 83

of 0.42. To start with, the oxygen ratio has been varied in an interval of ±30%.

Then, this ratio has been reduced to be focused in the gasiﬁcation area that has

been enclosed to the interval where methane production is low (high methane

composition correspond to pyrolysis) and sulphur oxides production does not

appear (oxides different from carbon leads to think about combustion; ER = 1isa

complete combustion). It fairly corresponds to the gasiﬁcation temperature range

commented before, between around 800 and 1,800C. Finally, the ER range

oscillates between 0.4 and 0.55. If representing the whole range of results (±30%)

for ER between 0.29 and 0.55, it is seen that there is a signiﬁcant change in syngas

composition around ER = 0.33.

As seen in Fig. 10, and in the H

/CO behaviour, there is a maximum in H

production around ER = 0.46. The CO

production tends to increase with an

increase in inlet oxygen (thus, Eqs. 15, 16 and 17 are of concern). As in all the

other SAs, methane is quite constant. The CGE tends to decrease as the ER

increases, because of the CO

increment, and CO decreases.

SA 4 corresponds to steam ratio variation. It varies between 0.15 and 0.4,

according to ELCOGAS real experiences. The main role of the steam is as a

temperature moderator. It is seen in Fig. 11 that the gasiﬁcation temperature

diminishes as the ratio increases. With the decrease of temperature, contrary at

what has been seen in Fig. 8, CO diminishes and CO

increases. Hydrogen also

increases, as the temperature is lower. The CGE is quite constant, but the H

/CO

(%vol

dry

)

H2 CO CH4 CO2

0,29 0,39 0,49 0,59 0,69 0,79 0,89 0,99 1,09 1,19

0,40 0,43 0,46 0,49 0,52 0,55

Raw gas composition

(%vol

dry

)

H2 CO CH4 CO2H

H2/CO CGE

900

1000

1100

1200

1300

1400

1500

1600

1700

1800

0,40 0,43 0,46 0,49 0,52 0,55

gasif

(°C)

Fig. 10 SA 3: Effects of ER variation

84 M. Pérez-Fortes and A. D. Bojarski

ratio increases. In endothermic reactions, according to what it has been discussed

in Sect. 3.3, as the temperature decreases, tends to react to the reactants side, thus

being Eqs. 22 and 25 of concern, where as the temperature increases, they tend to

react to the reactant side.

SA 5 leads with variations in feedstock composition. A certain amount of

petcoke is added to the coal according to the percentages seen in Fig. 12. As the

proportion of petcoke increases, as it has a LHV higher than coal, the T

gasif

increases. It implies a higher LHV

, therefore, the CGE increases with the per-

centage of petcoke. CO composition is the one that experiments the highest change

in tendency, increasing a 20% in the interval of study. H

increases, as well as

. O/C and H/O are the ratios derived from the feedstock inlet composition.

Higher values of C (lower O/C) imply higher values of caloriﬁc power. The same

as H/O increases.

In summary, as seen in this results analysis, the P

gasif

has a minimal inﬂuence in

the ﬁnal syngas parameters. Assuming a gasiﬁcation reactor where only T

gasif

changed (when it is heated externally), it seems that the syngas composition leads

to a more or less ‘ﬁxed composition’, where increasing the temperature does not

imply better results. On the contrary, if the temperature is increased because more

oxygen is introduced, in the gasiﬁcation zone, H

tends to decrease, whereas CO

tends to increase. Steam ratio increase leads to a gasiﬁcation temperature decrease.

/CO is favoured. The ﬁnal gas composition depends on the relations between

Fig. 11 SA 4: Effects of steam ratio variation

Modelling Syngas Generation 85

carbon, oxygen and hydrogen from the feedstock composition, because T

gasif

and

the mass balance are inﬂuenced. Equations 18–24 are the most important, being

the equations that imply CH

less signiﬁcant because in the gasiﬁcation zone, the

methane value is very low.

5 Conclusions

In this chapter, the equilibrium model minimising Gibbs free energy is used to

simulate the performance of a PRENFLO gasiﬁer with T

gasif

1450 8C, P

gasif

25 bar

and residence time 7.7 s. The model has been validated with ELCOGAS plant

data, showing acceptable agreement. This chapter highlights that the model ﬁtting

is highly dependant on the speciﬁc gasiﬁcation conditions, better at higher tem-

peratures and/or longer residence times, as well as dependant on the raw materials

introduced in the reactor. In this view, if co-gasiﬁcation with biomass is of con-

cern, tars and synergect effect should be known to adequately represent the gas-

iﬁcation behaviour of temperature, pressure, ER, steam ratio and feedstock

composition are studied. Feedstock composition is the most inﬂuential parameter.

Reactor pressure and temperature are ﬁxed by the ﬁnal syngas application and the

ashes melting point, respectively. That is why in the following chapters, the SAs

are performed by varying the ER and the steam ratio. The desired gasiﬁer tem-

perature is obtained by adjusting the ER ratio in each case (thus, playing with

exothermic and endothermic reactions).

Fig. 12 SA 5: Effects of feed composition variation

86 M. Pérez-Fortes and A. D. Bojarski

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Modelling Syngas Generation 87

Puigjaner L. (ed.) Syngas from Waste - Emerging Technologies

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