Introduction of a ternary diagram for comprehensive evaluation of gasification processes for ash-rich coal

Introduction of a ternary diagram for comprehensive evaluation of gasification processes for ash-rich coal

Fuel xxx (2012) xxx–xxx Contents lists available at SciVerse ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Introduction of a te...

927KB Sizes 0 Downloads 139 Views

Fuel xxx (2012) xxx–xxx

Contents lists available at SciVerse ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel

Introduction of a ternary diagram for comprehensive evaluation of gasification processes for ash-rich coal Martin Gräbner ⇑, Bernd Meyer Department of Energy Process Engineering and Chemical Engineering, Technische Universität Bergakademie Freiberg, Fuchsmühlenweg 9, Reiche Zeche, 09599 Freiberg, Germany

a r t i c l e

i n f o

Article history: Received 4 November 2011 Received in revised form 25 January 2012 Accepted 31 January 2012 Available online 15 February 2012 Keywords: Coal gasification Ternary diagram Ash rich coal Cold gas efficiency

a b s t r a c t The present paper addresses the development of a comprehensive thermodynamic approach for the evaluation of gasification processes. A ternary diagram is introduced for a South African coal with an elevated ash content of 25.3 wt.%(wf). The ternary diagram allows the evaluation of most of the commercially applied gasification technologies depending on the three variables O2, H2O and coal mass flow. Cold gas efficiency, dry CH4 yield, specific syngas production, H2/CO ratio, CO/C and CH4/C selectivity as well as temperature and carbon conversion were selected as performance measures. Based on literature data, generic models of the commercial Shell, Siemens, ConocoPhillips, HTW and GE coal gasifications systems were developed enabling an integration into the ternary diagram at standardized boundary conditions. The graphical approach indicates the existence of optimum configurations for the specific gasifier types and leads to an individual potential assessment. At a typical gasification pressure of 30 bar, a theoretical maximum cold gas efficiency of 87.4% was identified at a temperature of 980 °C for the above mentioned coal, whereas the maximum syngas yield of 2.09 m3(H2 + CO STP)/kg(waf) was located at 1135 °C. It is shown that only fluid-bed or two-stage processes have the potential to achieve these global maxima. The sensitivity of these maxima to varying ash contents from 5 to 45 wt.% and to coal rank is investigated as well. The study is concluded by the introduction of a simplified user diagram which was derived in order to drive a process towards the identified maxima. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Due to midterm depletion of oil and gas, coal gains importance not only as energy carrier but as feedstock for various chemical syntheses [1]. The available commercial gasification processes which have been presented earlier [2] must be evaluated if they are capable for such conversion strategies. The main challenge is the increasing ash content of the coal as reported from South Africa [3], India [4], Japan [5], and China [6]. The comprehensive assessment of gasification processes is difficult due to lots of independent variables such as coal composition and reactivity, temperature, pressure, H2O supply and other varying boundary conditions. A well-known approach, which was suggested first by Grout [7], is to split coal in its molar C–H–O composition plotting a ternary diagram. While Ghosh [8] used the diagram for coal rank indication, Stephens [9] and Battaerd and Evans [10] incorporated reacting gases and hydrocarbons as well. Recently, Li et al. [11] used the same diagram to illustrate carbon deposition isotherms for a gasification system. However, ⇑ Corresponding author. E-mail address: [email protected] (M. Gräbner).

regarding performance parameters, technology comparison and optimum identification, the C–H–O plot has not been used, although it has a significant potential to illustrate basic relations. It should be noted that in a C–H–O molar plot, the region of gasifier operation in the range of the triangle O2–H2O–CxHyOz would be very small. However, if the same O2–H2O–CxHyOz-system is used as corner points for a new molar based ternary diagram, recalculations of the coal flow eliminating sulfur, nitrogen, moisture and ash as well as recalculations of the technical oxygen flow eliminating nitrogen and argon will be necessary. In the present paper, we introduce a novel approach of plotting O2–H2O–coal mass flow in wt.% in a ternary plot. It allows the assessment of temperature, carbon conversion, cold gas efficiency, dry methane yield, specific synthesis gas production, H2/CO ratio as well as CO/C and CH4/C selectivity of the converted carbon in an easy way without recalculations of the input flows. With the disengagement from molar fractions, a distinct atomic ratio is not longer necessary for each stream. Hence, CxHxOz can be replaced by coal containing all impurities (e.g. mineral matter) and oxygen may include nitrogen as well. Consequently, the diagram is easy to use because the mass flows into a technical gasifier from practice can be applied directly.

0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2012.01.069

Please cite this article in press as: Gräbner M, Meyer B. Introduction of a ternary diagram for comprehensive evaluation of gasification processes for ashrich coal. Fuel (2012), doi:10.1016/j.fuel.2012.01.069

2

M. Gräbner, B. Meyer / Fuel xxx (2012) xxx–xxx

An ash-rich South African coal was selected for the investigation since elevated ash contents pose a challenge to most of the commercial gasification processes. Table 1 presents the coal composition and LHV showing an ash content of 25.3 wt.%. Since all diagrams are based on isobar, adiabatic equilibrium calculations, all figures represent the maximum achievable values of the distinct parameters. In the next step, technical gasifiers are incorporated in the diagram according to their O2–H2O–coal consumptions.

2. Theoretical and technological background 2.1. Ternary diagram setup The O2–H2O–coal ternary diagram is developed by means of equilibrium modeling applying minimization of Gibbs free enthalpy, e.g. in the software Aspen Plus [12]. Fig. 1 indicates the principal scheme of the ternary gasification diagram setup. It can be seen that the mass flow rates into the gasification systems are normalized to unity and treated as mass fractions in wt.% which serve as input parameters for the diagram. In order to concentrate information and maintain applicability, reasonable combinations of output parameters are identified leading to four types of ternary diagrams as presented in Fig. 1.

2.2. Location of technical gasifiers Higman and van der Burgt [13] provide detailed descriptions for the ConocoPhillips (E-Gas), General Electric (GE), Shell, Siemens, HTW (high-temperature Winkler) and Lurgi fixed-bed dry bottom (Lurgi FBDB) gasification technologies, which are selected to be integrated in the diagram. In a first step, for each entrained-flow and fluid-bed process a generic thermodynamic Aspen Plus model is developed. Deviations from equilibrium are included using user defined functions. Verification data for the models are given by Woods et al. [15] for ConcoPhillips, by McDaniel [16] for GE, by Rich et al. [17] for Shell, by Deutsche Babcock [18] for Siemens, and by Bellin et al. [19] for HTW. In a second step, unified boundary conditions were applied to the models to maintain comparability. From the two types of Lurgi fixed-bed gasifiers, the low-temperature Lurgi fixed-bed dry bottom (Lurgi FBDB) system was integrated in the study, because Modde and Krzack [20] provide data which was gained from operating experiences for a similar coal (German bituminous coal from Dorsten with 22.0 wt.%(wf) ash). For the high-temperature slagging British Gas/Lurgi (BGL) fixedbed gasifier, no data for ash-rich coal was available. Table 2 shows all the operating conditions and the physical states of all entering streams serving as boundary conditions for the models. In order to simplify the location of gasifiers and the analysis of the diagrams, four different domains A, B, C and D are introduced. These four domains apply for single stage processes and their boundaries are given mostly by technical limitations. For entrained-flow gasifiers, an upper temperature limit of 2000 °C and a lower temperature limit of 1440 °C (ash fluid temperature) must be maintained to ensure material lifetime and slag discharging conditions. In case of dry feeding systems, moderator steam can vary between 0 and 6 wt.%. Hence, the domain A for dry feed

1. Temperature and carbon conversion in equilibrium are combined offering a general overview and an easy location of gasifier domains. 2. Cold gas efficiency on lower heating value (LHV) basis and dry methane gas yield are fitted together because the high LHV of methane contributes significantly to the cold gas efficiency but limits gas quality in terms of synthesis applications or pre-combustion CO2 separation. 3. Syngas yield and H2/CO ratio are combined since the expected carbon utilization as well as the CO shift conversion efforts for a desired downstream process can be derived directly. 4. The selectivity of carbon gasified to CO, CH4 and CO2 permits carbon management, illustrating to which species the carbon is converted. In order to normalize the sum to 100%, the isolines refer only to the converted part of the carbon. Higher hydrocarbons (tars) are neglected. A pressure of 30 bar is selected due to the suitability for various chemical syntheses [13] and integrated gasification combined cycle (IGCC) power generation including CO2 capture as well [14].

Coal O2 H2O

O2

Gasifier - Siemens - Shell - HTW - GE - ConocoPhillips

Ternary gasifcation diagram (on wt% basis)

H2O

Results of generic models

Temperature & Cold gas efficiency carbon conversion & dry CH4 content

Table 2 Unified boundary conditions for gasification modeling (LHV – lower heating value, IP – intermediate pressure).

Coal Results of adiabatic equilibrium calculation

Syngas yield & H2/CO ratio

CO/C & CH4/C selectivity

Location of gasifiers, operating range and potential assessment Fig. 1. Schematic overview of the ternary gasification diagram setup.

Parameter

Value

Comment/reference

Pressure Temperature

30 bar 1550 °C

Thermal capacity Coal/N2 Coal/transport gas Solids in slurry Slurry temperature O2 purity O2 temperature Moderator steam Quench water

500 MW 25 °C 350 kg/m3(eff.) 65 wt.% 120 °C 95 vol.% 240 °C 37 bar/246 °C 37 bar/175 °C

[13,14] >100 K above ash fluid temperature for slagging systems LHV basis, equivalent to 2066 t/d +3 bar above reactor pressure [13] [21] [25] Residual: 3 vol.% Ar and 2 vol.% N2 +3 bar above reactor pressure Saturated from IP level Preheated for high gas moisture

Table 1 Ultimate analysis of South African high-volatile bituminous coal (waf – water and ash free, wf – water free, ar – as received, LHV – lower heating value). C wt.%(waf)

H

O

N

S

Ash wt.%(wf)

Moisture wt.%(ar)

LHV MJ/kg(wf)

79.6

4.1

13.3

2.1

0.9

25.3

6.0

22.39

Please cite this article in press as: Gräbner M, Meyer B. Introduction of a ternary diagram for comprehensive evaluation of gasification processes for ashrich coal. Fuel (2012), doi:10.1016/j.fuel.2012.01.069

3

M. Gräbner, B. Meyer / Fuel xxx (2012) xxx–xxx

2

H

O

(w

t% )

75

50

75

p

em .t ax

e ur at er

r fo

O H2

=

n co

2000

1500

2500 °C

°C

25

C

°C 1000

100

A

B

°C

m

D

50

3000 °C

st.

)

In Fig. 2, the temperature and carbon conversion is given while the regions for single-stage processes are marked. The Shell and Siemens systems group above the sticky ash temperature range on the O2–coal-axis indicating that no moderator steam is necessary due to the high ash content. In case of the Siemens system, little H2O is participating in the reaction because of the slight CO shift conversion in the full water quench. In the slurry feeding entrained flow domain B, the GE technology and the first stage of the ConocoPhillips gasifier appear. The point of ConocoPhillips is moved towards the O2–coal-axis since the slurry feed stream is upgraded by recycled char which is left over from the chemical quench [13,15]. If two stage processes occur (e.g. ConocoPhillips, Mitsubishi [2]) or independent gasifying agent inlets exist (e.g. Tsinghua two-stage oxygen gasifier), mixing lines can be drawn in the diagram. The law of reverse levers can be applied to identify the mixing points which is described in detail by Stephens [9]. Hence, for the ConocoPhillips gasifier three points appear which are located on one line. The central (mixing) point represents the second stage (gasifier outlet) and the point on the H2O–coal-axis represents the slurry point which is contacted with the product gas from the first stage. Consequently, the mixing point occurs outside the slurryfed domain and can theoretically move along the mixing line at varying slurry mass flows. If the coal had a higher ash fluid temperature, which might apply to other high ash coals, all slagging gasifier points would be shifted towards the 100 wt.% O2 point resulting in higher temperatures.

Lurgi FBDB GE Siemens Shell ConocoPhillips HTW

t%

3.1. Ternary diagram for temperature and carbon conversion

100 A Entrained flow (dry feed) B Entrained flow (slurry feed) C Fluidized bed D Fixed bed (dry ash) 25

(w

3. Results and discussion

0

O2

entrained-flow systems is defined. The slurry feed entrained-flow domain B refers to the concentration of solids in the coal–water slurry ranging from 50 to 70 wt.% [21]. For the fluid-bed gasifier domain C a maximum temperature of 1200 °C ( 80 K below softening temperature) is suggested to avoid undesired bed agglomeration. Although carbon conversions in most fluid-beds were very low because of lower gasification temperature, Adlhoch et al. [22] and Keller et al. [23] report from test runs, that carbon conversions >80% are feasible, which was selected as boundary. Accordingly, Bellin et al. [19] suggested a minimum exit gas temperature of 850 °C. The left boundary represents a maximum gasifying agents to coal mass ratio of 3 as, e.g. tested in the UGas system [24]. For completeness, the fixed-bed domain for dry ash conditions D was also integrated between 600 and 800 °C being aware that thermodynamic equilibrium is not reached at this temperature level. Since all iso-lines (e.g. temperature) are gained by idealized adiabatic equilibrium calculations at 30 bar, only the gasifier input mass flows are necessary to locate the individual systems in the ternary diagram. Consequently, the gasifier points in the diagram represent a hot, well-mixed reaction zone where equilibrium might be approached. Except for fixed-bed gasifiers, this zone is equivalent to the hot raw gas outlet before any heat recovery (e.g. GE, HTW) or quenching (e.g. Shell, Siemens). For two-stage processes like ConocoPhillips, more points can occur. If the ash or slag mass flows leave the hot reaction zone apart from the raw gas (e.g. HTW, Shell, ConocoPhillips), the gasifier points will refer to the raw gas flow. The integration of the ash in the calculations is explained in detail in Section 3.6. Since the generic gasifier models incorporate deviations from equilibrium, differences between the model results and the ternary diagram emerge regarding performance measures. Their magnitude is an indicator for the potential of process optimization and is therefore addressed in Section 3.5.

10

0%

500 °C

Carbo n

% 8 0 70 % 0 % 6

5

0%

40

conve rsion

%

30

%

20

%

0 0

25

50

75

100

South African Coal (wt%) Fig. 2. Ternary diagram for temperature and carbon conversion (30 bar).

The fluidized bed region C indicates an operation in carbon-rich bed inventory mode (carbon conversion <100 %) or ash-rich bed inventory mode where the HTW gasifier operates at carbon-rich conditions. The broad extension of the domain indicates the high flexibility in gasifying agent supply for the fluidized-bed region. It must be noted that the plotted carbon conversion refers to equilibrium carbon (e.g. HTW) and is not identical to non-converted carbon of low reactivity leaving a gasifier (e.g. GE). These minor quantities of inert carbon can be subtracted from the feed coal mass flow since it is not taking part in the reaction. In the fixed-bed domain D, the Lurgi FBDB gasifier is integrated according to its consumption parameters showing the highest H2O consumption amongst all evaluated systems. As a result of combining slagging and fixed-bed conditions, a potential BGL gasifier point would be located outside domain D shifted towards higher O2 and lower H2O conditions. A great advantage of the diagram is to asses directly the effect of additional O2, H2O or coal injection with the help of mixing lines. For example, if it is considered to upgrade the Lurgi FBDB product gas by oxygen injection, the mixing line between the gasifier point and the top corner (O2) of the diagram applies following the law of reverse levers for the intercepts as in every ternary diagram. 3.2. Ternary diagram for cold gas efficiency and dry methane yield Fig. 3 shows the cold gas efficiency which is defined on lower heating value basis [13]. A tongue-shaped region of elevated cold gas efficiency (>85%) can be observed surrounding the 100% carbon conversion iso-line. Along this line, a maximum cold gas efficiency of 87.4% was identified at 980 °C being inside the fluid-bed gasifier domain C indicated by the hexagon. Since any moderator steam injection to a dry feed single stage entrained-flow process would cause losses in cold gas efficiency, no flexibility or significant potential can be derived from domain A. If very optimistic stable slurry concentrations of 70 wt.% solids were assumed, still cold gas efficiencies >80% would not be possible for single stage slurry feed entrained flow processes in domain B. At lower temperatures again an increase in cold gas efficiency can be determined. However, kinetic limitations and tar formation might offset the theoretical equilibrium values below 800 °C. Hence, the indicated equilibrium cold gas efficiency of 85% is in practice not achieved by the Lurgi

Please cite this article in press as: Gräbner M, Meyer B. Introduction of a ternary diagram for comprehensive evaluation of gasification processes for ashrich coal. Fuel (2012), doi:10.1016/j.fuel.2012.01.069

M. Gräbner, B. Meyer / Fuel xxx (2012) xxx–xxx

0 Lurgi FBDB GE Siemens Shell ConocoPhillips HTW

100 Entrained flow (dry feed) Entrained flow (slurry feed) Fluidized bed 25 Fixed bed (dry ash)

%

Cold

30

0%

70

%

A 0.1

B

1.0 2.5

C

25

5 7

50

D

10

%

30

20

100

%

40 vol% (dry gas) CH

4

50

0

75

100

South African Coal (wt%) Fig. 3. Ternary diagram for cold gas efficiency and dry methane yield (30 bar).

FBDB gasifier. Besides cold gas efficiency, the dry methane content of the product gas in vol.% is exhibited. Fig. 3 shows that in the carbon-rich region (carbon conversion <100%) the dry methane yield is mostly between 1 and 20 vol.%. In contarst, high cold gas efficiencies (>80%) accompanied by methane contents of <0.1 vol.% can be reached left of the 100% carbon conversion line, which is typical for entrained-flow technologies. Regarding technology potential, the HTW gasifier or other fluidbed processes (e.g. U-Gas [24]) show the highest flexibility and potential for further optimization towards maximum cold gas efficiency. But also the ConocoPhillips system shows a certain potential since the mixing point inside the fluid-bed domain may be shifted towards increased cold gas efficiency as well. 3.3. Ternary diagram for syngas yield and H2/CO ratio As a further measure to assess gasification, the synthesis gas yield in m3(H2 + CO STP)/kg(waf) was selected. The water and

0 100

Lurgi FBDB GE Siemens Shell ConocoPhillips HTW

A Entrained flow (dry feed) B Entrained flow (slurry feed) C Fluidized bed D Fixed bed (dry ash)

CO

75 1.25 H2/CO= 2.0

0.75 1.0 1.5

C

1.75

% 90

75

1 .5

3 STP)/kg(wa 0.5 m (H 2+CO

25

0.1%

25

10% 20% 40% conversion to CH

4

100

100

1% 2.5% 5%

C

D

f)

A

B

1.0

D

0

70

A

50

Re sid ua l to

B

1.5 5 1.7 2. 0

50 %

O

0. 5

O 2

0.25 1.0 0.5

50

75

) t% (w

50

) t% (w

50

Lurgi FBDB GE Siemens Shell ConocoPhillips HTW

O2

O2

(w t% )

75

100

A Entrained flow (dry feed) B Entrained flow (slurry feed) C Fluidized bed D Fixed bed (dry ash) 25

(w t% )

25

H

Fig. 5 reveals that only the dry feed entrained-flow processes (Shell and Siemens) can yield product gases where more than 90% of the coal’s carbon is converted to CO. For all other examined processes, less than 80% CO/C selectivity was observed. In case of the slurry fed GE system mainly CO2 emerges whereas in terms of the ConocoPhillips and HTW technologies also considerable amounts of methane accompanied by CO2 can be expected. Comparing the discussed syngas yield maximum from Fig. 4 to the equivalent area in Fig. 5, the decreasing CO/C selectivity towards the H2O corner illustrates the conversion of CO to CO2 producing H2. Again the fluid-bed domain provides the highest flexibility and potential for further optimization especially in the area close to the O2–coal-axis.

H

0

3.4. Ternary diagram for CO/C and CH4/C selectivity

2

25

%c on v ers ion to C 30 % O

0

10

%

is m ain ly

85

0%

10 0%

(w t%) O 2

H

50 5

75

t%) (w

50

O2

Ga sE fficie 10 ncy %

75

2

A B C D

ash free basis was suggested by Bellin et al. [19] for comparison of varying ash contents. The molar H2/CO ratio is decisive determining the gas treatment steps. Fig. 4 presents the according plot for syngas yield and H2/CO ratio. The highest syngas yield of 2.09 m3(H2 + CO STP)/kg(waf) was identified at 1135 °C marked by a hexagon in the diagram. Accordingly, the zone where syngas yield is higher than 2.0 m3(H2 + CO STP)/kg(waf) is small resulting in the exclusion of nearly all entrained-flow gasifiers from that area. According to the considered technologies, only the ConocoPhillips mixing points are located in the syngas yield area >2.0 m3(H2 + CO STP)/kg(waf) if equilibrium is assumed. Unexpectedly, most of the fluid-bed domain C overlaps with syngas yields above 2.0 m3(H2 + CO STP)/kg(waf) indicating that not only dryfed entrained flow-processes can deliver a high syngas yield as broadly concluded. Additionally, the elevated amount of syngas can be supplied by fluid-bed processes accompanied by much higher H2/CO ratios up to 1.5 at 2.0 m3(H2 + CO STP)/kg(waf). The observed extension of the high syngas yield area towards the H2O corner reflects basically the proceeding of the non-catalytic water gas shift reaction at H2O excess conditions. In conclusion, the CO shift conversion effort to meet the requirements of syntheses can be significantly reduced for an optimized fluid-bed system in comparison to dry-fed entrained-flow processes with H2/CO ratios of 0.25–0.5 for the same gas production.

%

4

0

0 25

50

75

100

0

25

50

75

100

South African Coal (wt%)

South African Coal (wt%)

Fig. 4. Ternary diagram for syngas yield and H2/CO ratio (30 bar).

Fig. 5. Ternary diagram for for CO/C and CH4/C selectivity (30 bar).

Please cite this article in press as: Gräbner M, Meyer B. Introduction of a ternary diagram for comprehensive evaluation of gasification processes for ashrich coal. Fuel (2012), doi:10.1016/j.fuel.2012.01.069

5

M. Gräbner, B. Meyer / Fuel xxx (2012) xxx–xxx

+4.6

Shell Maximum for slurry feed slagging systems (B)

Siemens

+3.7

Maximum for dry feed slagging systems (A)

Global maximum (C)

+14.1 %-pts.

GE

possible percentage of increase relating the domain specific maximum to the generic model results. The same overall tendencies are observed like in the case of cold gas efficiency. The only difference is that the HTW gasifier shows the highest potential due to its comparatively low carbon conversion and high CH4 yield [19]. Due to the high potential of the HTW gasifier, enhanced concepts like Power-HTW [26] or the internal circulation gasifier (INCI) [27] have been suggested earlier.

+13.7 %-pts.

CoP

3.6. The influence of ash content +10.0 %-pts.

60

65

70

75

80

85

90

Cold gas efficiency (%) Fig. 6. Cold gas efficiency potential analysis (30 bar).

3.5. Technology potential analysis The comparison of the results of the generic gasifier models to the idealized indications of the ternary diagrams permits the evaluation of distinct potentials for each technology. Fig. 6 exhibits the cold gas efficiency results obtained from the generic models indicated by the symbols. The maximum possible cold gas efficiency values obtained from the ternary diagram’s domain limits A, B, C are inserted as well. Hence, the difference between the gasifier point and the domain limits represents the optimization potential for each technology. Although ash rich coal is processed, the Siemens (78.8%) and the Shell (77.9%) systems show the highest cold gas efficiency closely followed by the HTW gasifier (77.4%). The two-stage slurry fed ConocoPhillips (CoP) system achieves a cold gas efficiency of 73.7% while the single-stage GE system has only 60.9% due to poor single pass carbon conversion [16]. Fig. 6 also indicates the possible increase in cold gas efficiency in %-pts. if adiabatic equilibrium is approached. The highest potential is obvious for the GE system accompanied by a low maximum. The Siemens and Shell systems show little potential on an already high level. The ConocoPhillips and HTW systems can theoretically approach the global maximum exceeding all other systems. The same analysis is carried out for the syngas yield presented in Fig. 7. Again the Siemens (1.91) and Shell (1.89) systems show the best performance followed in a distance by ConocoPhillips (1.76), HTW (1.64) and GE (1.52). The potential is shown by the

t% ) (w O 50

Global maximum (C)

50

25

+18.7 %

CoP

Maximum cold gas efficiency

) t%

+18.4 %

GE ConocoPhillips Shell Siemens HTW

75

(w

GE

+4.7 %

25

Maximum for dry feed slagging systems (A)

0

O2

Siemens

Maximum for slurry feed slagging systems (B)

Coal ash content / 5.0 wt% (wf) 25.3 wt% (wf) / / 45.0 wt% (wf)

H

+5.8 %

Shell

In order to investigate the influence of the ash content, a part of the ternary diagram is enlarged where coal mass flow is between 25 and 100 wt.% including all modeled gasifiers. The ash content of the South African coal is varied from 5 to 45 wt.%(wf) by scaling the reference case from Table 1. The ash is respected in the calculation of the ternary diagram and in the gasifier models as a nonreacting stream. It has a constant specific heat capacity of 1.05 kJ/(kg K) and a melting enthalpy which increases between the ash softening and fluid temperature to a final value of 678.6 kJ/kg. The value was obtained from the FactSage database [28] applying the measured ash composition under reducing conditions. Corresponding to Figs. 2 and 3 only the 80% cold gas efficiency and ash fluid temperature (1440 °C) iso-lines are shown in Fig. 8 for three different ash contents. The gasifier points are displayed and consistent with the ash content in color. The maximum cold gas efficiency points are displayed as well. It can be seen that the area of cold gas efficiencies >80 % as well as the ash fluid temperature iso-lines are shifted towards the 100 % coal corner due to the increased ash fraction of the coal mass flow. Another effect is that the same area becomes smaller with increasing ash content due to the energy consumption by ash heating. The location of the single stage slagging gasifiers is determined by the necessity to operate well above the ash fluid temperature. Consequently, the Siemens and Shell systems are located within the high cold gas efficiency area at 5 wt.%(wf) ash. They cannot approach the same area at 45 wt.% because the area is shifted towards lower temperatures. The shift of the GE and ConocoPhillips systems with increasing ash content is related to the constant slurry solids

2

HTW

Ash fluid temperature 80 % Cold gas efficiency

+27.4 %

HTW

75 25

1.5

1.6

1.7

1.9

2.0

2.1

Syngas Yield (m³(H2+CO STP)/kg(waf)) Fig. 7. Syngas yield potential analysis (30 bar).

2.2

0 50

75

100

South African Coal with variable ash content (wt %) Fig. 8. Sensitivity of the 80% cold gas efficiency and ash fluid temperature iso-lines to coal ash content.

Please cite this article in press as: Gräbner M, Meyer B. Introduction of a ternary diagram for comprehensive evaluation of gasification processes for ashrich coal. Fuel (2012), doi:10.1016/j.fuel.2012.01.069

M. Gräbner, B. Meyer / Fuel xxx (2012) xxx–xxx

3.7. The influence of coal rank To assess the influence of the coal rank, a Pittsburgh #8 bituminous coal was selected which has an ash content of 10.2 wt.%(wf) and a LHV of 34.0 MJ/kg(waf) while the LHV of the reference coal is 29.9 MJ/kg(waf). The composition and ash data were taken from Miller and Tillman [29]. A lower ash fluid temperature of 1340 °C permits slagging conditions at 1450 °C. The same analysis Coal ash content / 5.0 wt% (wf) / 25.3 wt% (wf) / 45.0 wt% (wf)

0

70 Reference case

60

Shell Siemens GE-R ConocoPhillips HTW

50

0

10

40

2.0

Thermodynamic maximum

1.8

1.6 Shell Siemens GE-R ConocoPhillips HTW

1.4

1.2 0

10

Reference case

20

30

40

50

Ash content (wt% (wf)) Fig. 11. Syngas yield at varying ash content derived from gasifier modeling.

Coal type / South African / Pittsburgh #8

GE GE ConocoPhillips ConocoPhillips Shell Shell Siemens Siemens HTW HTW

75

80 % Cold gas efficiency

50

(w

O2

t% )

0

O

(w

(w

H

) t%

) t%

50

50

50

2.2

25

2

30

Fig. 10. Cold gas efficiency at varying ash content derived from gasifier modeling.

O2

O

20

Ash content (wt% (wf))

2

t% )

80

50

Ash fluid temperature

(w

Thermodynamic maximum

GE ConocoPhillips Shell Siemens HTW

75

25

H

90

Cold gas efficiency (%)

concentration of 65 wt.%. Hence, the decreasing energy density of the slurry leads to an exclusion from the high cold gas efficiency area at high ash contents. The analysis of the diagram shows that the HTW gasifier is able to follow the elevated cold gas efficiency area due to its operation at dry ash conditions. Furthermore, it approaches the cold gas efficiency maximum with increasing ash content. The same analysis is carried out for the 2.0 m3(H2 + CO STP)/ kg(waf) iso-line in Fig. 9. Again, the shift towards the 100% coal corner with increasing ash content is obvious. Due to the increasing combustion of syngas for ash heating, the area of syngas yields >2.0 m3(H2 + CO STP)/kg(waf) is significantly reduced with increasing ash content. All slagging gasifiers cannot approach the high syngas yield area at 45 wt.%(wf) ash, since it is located below the ash fluid temperature. Also the HTW gasifier does not operate close to the syngas yield maximum. Since the ternary diagram demonstrates only the thermodynamic potential, the verified gasifier models were used to accomplish a sensitivity analysis of the ash content. Fig. 10 exhibits the results for cold gas efficiency which are in general consistent with the observations from Fig. 8. While the slurry fed GE system shows the strongest decrease of cold gas efficiency with increasing ash content, the effect is mitigated for the ConocoPhillips gasifier due to the chemical quench. The dry fed systems (Shell, Siemens, HTW) operate at the same cold gas efficiency decreasing slightly with increasing ash content. At 45 wt.%(wf) ash, the HTW gasifier exceeds all other systems. The same tendencies can be found for the syngas yield shown in Fig. 11. The only exception is the HTW gasifier which exhibits the lowest syngas yield at 5 wt.%(wf) slightly increasing with increasing ash content. This can be explained by the increasing CH4 conversion caused by higher O2 consumption at higher ash content.

Syngas yield (m³(H2+CO STP)/kg(waf))

6

25

25

Ash fluid temperatures 3

2.0 m (H2+CO STP)/kg(waf)

Maximum syngas yield

Maximum cold gas efficiency 75

75 25

0

0 50

75

100

25

50

75

100

South African Coal with variable ash content (wt %)

Coal (wt %)

Fig. 9. Sensitivity of the 2.0 m3(H2 + CO STP)/kg(waf) and ash fluid temperature isolines to coal ash content.

Fig. 12. Sensitivity of the 80% cold gas efficiency and ash fluid temperature iso-lines to coal rank.

Please cite this article in press as: Gräbner M, Meyer B. Introduction of a ternary diagram for comprehensive evaluation of gasification processes for ashrich coal. Fuel (2012), doi:10.1016/j.fuel.2012.01.069

7

M. Gräbner, B. Meyer / Fuel xxx (2012) xxx–xxx

0

Coal type / South African / Pittsburgh #8

25

t% )

Maximum syngas yield

50

2

) t%

O

(w

(w

O2

H

accompanied by the higher ash content make any moderator redundant. Hence, any steam addition would cause a temperature drop which must be counteracted by increased O2 supply in order to maintain slagging conditions at 1550 °C. Consequently, any steam injection to dry feed single stage slagging processes causes losses in cold gas efficiency as stated earlier. Due to the extended area of high cold gas efficiency for Pittsburgh #8 coal, all gasifiers operate at a higher level of cold gas efficiency while ConocoPhillips and HTW approach the maximum. Looking at the syngas yield in Fig. 13, the beneficial effect of coal rank is significant. The area of syngas yields >2.0 m3(H2 + CO STP)/ kg(waf) experiences a broad extension incorporating all gasifiers except the HTW system.

GE GE ConocoPhillips ConocoPhillips Shell Shell Siemens Siemens HTW HTW

75

50

25

Ash fliud temperatures

3.8. Optimum user diagram

3

2.0 m (H2+CO STP)/kg(waf)

50

100

75

Coal (wt %) Fig. 13. Sensitivity of the 2.0 m3(H2 + CO STP)/kg(waf) and ash fluid temperature iso-lines to coal rank.

procedure as for the varied ash content was carried out. The 80% cold gas efficiency and the ash fluid temperature iso-lines are presented in Fig. 12. Comparing the Pittsburgh #8 coal iso-lines to the blue reference coal iso-lines, a shift is observed which can be partly traced to the altered ash content. However, the change in the shape of the curves refers to the change in coal rank. Regarding the gasifier points, it can be seen that for the dry feed single stage entrained flow processes steam injection can help to increase cold gas efficiency in case of Pittsburgh #8 coal. Here, the steam acts as temperature moderator. Otherwise the temperature would exceed 2000 °C for 99.9% carbon conversion under dry conditions. In case of the South African coal, the lower rank

Maximum H2+CO yield

(m³(H2+CO STP)/kg(input, waf))

1 bar 0

1

2

3

5

100 bar 6

0

Maximum 1

2

3

4 0.55

2.3 2.2

0.50

2.1 2.0

0.45

1.9

0.40

1.8 0.35

1.7

0.30

1.6

1400

Temperature (°C)

30 bar 4

Oxygen consumption O2/Coal (m³(STP)/kg)

25

In order to exploit the identified potential, a simplified user diagram was developed for the specific syngas yield as exhibited in Fig. 14. The user diagram allows identification of the maximum possible syngas yield for pressures between 1 and 100 bar and temperatures between 600 and 1500 °C for the selected South African coal. The diagram is derived from the ternary diagram by setting the temperature and pressure to constant values and finding the maximum syngas yield. For the identified point the steam/oxygen ratio in kg/m3(STP), the molar H2/CO ratio, the temperature in °C and the oxygen consumption O2/coal in m3(STP)/kg (ar) were extracted from the diagram or calculated from the flow streams. The relations are plotted in a quad diagram where monotonic correlations are used to link the individual curves in the diagrams. The dashed curves represent the maximum syngas yield path at varying pressure. Hence, it is possible to draw connection lines amongst the curves (see arrows in Fig. 14). All information to adjust the maximum syngas yield for a given pressure can be read off easily from the plot. The arrows demonstrate the utilization of the diagram for a 30 bar process. The maximum syngas yield being 2.09 m3(H2 + CO STP)/kg(waf) is selected as starting point which can be extended along the H2O/O2-line of 0.5 kg/m3(STP)

0

1400

Sticky ash zone

1200

1200

1000

1000

800

800

Temperature (°C)

75

600

600 0

1

2

3

4

5

Steam/oxygen H2O/O2 (kg/m³(STP))

6

0

1

2

3

4

H2/CO ratio (mol/mol)

Fig. 14. User diagram for optimum syngas yield for South African coal.

Please cite this article in press as: Gräbner M, Meyer B. Introduction of a ternary diagram for comprehensive evaluation of gasification processes for ashrich coal. Fuel (2012), doi:10.1016/j.fuel.2012.01.069

8

M. Gräbner, B. Meyer / Fuel xxx (2012) xxx–xxx

to a temperature of 1135 °C. At constant temperature a molar H2/ CO ratio of 0.6 with an oxygen consumption of 0.385 m3(STP)/kg can be found. The information might be used to design a new system or to adjust an existing gasifier, e.g. the HTW system, in order to increase the syngas yield towards the theoretical maximum. 4. Conclusion A new type of ternary diagram for comprehensive evaluation of gasification processes was developed using the example of ash-rich South African coal. Depending on the three independent variables O2, H2O, and coal mass flow, an assessment of temperature, carbon conversion, cold gas efficiency, dry methane content, syngas yield, H2/CO ratio and selectivity of carbon conversion was accomplished. The diagram allows one to distinguish operating regions for single-stage entrained-flow, fluidized-bed and dry ash fixedbed processes and provides a wide-spread overview at one glance. It is capable to evaluate multi-stage processes by simple mixing lines. The ConocoPhillips, GE, Shell, Siemens, and HTW technologies were incorporated in the diagram as a result of generic model setup. The Lurgi fixed-bed dry bottom gasifier was located based on operating experiences of similar German ash-rich hard coal [20]. The main findings are as follows:  The maximum theoretical cold gas efficiency at 30 bar of 87.4% was identified at 980 °C and the maximum syngas yield of 2.09 m3(H2 + CO STP)/kg(waf) was found at 1135 °C.  Only fluid-bed processes (e.g. HTW) and two-stage processes (e.g. ConocoPhillips) have the potential to reach the global maximum.  Fluid-bed processes indicate a potential of H2/CO ratios up to 1.5 at specific syngas productions of 2.0 m3(H2 + CO STP)/ kg(waf) incorporating a part of the CO shift conversion into the gasifier.  The generic models of mature Siemens and Shell technology predict cold gas efficiencies and syngas yields close to the thermodynamic maximum while in all other cases significant improvements are possible.  The variation of the ash content between 5 and 45 wt.%(wf) revealed a shift of the cold gas efficiency and syngas yield maxima to lower temperatures with increasing ash content, which is unfavorable for all single stage slagging entrained flow processes.  The comparison of the South African coal to a Pittsburgh #8 coal of higher rank exhibits that higher rank and lower ash content cause a significant extension of the high syngas yield and cold gas efficiency areas permitting higher operational flexibility and steam injection for cold gas efficiency increase.  A simplified user diagram for a pressure range from 1 to 100 bar and a temperature range from 600 to 1500 °C was developed in order to configure processes to achieve optimum performance.

Acknowledgements We like to thank the Ministry of Science and the Arts of the Free State of Saxony (R&D number 12272-1979) as well as the German Federal Ministry of Economics and Technology for the financial support of the present work (R&D number 0327865). Special thanks is regarded to P.A. Nikrityuk and A. Laugwitz for various hints and productive discussions.

References [1] Pardemann R, Meyer B. Status and perspectives of coal utilisation in power plants including gasification. Chem Ing Tech 2011;83(11):1–16. doi:10.1002/ cite.201100074. [2] Gräbner M, Meyer B. Coal gasification – Quo Vadis? World of Mining-Surf Undergr 2010;62(6):355–62. [3] Everson RC, Neomagus HW, Kaitano R, Falcon R, van Alphen C, du Cann VM. Properties of high ash char particles derived from inertinite-rich coal: 1. Chemical, structural and petrographic characteristics. Fuel 2008;87(13– 14):3082–90. doi:10.1016/j.fuel.2008.03.024. [4] Iyengar R, Haque R. Gasification of high-ash indian coals for power generation. Fuel Process Technol 1991;27(3):247–62. [5] Kurose R, Ikeda M, Makino H. Combustion characteristics of high ash coal in a pulverized coal combustion. Fuel 2001;80(10):1447–55. doi:10.1016/S00162361(01)00020-5. [6] Liu G, Zheng L, Gao L, Zhang H, Peng Z. The characterization of coal quality from the Jining coalfield. Energy 2005;30(10):1903–14. doi:10.1016/ j.energy.2004.09.003. [7] Grout FF. The composition of coals. Econ Geol 1907;2(3):225–41. doi:10.2113/ gsecongeo.2.3.225. [8] Ghosh T. Change in coal macerals. Fuel 1971;50(2):218–21. doi:10.1016/00162361(71)90011-1. [9] Stephens JF. Coal as a C–H–O ternary system. 1. Geochemistry. Fuel 1979;58(7):489–94. doi:10.1016/0016-2361(79)90166-2. [10] Battaerd HA, Evans DG. An alternative representation of coal composition data. Fuel 1979;58(2):105–8. doi:10.1016/0016-2361(79)90233-3. [11] Li X, Grace J, Watkinson A, Lim C, Ergüdenler A. Equilibrium modeling of gasification: a free energy minimization approach and its application to a circulating fluidized bed coal gasifier. Fuel 2001;80(2):195–207. [12] ASPEN Plus, Version 2006. Aspen Technology Inc., 200 Wheeler Road Burlington, MA, USA; 2006. [13] Higman C, van der Burgt M. Gasification. New York: Elsevier Science; 2003. doi:10.1016/B978-075067707-3/50018-8. [14] Gräbner M, Morstein O, Rappold D, Günster W, Beysel G, Meyer B. Constructability study on a german reference IGCC power plant with and without CO2-capture for hard coal and lignite. Energy Convers Manage 2010;51(11):2179–87. [15] Woods MC, Capicotto PJ, Haslbeck JL, Kuehn NJ, Matuszewski M, Pinkerton LL, et al. Cost and performance baseline for fossil energy plants. Bituminous coal and natural gas to electricity, vol. 1. Tech. Rep. DOE/NETL-2007/1281; National Energy Technology Laboratory; 2007. [16] McDaniel J. Polk power station 250 MW IGCC. Compact Course Gasification, TU Bergakademie Freiberg; 10.-12. November 2008. [17] Rich JWJ, Hoppe R, Choi GN, Hennekes RJ, Heydenrich R, Hooper M, et al. WMPI-waste coal to clean liquid fuels. Gasification Technologies Conference; 2003. [18] Deutsche Babcock. Kombikraftwerk mit GSP-Flugstromvergasung. Deutsche Babcock Werke AG Brochure; 1992. [19] Bellin A, Dehms G, Karkowski G, Nassenstein C, Schrader L, Schumacher HJ. Kohlevergasung im Hochtemperatur-Winkler-Vergaser. Tech. Rep. FK 03E1092C, ISBN 3-926732-07-5; Rheinbraun AG; 1988. [20] Modde P, Krzack S. Die Veredlung und Umwandlung von Kohle, Technologien und Projekte 1970 bis 2000 in Deutschland. Gaserzeuger mit Drehrost. DGMK Deutsche Wissenschaftliche Gesellschaft für Erdöl, Erdgas und Kohle e.V.; 2008. p. 307–9. doi:10.1002/cite.200950041 [21] Hornick MJ, McDaniel JE. Tampa electric polk power station integrated gasification combined cycle project-final report. Tech. Rep. DE-FC-2191Mc27363; Tampa Electric Company, Polk Power Station; 2002. [22] Adlhoch W, Keller J, Herbert P. The development of the HTW coal gasification process. In: 9th EPRI Conference of coal gasification power plants; 1990. [23] Keller H, Adlhoch W, Vierrath H. The high temperature Winkler (HTW) coal gasification process – a proven technology for the production of syngas and fuel gas. In: 19th World GAS Conference; 1994. [24] Mason D, Patel J. Chemistry of ash agglomeration in the U-GasÒ process. Fuel Process Technol 1980;3(3–4):181–206. [25] Valenti M. Bringing coal into the 21st century. Mech Eng 1995;117(2). [26] Gräbner M, Ogriseck S, Meyer B. Numerical simulation of coal gasification at circulating fluidised bed conditions. Fuel Process Technol 2007;88(10):948–58. [27] Gräbner M, Uebel K, Messig D, Meyer B. Development and modelling of 3rd generation gasifiers for low-rank and high-ash coals. Int Conf Coal Sci Technol 2009. [28] Bale C, Chartrand P, Degterov S, Eriksson G, Hack K, Mahfoud RB, et al. Factsage thermochemical software and databases. Calphad 2002;26(2):189–228. [29] Miller BG, Tillman DA. Coal characteristics. In: Miller BG, Tillman DA, editors. Combustion engineering issues for solid fuel systems. Burlington: Academic Press; 2008. p. 33–81. doi:10.1016/B978-0-12-373611-6.00002-1. ISBN 9780-12-373611-6

Please cite this article in press as: Gräbner M, Meyer B. Introduction of a ternary diagram for comprehensive evaluation of gasification processes for ashrich coal. Fuel (2012), doi:10.1016/j.fuel.2012.01.069