Review article

An understanding of the behaviour of anumber of element phases impacting on a commercial-scale Sasol-Lurgi FBDB gasifier

J.R. Bunta, , , F.B. Waandersb

a Sasol Technology (Pty) Ltd., Box 1, Sasolburg 1947, South Africab School of Chemical and Minerals Engineering, North West-University, Potchefstroom 2520, South AfricaReceived 24 August 2007. Revised 22 November 2007. Accepted 27 November 2007. Available online 26 December 2007.http://dx.doi.org/10.1016/j.fuel.2007.11.014, How to Cite or Link Using DOICited by in Scopus (5)Permissions & Reprints

AbstractChemical properties of coal which impact on gasification performance relate to those processes which do effect a change in chemical constitution, these in turn may lead to changes in physical properties such as particle size distribution and surface area of the coal. Turn-out samples obtained from a commercial-scale Sasol-Lurgi fixed-bed dry bottom (FBDB) gasifier were characterized to understand and interpret the internal chemical property behaviour and are discussed in relation to the residual C, H, N, S and O distribution profiles obtained. Thermodynamic equilibrium simulation of the organic and inorganic speciation behaviour occurring within a fixed-bed gasifier was modelled using the Fact-Sage simulation package, and used to support the measured ultimate analysis profile data obtained.

The measured gasifier ultimate analysis profiles provided good insight into understanding the development of aromaticity of the char, expressed by the carbon:hydrogen ratio calculated on a mass basis. Equilibrium compositional profiles calculated for C, H, N, S and O provided discernment regarding the speciation and partitioning behaviour occurring within the fixed-bed-reactor. Fact-Sage thermodynamic equilibrium modeling of the gasifier related to the ultimate analysis results, was found to be useful in identifying an oxygen scavenging effect created by the mineral transformation behaviour occurring during reduction. It was found that oxygen-containing species such as Mg2Al4Si5O18 (corderite)

and Fe2Al4Si5O18 (ferro-corderite) form within the reduction zone. It would appear that mineral composition is a more fundamental property than merely ash content in the gasification process, when viewed on an oxygen consumption basis.

KeywordsSasol-Lurgi fixed-bed dry bottom gasification; “Turn-out” sampling methodology; Reaction zones; Element phases C, H, N, S and O

1. Background and introductionWhen coal is heated in an inert atmosphere, the coal generally undergoes a kind of de-polymerization reaction which leads to the formation of a meta-stable intermediate product. This de-polymerization may be due to the scission of e.g. methylene (–CH2–) bridges, or ether bridges (–O–). Free radical species are formed due to the thermal cracking of the linkages between the aromatic clusters. These free radical species are stabilized either through the re-arrangement of atoms within a fragment or by collision with other species. The resultant stabilised structure, depending on the vapour pressure, may either evolve as a volatile component (gas or tar) or remain as part of the residual char [1].

In the drying zone, the coal loses all of its moisture and this drying process results in an endothermic process. The temperature of the exit gas will be highly affected by the moisture content present in the feed coal. When the dried coal reaches a temperature of about 350–400 °C, it starts to de-volatilize with the production of gases, oils and tars [1], [2] and [3].

The process whereby tars, oils and hydrocarbon gasses (e.g. methane and ethane) are formed, is often referred to as the primary pyrolysis of coal; the product being referred to as “semi-char”. The temperature range in which semi-char is formed is typically between 400 and 600 °C. Upon further heating, the coal de-volatilizes further with the evolution of mainly hydrogen, to form the char. In the pyrolysis zone the coal is heated in an inert atmosphere, to a temperature of 700 °C. The coal undergoes

pyrolysis or destructive distillation, due to the action of heat and thus decomposes. The reactivity of a given char is a measure of the rate at which the specific char will react with a given reactant, under well defined conditions of temperature and pressure. The Boudouard reaction and rate of reaction is given in Eqs. (1) and (2).

(1)

(2)R(CO2)=(dX/dt)·[1/(1-X)]where R(CO2) is the C gasification rate (or reactivity expressed as inverse time, s−1 or h−1), X is the C conversion factor, and t is the time.

R is sometimes referred to as the “specific” or “intrinsic” gasification rate. The formula implies that the moles of carbon gasified per unit time (under conditions of constant temperature, pressure and gas flow) is directly proportional to the amount of carbon present at any point in time. This means that R should remain constant with increasing conversion [1].The most important chemical reactions relevant to the fixed-bed gasification process (involving C, H and O) are similar to those of gas reforming. Both take place at a relatively high temperature (approximately 1000 °C or more), which is as a result of the exothermic combustion (oxidation) reactions which are required to drive the endothermic reduction reactions.Active sites in char (normally depicted as C) are per definition those points on the surface of the coal where certain reactions can occur [4]. Reactants such as oxygen, carbon dioxide and water (steam) do not readily react with carbon, unless they are able to dissociate in order to chemisorb on the carbon surface. This dissociation occurs primarily on the edges of the aromatic layers of the char, or otherwise at any point where the chemical structure shows a defect [1]. These reactions may be visualised as follows:(3)

(4)C(O)→CO+C(char)

or(5)

(6)C(O)→CO+C(char)

In both cases the reactant dissociates with the adsorption of an oxygen atom on the carbon active sites. In the second step, the carbon–oxygen complex is desorbed from the bulk of the char structure, creating a new active site.

During high temperature gasification of coal (or other solid fuels), most of the sulphur constituent is released and converted to hydrogen sulphide (H2S), as well as a small amount of carbonyl sulphide (COS), due to the reduced oxygen environment in the gasifier. The concentration levels of these so-called acid gases, in the raw syngas exiting the gasifier, are almost entirely dependent on the levels of sulphur in the coal [5]. The H2S and COS contaminants are almost entirely removed from the syngas in the acid gas removal equipment, prior to combustion or fuel conversion processes [6].The gasification process differs significantly from combustion with respect to the impact of chemically bound nitrogen in solid fuels, like coal. Gasification, because it operates with a deficiency of oxygen, converts most of the fuel nitrogen into harmless nitrogen gas (N2). While a small portion is converted to ammonia (NH3) and hydrogen cyanide (HCN), these water-soluble species are removed during fuel gas cooling and cleaning, and are usually converted to nitrogen in the sulphur recovery process [6].From various unpublished test gasifier runs at Sasol, an understanding of the oxygen consumption on the basis of ash content has still not been clearly explained. Even when using statistically designed experimental runs, the lack in closures with respect to oxygen balance could not always be clarified.2. Objective of this studyIn a sequential (axial) sampling “turn-out” methodology, as proposed by Krishnudu et al., [7] and [8], a Sasol-Lurgi FBDB MK IV gasifier was sampled, in order to present samples to accurately describe operational aspects occurring within the reaction zones of the reactor. Characterization of the C, H, N, S and O properties of the sample increments are expected to provide some understanding of the chemical partitioning behaviour of these elements within the fixed-bed gasifier: i.e. drying, pyrolysis, reduction and combustion (ash-bed) zones.

The measured profiles obtained by means of ultimate analysis measurements will be discussed in this paper, together with the partitioning and speciation behaviour predicted by the Fact-Sage thermodynamic equilibrium model for these elements. The modeling data is shown mainly to provide the reader with an understanding of the main gas phase chemical reactions occurring within the gasifier. This data should not be directly compared to the elemental ultimate analysis profiles, as the effect of mineral transformation behaviour is included in the model, while volatile matter is excluded. Steam (H2O) and oxygen (O2) were used as gasification agents in the model.

3. Experimental

3.1. Gasifier sampling methodologyA sampling plan was required for the gasifier turn-out exercise with the number of samples required, being calculated according to the specifications of a statistical model. In order to determine the C, H, N, S and O profiles it was necessary to collect samples at different levels across the length of the gasifier. In this case, a minimum sample increment size was calculated using the statistical design-expert software.

After, de-pressurizing the gasifier by cutting off the supply of oxygen and steam, followed by water quenching, the ash-grate speed of the reactor was controlled at minimal revolutions per hour while discharging the gasifier contents. This gasifier sampling procedure was similar to the method proposed earlier by Krishnudu et al. [7] and [8]. Each sample was further sub-sampled and contained in 200 l drums for characterization purposes.

3.2. Analytical characterization methods utilizedEach of the gasifier turn-out samples obtained were thoroughly homogenized after measuring the bulk density and then riffled and split into equal quarters. Two of the representative quarters, in each case, were combined and stage-crushed to below 1 mm for analytical purposes, the other two quarters were combined for particle size determination. The analytical methods used in the sample characterization will be discussed in the following section.

3.2.1. Ultimate analysis

Carbon, hydrogen and nitrogen were determined using the ASTM

D5373 procedure, total sulphur using ASTM D4239 and the oxygen content was calculated by difference. Results were normalized to a constant ash content for discussion purposes.

3.3. Fact-Sage modelingThe current ASPEN thermodynamic model used for gasification purposes at Sasol only simulates the organic component behaviour during fixed-bed gasification [9], [10] and [11]. The purpose of using the Fact-Sage model will be to predict the changes in partitioning and speciation behaviour in the drying, pyrolysis, reduction and combustion processes occurring within the fixed-bed gasifier with a focus on both the inorganic elements and organic components.

To simulate the gasification process as close as possible to the actual gasification process, similar flows (kg/h) and conditions (temperature, pressure and mass flows) as measured during stable operation were used in the modeling campaign. The coal consists of: moisture, fixed carbon, volatiles and minerals and the coal feed is thus he sum of these four components. The input into Fact-Sage has to however be in elemental form, i.e. carbon (C), hydrogen (H), nitrogen (N), sulphur (S) and oxygen (O). The mass flow for the fixed carbon and volatile matter are normalized to an elemental composition similar to that of an ultimate analysis. The ash flow consists of different mineral species, and the ash flow was normalized to a mass flow for the different mineral species. The gasification process is schematically given in Fig. 1 together with the typical reaction zones with respect to the gasifier temperature profile [12].

<img class="figure large" border="0" alt="Full-size image (35K)" src="http://ars.els-cdn.com/content/image/1-s2.0-S0016236107005170-gr1.jpg" data-thumbsrc="http://ars.els-cdn.com/content/image/1-s2.0-S0016236107005170-gr1.sml" data-fullsrc="http://ars.els-cdn.com/content/image/1-s2.0-S0016236107005170-gr1.jpg">Fig. 1. Typical reaction zones in the Sasol-Lurgi MK IV FBDB gasification process with respect to the temperature profile [12].

Each zone was treated as a separate zone for modeling purposes in the Fact-Sage modeling. The model operates on the principle

that the coal (inorganic and organic components) flow from the top downwards in the gasifier; and the gas flows upwards into the zone that is being modeled, i.e. the coal that flows into the drying and de-volatilization zone is brought into contact with the gas that flows out of the gasification zone – thus a counter-current effect.

Schematically this implies that the up-flowing gas from the gasification zone reacts with the down-flowing coal and mineral matter into the gasifier. The discussion of the results, although integrated as one unit, will be done in three sections: (1) drying and de-volatilization, (2) reduction (gasification) and (3) combustion and ash-bed zones. The emphasis of this paper will be on the partitioning and speciation behaviour of the C, H, S, N and O species, trends and variations impacting on the mineral species, rather than on the actual flows.

The drying and de-volatilization zone only took into account the temperature range from 25 °C to 725 °C, while the reduction zone temperature ranged from 725 °C to 1316 °C and the combustion ash-bed zone temperature ranged from 1340 °C to 340 °C. The input characteristics of the feed coal to the model in this case was identical to the work conducted by Van Dyk [10], with the exception of the trace element composition which was normalized, based on the mineral composition considered as ash by the proximate analysis. The mineralogical composition of the ash component (wt.dolomite (10.1%), calcite (6.7%), pyrite (4%), muscovite/illite (2.9%), microcline (1.9%), gypsum (1.1%). apatite (0.5%) and anatase (0.3%). Minor quantities of chlorine and fluorine were also present.

Carbon was added as follows to the model: (100%) in the feed coal and treated in the drying and pyrolysis zones of the gasifier; (87%) as input to the reduction zone; and (26%) addition to the combustion and ash-bed zone of the gasifier. These additions were necessary, since under the thermodynamically controlled conditions simulated by the model, all of the carbon would be consumed in the various reaction zones and the gaseous reactions involving carbon, i.e. CO2 and CO reactions would not be formed. The carbon input values used in the Fact-Sage model were predicted from the current Sasol Aspen gasification model [9].

4. Results

The Sasol-Lurgi MK IV FBDB gasifier residual carbon, hydrogen, nitrogen, sulphur and oxygen profiles given on a mass basis are shown in Fig. 2 and Fig. 4 with trend lines included on the graphs in order to guide the eye. A statistical analysis of the data, with respect to the four distinctive gasification reaction zones identified, i.e. (1) drying, (2) pyrolysis, (3) reduction and (4) combustion (ash-bed) are incorporated throughout, in the form of a zone average value. These zones are integrated with error bars, showing the confidence intervals at a 95% confidence level. In order to better understand the speciation and partitioning behaviour occurring during fixed-bed gasification, Fact-Sage thermodynamic equilibrium modeling was employed, providing equilibrium compositional profiles. This data is shown mainly to provide the reader with an understanding of the main gas phase chemical reactions occurring within the gasifier. This data should not be directly compared to the elemental ultimate analysis profiles, as the effect of mineral transformation behaviour is included in the model, while volatile matter is excluded and steam (H2O) and oxygen (O) are used as gasification agents in the model.

<img class="figure large" border="0" alt="Full-size image (35K)" src="http://ars.els-cdn.com/content/image/1-s2.0-S0016236107005170-gr2.jpg" data-thumbsrc="http://ars.els-cdn.com/content/image/1-s2.0-S0016236107005170-gr2.sml" data-fullsrc="http://ars.els-cdn.com/content/image/1-s2.0-S0016236107005170-gr2.jpg">Fig. 2. Chart depicting the relationship between the ultimate carbon and fixed carbon distribution profiles on a normalized kg/100 kg ash basis for the Sasol-Lurgi MK IV FBDB gasifier.

4.1. Carbon (C) distribution profileFrom Fig. 2 it is clear that the average ultimate carbon content is significantly different between the four reaction zones and decreases from an average value of 170 kg/100 kg ash to about 10 kg/100 kg ash. It is important to note that the fixed carbon (FC) content and the ultimate carbon content approach the same normalized range within the reduction zone and the combustion zones. The carbon associated with the volatile matter in the case

of the amount of ultimate carbon is included in the analysis, therefore the difference between the two, lies only in the drying and pyrolysis zones of the gasifier as expected, thus providing credibility to the sampling methodology employed in this study.

When comparing the model input carbon values (predicted using Aspen) [9] with the measured carbon distribution given in Fig. 2, it is clear that there is an error in the carbon apportionment to the reduction zone, i.e. 87% predicted versus 67% measured at the end of the pyrolysis zone. From the measured profile, 87% of the carbon indeed remains after the fast pyrolysis step, but carbon conversion continues within the “slow pyrolysis with some gasification” regime within the pyrolysis zone of the gasifier. The carbon allocation to the combustion and ash bed zone is correct, i.e. 26% predicted by Aspen and 26% measured in the Sasol-Lurgi MK IV FBDB gasifier.

The Fact-Sage calculated carbon equilibrium compositional profiles in the drying, pyrolysis and reduction zones of the gasifier are given in Fig. 3. From the predictive carbon speciation data for the drying and pyrolysis zones, it is apparent that trace quantities of methane (CH4) and CO2 are formed at 125 °C. The predicted value for methane is in agreement with laboratory scale pyrolysis test work results obtained by Morgan [4] who found that methane evolved at a temperature of 100 °C from experiments conducted on a high ash South African bituminous coal using a thermo-gravimetric (TGA) apparatus coupled to a GC operating at atmospheric pressure in a nitrogen atmosphere. As the solid carbon further reacts within this zone (Fig. 3), the CH4 and CO2

contents increase to ca. 20% and 30% at a temperature of 525 °C respectively while CO begins to form. Small quantities of COS (0.1%) are also formed from the transformation of pyrite and other base metal sulphides, appearing from ca. 525 °C. The predicted COS results from this study are in agreement with those found by Chu et al., [13] who measured a similar concentration of COS appearing at 500 °C in laboratory scale pyrolysis experiments conducted on Shenhua coal. The measured CO2 concentration (28.2%) in the raw gas at the time of de-commissioning of the Sasol-Lurgi MK IV FBDB gasifier validates the modeled result of 30% CO2 estimated to be present in the raw gas exiting the gasifier at a temperature of 550 °C. At the end of the pyrolysis zone (725 °C), the model predicts that the gaseous carbon-containing species CO2, CH4, CO and COS exist at

concentrations of 39%, 17%, 15% and 0.2% respectively, while the solid carbon concentration is estimated to be less than 40%. These predicted results are in reasonable agreement with measured results reported by Anastai [6] which show the typical compositions of raw synthesis gas for the Sasol 1 complex (on a volume % basis) to be: CO2 (31.4%), CH4 (10.2%), and CO (17.1%).

<img class="figure large" border="0" alt="Full-size image (39K)" src="http://ars.els-cdn.com/content/image/1-s2.0-S0016236107005170-gr3.jpg" data-thumbsrc="http://ars.els-cdn.com/content/image/1-s2.0-S0016236107005170-gr3.sml" data-fullsrc="http://ars.els-cdn.com/content/image/1-s2.0-S0016236107005170-gr3.jpg">Fig. 3. Chart showing the equilibrium composition profile calculated for carbon in the drying, pyrolysis and reduction zones of the gasifier.

The carbon species (excludes solid carbon due to chemical equilibrium) within the reduction zone at a temperature of 825 °C (Fig. 3) and the CH4 content is reduced from 17% at 725 °C, to 3% at 925 °C, disappearing completely at 1125 °C. The CO2

content is reduced from 39% at 725 °C to 24% at 1316 °C due to the Boudouard reaction (see Eq. (1)), yielding an increase in CO. The CO content subsequently increases from 15% at 725 °C to 76% at the completion of gasification. Trace amounts of COS and H2CO (not shown on the graph due to scale) also co-exist in the gas phase together with CO and CO2 within the reduction zone of the gasifier.

In the combustion and ash-bed zone of the gasifier, the CO2

concentration increases to 97% and the CO concentration decreases to less than 3% within this oxygen-rich environment at a temperature of 640 °C. Upon further cooling, CH4 is predicted to occur due to the reaction of C and H2 from steam and CO2, increasing in concentration within the ash-bed to 11% at 340 °C. At this temperature, CO is shown to not be present and the CO2

concentration is predicted to be less than 90%.

The normalized residual hydrogen, nitrogen, sulphur and oxygen

profiles from the Sasol-Lurgi MK IV FBDB gasifier are graphically depicted in Fig. 4 and the species will be discussed separately.

<img class="figure large" border="0" alt="Full-size image (36K)" src="http://ars.els-cdn.com/content/image/1-s2.0-S0016236107005170-gr4.jpg" data-thumbsrc="http://ars.els-cdn.com/content/image/1-s2.0-S0016236107005170-gr4.sml" data-fullsrc="http://ars.els-cdn.com/content/image/1-s2.0-S0016236107005170-gr4.jpg">Fig. 4. Chart depicting the hydrogen, nitrogen, sulphur and oxygen distribution profiles on a normalized kg/100 kg ash basis for the Sasol-Lurgi MK IV FBDB gasifier.

4.2. Hydrogen (H) distribution profileFrom the data shown in Fig. 4 it can be seen that the Sasol-Lurgi MK IV FBDB gasifier normalized hydrogen content decreased from: about 9 kg/100 kg ash to almost zero from the drying to the combustion (ash-bed) zones, respectively. The hydrogen content in the feed coal was 10.3 kg/100 kg ash, with a 20% reduction occurring within the drying zone area of the gasifier to a level of 8.3 kg/100 kg ash at the start of the pyrolysis zone. This reduction in hydrogen can be ascribed to a loss of water and hydrogen-containing surface functional groups. A sharp release of hydrogen is observed within the pyrolysis zone of the gasifier, where 84% of the coal-bound hydrogen is volatilized from the resultant coal char.

This finding is in agreement with the work of Solomon et al. [14] who used the dry ash-free hydrogen content as a measure of the extent of pyrolysis (hydrogen content decreases with an increase in temperature). It was found that, for a range of coal types, that there was a decreasing reactivity in air with decreasing hydrogen content, and that most of the change occurred below 2.5% hydrogen, i.e. after the evolution of aliphatic hydrogen was complete. In the case of the results from the present study, the hydrogen content on an ash-free basis decreased from 4.2% in the feed coal to 1% at the end of pyrolysis. The coal char CO2

reactivity was also found to decrease with a decrease in (daf) hydrogen content up to 2 wt.% hydrogen, which confirms the lab-

scale results of Solomon et al. [14]. Thereafter, the coal char reactivity remained constant up to 1 wt.% hydrogen content in the “slow pyrolysis with gasification” region, within the pyrolysis zone of the SL MK IV FBDB gasifier. During reduction, the coal char CO2 reactivity was shown to increase significantly with a minimal decrease in hydrogen content, before pore collapse and diffusion limitations finally decrease the reactivity to a constant 0.75 wt.% hydrogen content within the combustion zone [15].

The hydrogen content further decreased in the ash-bed (Fig. 4), where after a further reduction occurs, relating to an overall 99% partitioning of hydrogen to the gas phase at the ash-grate position. As shown in Fig. 4, some of the hydrogen is associated with carbon in the form of CH4, particularly during cooling within the ash bed.

When coal is heated and volatiles are evolved, hydrogen-rich side chains are lost and the remaining char structure becomes more aromatic in nature [4]. This explains the shape of the C:H ratio versus temperature curve, i.e. as the coal tends towards a graphite-like structure, the amount of hydrogen in the coal decreases [4]. In Fig. 5 the Sasol-Lurgi MK IV FBDB gasifier measured trend results obtained for the carbon to hydrogen ratio on a mass basis is shown.

<img class="figure large" border="0" alt="Full-size image (31K)" src="http://ars.els-cdn.com/content/image/1-s2.0-S0016236107005170-gr5.jpg" data-thumbsrc="http://ars.els-cdn.com/content/image/1-s2.0-S0016236107005170-gr5.sml" data-fullsrc="http://ars.els-cdn.com/content/image/1-s2.0-S0016236107005170-gr5.jpg">Fig. 5. Chart showing the carbon:hydrogen ratio distribution profile for the Sasol-Lurgi MK IV FBDB gasifier expressed on a normalized kg/100 kg ash content basis.

From Fig. 5 it can be observed that the carbon to hydrogen ratio increased from 20:1 to reach a maximum of about 135:1 in the reduction zone, finally decreasing at the ash-bed region, with the carbon to hydrogen ratio relatively constant within the drying zone area of the gasifier. The inferred “aromaticity” shows some

scatter in the reduction and combustion zones, due to the carbon conversion reactions occurring within this highly reactive zone. This finding is supported by the experimental work of Morgan [4] who found a three-fold increase in the C:H ratio as function of increasing temperature using TGA pyrolysis experiments on high ash inertinite-rich bituminous coal. The pyrolysis conditions were conducted at a heating rate of 10 °C/min, soak time of 1 min, atmospheric pressure and nitrogen atmosphere. However, In contrast, it has been pointed out by Berkowitz [16] that the polyadamatane structural model for coal shows that high C:H ratio can also be consistent with aliphatic structure.

The Fact-Sage calculated hydrogen equilibrium compositional profiles for the drying, pyrolysis, and reduction zones of the gasifier are given in Fig. 6. It is apparent that crystal bonded water is the most significant hydrogen-containing species present at 125 °C, as well as minor quantities of H2S and steam (H2O gas). CH4 (31%) and steam (66%) are the major hydrogen-containing species formed up to 225 °C, together with small quantities of OH released by some of the minerals (not shown due to scale). H2 gas evolved from ca. 325 °C (1%), rising to 28% with an increase in temperature up to 725 °C within the pyrolysis zone together with 34% methane, 38% hydrogen in the form of steam, as well as 0.64% of the hydrogen related to H2S. These results are in some agreement with the raw gas composition results reported by Kristiansen [17] for a moving bed gasifier operating on Texas lignite, i.e. 50% H2O, 20% H2 and 0.3% H2S. From the reduction zone equilibrium profile for hydrogen, it is clear that the H2 and steam contents remained almost constant in the temperature regime of 925 °C to 1316 °C, i.e. 50% and 60% respectively, once methane and ammonia (not shown due to scale) are depleted.

<img class="figure large" border="0" alt="Full-size image (40K)" src="http://ars.els-cdn.com/content/image/1-s2.0-S0016236107005170-gr6.jpg" data-thumbsrc="http://ars.els-cdn.com/content/image/1-s2.0-S0016236107005170-gr6.sml" data-fullsrc="http://ars.els-cdn.com/content/image/1-s2.0-S0016236107005170-gr6.jpg">

Fig. 6. Chart depicting the equilibrium composition profile calculated for hydrogen in the drying, pyrolysis and reduction zones of the gasifier.

In the combustion and ash-bed zone of the gasifier, the steam concentration increases to 94% and the H2 concentration decreases to 63% within this oxygen-rich environment at a temperature of 540 °C. Upon further cooling, CH4 is predicted to occur, due to the reaction between C and hydrogen from steam and reduction of CO2, increasing in concentration within the ash-bed to 3% at 340 °C. At this temperature the H2, steam and H2S concentrations are predicted to be below 1%, 96% and 0.1% respectively together with trace quantities of NH3 (not shown due to scale).

4.3. Nitrogen (N) distribution profileFrom the data shown in Fig. 4 it can be seen that the Sasol-Lurgi MK IV FBDB gasifier normalized nitrogen content ranged from: 4.1 to 4.6, 1.8 to 3.5, 0.7 to 1.0 and 0.2 to 0.4 kg/100 kg ash content in the drying, pyrolysis, reduction and combustion (ash-bed) zones, respectively. The average residual nitrogen content is thus significantly different between the four reaction zones within the gasifier.

The nitrogen appears to be deeply imbedded within the coal structure, as only 5% of the nitrogen is evolved within the drying zone area of the gasifier. Within the “slow pyrolysis with gasification” region, the coal-bound nitrogen begins to de-volatilize from the rings during polymerization [2]. At the combustion zone position, 89% of the nitrogen has partitioned into the gas phase. The release of nitrogen, when compared to hydrogen, occurred at a slower rate and 10% remained in the char at the ash-bed position for the nitrogen species.

The equilibrium compositional profile for nitrogen in the drying, pyrolysis and reduction zones of the gasifier is given in Fig. 7. It can be seen that almost all of the nitrogen in the system speciated as N2, the balance in the form of NH3. It can also be observed that the nitrogen speciation behaviour is the inverse in the reduction zone of the reactor when compared to the drying and pyrolysis zones. Within the combustion and ash-bed zone of the gasifier, N2 is still the major nitrogen species present, declining in concentration slightly with a decrease in temperature together with an observed increase in the NH3 concentration near the ash-grate. The raw gas composition data reported by

Kristiansen [17] for an oxygen-blown moving bed gasifier operating on Texas lignite typically contains about 0.1% N2 and 0.3% as (NH3 + HCN) on a volume percentage basis.

<img class="figure large" border="0" alt="Full-size image (41K)" src="http://ars.els-cdn.com/content/image/1-s2.0-S0016236107005170-gr7.jpg" data-thumbsrc="http://ars.els-cdn.com/content/image/1-s2.0-S0016236107005170-gr7.sml" data-fullsrc="http://ars.els-cdn.com/content/image/1-s2.0-S0016236107005170-gr7.jpg">Fig. 7. Chart showing the equilibrium composition profile calculated for nitrogen in the drying, pyrolysis and reduction zones of the gasifier.

4.4. Total sulphur (S) distribution profileFrom the data shown in Fig. 4 it can be observed that the Sasol-Lurgi MK IV FBDB gasifier normalized total sulphur content ranged from about 3 kg/100 kg ash content to about 0.53 kg/100 kg ash content from the drying to the combustion (ash-bed) zone, showing that the average coal sulphur content decreases from the first to the fourth reaction zones within the gasifier.

Unfortunately the different forms of sulphur, i.e. pyritic, sulphatic, organically bound, etc. were not quantified in this study, so no deeper insight into their de-volatilization mechanics is possible. When observing the total sulphur profile given in Fig. 4, it is apparent that the feed coal contained 4 kg S/100 kg ash content. Based on knowledge of the Sasol coal supply (SCS) coal blends used for gasification in Secunda, 50% of the sulphur is in the form of pyrite and the other 50% sulphur is typically organically bound [18] and [19].

From Fig. 4 it can also be seen that approximately a third of the coal-bound sulphur is emitted within the drying zone area of the gasifier. At the onset of “slow pyrolysis with some gasification”, about half of the sulphur present in the feed coal has already volatilized. A steady reduction in sulphur is observed through the reduction zone and at the combustion zone the total sulphur content in the char is still 0.79 kg/100 kg ash content, relating to

an evolution of only 79% sulphur at this stage of the gasification process. At the ash-grate, 92% conversion of sulphur has occurred.

The equilibrium compositional profile for sulphur in the drying, pyrolysis and reduction zones of the gasifier (Fig. 8) reflects the sulphur distribution percentage within the temperature range of 25–1316 °C. For the purpose of the model, the sulphur was assumed to be inorganically bound in the form of pyrite at 25 °C, but other inorganic S associations with solid CaSO4(H2O)2

(16.76%) and BaSO4 (0.12%) are also shown to exist based on other S-based relationships used in the model input. Only 63% of the sulphur used in the model input was in the form of pyrite (FeS2) at a temperature of 125 °C, together with about 17% H2S, as well as trace associated mineral species related to base metal sulphides (not shown due to scale). At 325 °C, pyrite is transformed to FeS (41%), while the amount of H2S increases to 57%. At 825 °C and within the reduction zone of the gasifier, FeS decomposes, producing even more H2S (98%) as well as COS (1.7%).

<img class="figure large" border="0" alt="Full-size image (41K)" src="http://ars.els-cdn.com/content/image/1-s2.0-S0016236107005170-gr8.jpg" data-thumbsrc="http://ars.els-cdn.com/content/image/1-s2.0-S0016236107005170-gr8.sml" data-fullsrc="http://ars.els-cdn.com/content/image/1-s2.0-S0016236107005170-gr8.jpg">Fig. 8. Chart showing the equilibrium composition profile calculated for sulphur in the drying, pyrolysis and reduction zones of the gasifier.

The calculated transformational behaviour of pyrite under these reducing conditions that are present within the gasifier, thus support the Mossbauer Spectroscopic findings of Waanders and Govender [20], where iron and sulphur mineral associations in coal, as well as transformation during gasification were studied, revealing similar results although the pyrite transformation occurred at somewhat higher temperatures.

In the reduction zone (Fig. 8), H2S is still the dominant sulphur

species (>95%), but the concentration of COS increased to 2.5% at 1316 °C, and some SO, SO2 and SO3 gas were also produced due to some melting of trace quantities of MgS, CaS, FeS and MnS species that enter the slag, occurring at 1025 °C (not shown due to scale).

In the combustion and ash-bed zone of the gasifier, the H2S and COS concentrations decrease to 85% and 0.9% respectively at a temperature of 1225 °C, together with the formation of 13% SO2, 1% SO, 0.3% HS, 0.2% S2 and trace quantities of SO3 within this oxygen-rich environment of the gasifier Upon further cooling the SO2, SO, HS, S2 and SO3 concentrations decline and 99.8% H2S together with trace quantities of MgS, CaS, FeS and MnS species remain (not shown due to scale) are predicted to occur within the ash-bed of the gasifier at a temperature of 340 °C.

The Fact-Sage model predictions for sulphur obtained from this study are also in good agreement with the XRD findings of Skhonde et al. [19] who studied the various sulphur-bearing components as they were exposed to the conditions of the Sasol-Lurgi MK IV gasifier. It was found that pyrite in the coal structure undergoes various changes, being converted to pyrrhotite and then to various oxides of iron, with subsequent loss of sulphur to form H2S. A low proportion of the sulphur species, including the organically bound sulphur was encapsulated by a melt which was formed by the interaction between included kaolinite and included fluxing minerals (pyrite, calcite and dolomite) at elevated temperature and pressure.

4.5. Oxygen (O) distribution profileFrom Fig. 4 the normalized oxygen content changed from: 27 to 29, 5 to 18, 0.6 to 1.4 and 0.3 to 1.1 kg/100 kg ash content in the drying, pyrolysis, reduction and combustion (ash-bed) zones respectively, with a significant difference exhibited between the drying, pyrolysis and reduction zones of the gasifier, while the coal-bound oxygen content in the reduction and combustion zones are not significantly different from one another. It can also be seen that the feed coal had an oxygen content of 31 kg/100 kg ash content and at the start of pyrolysis, the oxygen content was slightly reduced to 27 kg/100 kg ash content, probably due to loss of oxygen from water and the surface functional groups in the upper drying zone of the gasifier. The reduction in the oxygen content of the heat- affected coal is very

dramatic within the pyrolysis zone, where ca. 90% of the coal-bound oxygen partitions into the raw gas and tar, consistent with laboratory results by Neoh and Gannon [21] who reported for a wide rank of coals under conditions of high heating rate and high temperature, that hydrogen and oxygen from coal are preferentially distributed in the volatile phase. The coal-derived oxygen was reduced to 1.9 kg/100 kg ash content at the end of the pyrolysis zone, and only minor oxygen retention is observed (0.3 kg oxygen/100 kg ash content) at the ash-grate.

The scatter in the oxygen trend data observed in the lower half of the gasifier may possibly be ascribed to oxy-sorption reactions occurring within the exothermic oxidation zone, resulting in mineral transformations, and will be further discussed in the Fact-Sage modeling discussion for oxygen.

The equilibrium compositional profile for oxygen in the drying pyrolysis and reduction zones of the gasifier is given in Fig. 9. From the graph it can be observed that surface and crystal moisture are the dominant oxygen species that existed at 125 °C, together with some CO2 evolution (9%). At a higher temperature (225 °C), water gas (53%), CO2 (36%) and OH decomposition from K, Mg and Ca silicates, as well as from koalinite, occurs (not shown due to scale). The major percentage gas phase oxygen species present at the end of pyrolysis (725 °C) are: CO2 (49%), steam (31%), CO (12%), COS (0.1%) and the balance (8%) of the oxygen species reflected in the mineral solid transformation processes occurring.

<img class="figure large" border="0" alt="Full-size image (42K)" src="http://ars.els-cdn.com/content/image/1-s2.0-S0016236107005170-gr9.jpg" data-thumbsrc="http://ars.els-cdn.com/content/image/1-s2.0-S0016236107005170-gr9.sml" data-fullsrc="http://ars.els-cdn.com/content/image/1-s2.0-S0016236107005170-gr9.jpg">Fig. 9. Chart depicting the equilibrium composition profile calculated for oxygen in the drying, pyrolysis and reduction zones of the gasifier.

The model-predicted oxygen speciation behaviour in the

reduction zone of the gasifier shows that the CO2 concentration decreased with increasing temperature, up to 1316 °C, while the CO and steam (oxygen contribution) content increased with increasing temperature. Some minerals containing oxygen, melted at 925 °C forming a slag (not shown due to scale). For normal gasifier operation excess steam is required in order to prevent excessive clinker formation [10].

From the data given in Fig. 10 the mineralogical behaviour with respect to oxygen distribution is shown for the drying, pyrolysis and reduction zones of the gasifier. At 25 °C the major oxygen-containing minerals present in the feed coal are solid quartz (SiO2), aluminium oxide monohydrate [Al2O3(H2O)], muscovite KAl3Si3O10(OH)2 hydroxyapatite (Ca5HO13P3) and dolomite [CaMg (CO3)2]. As the temperature increases to 225 °C, most of these species are no longer thermodynamically stable and calcite (CaCO3) appears as the major oxygen-containing mineral present together with quartz. At 325 °C, quartz, calcium aluminium silicate (CaAl2Si2O8) and cordierite (Mg2Al4Si5O18) are shown to be stable, increasing in concentration up to 2%, 3% and 2%, respectively at the completion of pyrolysis at 725 °C.

<img class="figure large" border="0" alt="Full-size image (51K)" src="http://ars.els-cdn.com/content/image/1-s2.0-S0016236107005170-gr10.jpg" data-thumbsrc="http://ars.els-cdn.com/content/image/1-s2.0-S0016236107005170-gr10.sml" data-fullsrc="http://ars.els-cdn.com/content/image/1-s2.0-S0016236107005170-

gr10.jpg">Fig. 10. Chart showing the equilibrium composition profile calculated for oxygen in the drying, pyrolysis and reduction zones of the gasifier, highlighting the mineral transformation behaviour and oxygen scavenging effect within a liquid slag.

Within the reduction zone it is clear that Fe2Al4Si5O18 (ferro-cordierite), CaAl2Si2O8 (feldspar), Mg2Al4Si5O18 (corderite) and quartz melt, forming a slag phase between 1125 °C and 1316 °C. From Mossbauer analyses, the Fe species that formed was identified as hematite and the slag included a Fe-containing glass [22]. The sum of oxygen contained in the liquid slag is 5%; with a further 2.5% related to mineral transformation bonding.

Mineralogical characterization of the Sasol-Lurgi MK IV gasifier samples is expected to provide valuable insight into the slagging behaviour of the minerals present, with the possible relationship to the ash fusion temperature (AFT) profile.

In the combustion and ash-bed zone of the gasifier, the oxygen distribution shows that the CO content decreases from 32% to 1% and the water gas (steam) concentration increases from 39% to 72% with a decrease in temperature from 1340 °C to 1240 °C. The CO content then decreases steadily with a decrease in temperature, disappearing completely at 440 °C while the CO2

and water gas (steam) concentrations remain constant throughout the cooling cycle; i.e. 20% and 72%, respectively. Model predictions also calculate that ca. 4% CaAl2Si2O8(s2) (anorthite) crystallizes from the melt together with 2% solid Mg2Al4Si5O18 (cordierite) and 2% mullite (not shown due to scale). These major oxygen-containing solid phases remain constant throughout the ash-bed of the gasifier.

5. ConclusionsThe Sasol-Lurgi MK IV FBDB gasifier ultimate analysis profiles provided good insight into understanding the development of aromaticity of the char, expressed by the carbon: hydrogen ratio on a mass percentage basis. Equilibrium compositional profiles calculated for C, H, N, S and O provided good discernment regarding the gas phase speciation and partitioning behaviour occurring within the fixed-bed-reactor. Fact-Sage thermodynamic equilibrium modeling of the Sasol-Lurgi FBDB gasifier related to the ultimate analysis results was found to be useful in identifying an oxygen scavenging effect created by the mineral

transformation behaviour occurring during reduction.

It was found that large oxygen-containing species such as Mg2Al4Si5O18 (corderite) and Fe2Al4Si5O18 (ferro-corderite) form within the reduction zone; the sum of oxygen scavenging related to mineral transformation and oxygen in the slag phase is ca. 7%. Oxygen is one, if not the most expensive input cost to the gasification process and 2.5% of the gasification input oxygen was found, in this study, to be consumed in the mineral transformation (non-slag) phase.

It would appear that mineral composition is a more fundamental property and more a function of mineral composition than merely ash content that is significant in the gasification process, when viewed on an oxygen consumption basis.

AcknowledgementsThis work was jointly sponsored by Sasol Technology R+D together with Sasol-Lurgi and the Sasol Synfuels Gas Production Division (Secunda). The author gratefully acknowledges their support to publish this work.