- Email: [email protected]
Low temperature entrained flow pyrolysis and gasification of a Victorian brown coal
Fuel 154 (2015) 107–113
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Low temperature entrained flow pyrolysis and gasification of a Victorian brown coal Joanne Tanner a, Kazi Bayzid Kabir a,1, Michael Müller b, Sankar Bhattacharya a,⇑ a b
Monash University, Department of Chemical Engineering, Wellington Rd, Clayton 3800, Australia Institute for Energy and Climate Research (IEK-2), Leo-Brandt-Str. 1, 52425 Jülich, Germany
h i g h l i g h t s The entrained flow pyrolysis and gasification of a Victorian brown coal was studied. Entrained flow processing of this fuel was proven to be technically viable. Pyrolysis up to 1000 °C resulted in high surface area char suitable for gasification. Tars and syngas contaminants decreased with increasing pyrolysis temperature. Char conversion and syngas yield increased with increasing gasification temperature.
a r t i c l e
i n f o
Article history: Received 26 November 2014 Received in revised form 25 February 2015 Accepted 25 March 2015 Available online 4 April 2015 Keywords: Brown coal Pyrolysis Entrained flow gasification
a b s t r a c t The entrained flow gasification of Victorian brown coals is of interest for its potential use as a syngas feedstock in established commercial processes. To determine the applicability of entrained flow gasification to a Victorian brown coal, and to establish the scope for further investigation, coal pyrolysis and char gasification trials were conducted using a vertical entrained flow reactor. Pyrolysis between 800 and 1000 °C resulted in high surface area char, and tar production decreased with increasing temperature. Char conversion and syngas yield increased with increasing gasification temperature. Pyrolysis tars and contaminants in the syngas were also evaluated to determine their potential impact on the industrial process. The concept of entrained flow gasification of Victorian brown coals was shown to be operationally sound, with minimal gas and liquid phase contaminants. However, the temperature and residence times chosen for this study were insufficient to achieve complete conversion of the fuel. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Entrained flow (EF) technologies are currently state of the art for coal-to-products processes, as they can achieve higher carbon conversion and a cleaner syngas product than the previously more prolific moving and fluidised bed technologies [1]. In an EF gasifier, pulverised coal is entrained in a stream of gas, and a series of pyrolysis, gasification and gas-phase reactions result in a syngas product composed predominantly of H2 and CO with fuel-dependent concentrations of CO2, CH4, H2O, and H2S. The syngas can be cleaned and utilised directly for power generation, as in the IGCC process [2], or further converted into liquid fuels or chemical products by various commercial processes [3]. Some by-products of the coal gasification process include tars and gas- and solid-phase ⇑ Corresponding author. Tel.: +61 3 9905 9623; fax: +61 3 9905 5686. E-mail address: [email protected] (S. Bhattacharya). Present address: Department of Chemical Engineering, BUET, Dhaka 1000, Bangladesh. 1
http://dx.doi.org/10.1016/j.fuel.2015.03.069 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.
inorganic species. Any evolved tar species represent a reduction in process efficiency in terms of overall conversion to syngas as carbon and hydrogen are bound in the tars and therefore unavailable for conversion to the desirable products – H2 and CO. The behaviour, efficiency and by-products of Victorian brown coals (VBC) treated under entrained flow gasification conditions is of interest to determine the potential to convert this abundant, low quality resource into high quality, value added products for domestic and international markets. Data for various gasification processes is available for similar European [4] and US coals [5]; however it is difficult to apply data obtained using these fuels to predict the behaviour of VBC due to the differences in coal properties such as coal rank and composition, which affect the complex suite of reactions which occur [6]. Additionally, while a considerable amount of work has been done assessing fluidised bed gasification of Victorian brown coals at both ambient and high pressure [7,8], data for process design and optimisation concerning the entrained flow gasification of these fuels is not available.
108
J. Tanner et al. / Fuel 154 (2015) 107–113
The purpose of this study was therefore to conduct a proof-ofconcept investigation to determine the potential applicability of entrained flow gasification to a Victorian brown coal. By undertaking low temperature (800–1000 °C) coal pyrolysis and char gasification trials using a drop tube furnace (DTF) operated under entrained flow conditions, a baseline and scope for further investigation has been established. Numerous studies have been performed for other coals using DTF apparatus as entrained flow reactors (EFR) to simulate various aspects of industrial gasification processes [6,9,10] as they can be operated with sufficiently short residence times under isothermal conditions. In order to gain a clear understanding of the potential effect of various parameters on the process operability and products from the entrained flow gasification of Victorian brown coal, and to provide a baseline for further investigations, the two main process steps – coal pyrolysis and subsequent char gasification – were studied separately, and the implications of the results with respect to the industrial process considered.
2. Materials and methods 2.1. Raw materials Morwell brown coal was used in this study. The sample was airdried, then pulverised and sieved to obtain particles within the size range 90–106 lm, representative of those commonly used in industrial EF gasifiers. The coal properties are presented in Table 1.
2.2. Coal pyrolysis and gasification Pyrolysis and gasification experiments were conducted using an electrically heated vertical entrained flow reactor (EFR), as described in [11], at furnace temperatures of 800 °C, 900 °C and 1000 °C. The samples were oven-dried at 60 °C for over 1 h (final moisture content 2–3%) prior to feeding to the reactor. The particles were entrained under a total of 5.0 L/min of gas with 10% of the flow being diverted to purge the vibratory feed system, and the balance delivered directly to the reactor. A schematic of the apparatus is presented in Fig. 1. To determine the gas flowrate required to achieve entrainment of the coal or char particles, the terminal velocity for a particle of the maximum size employed for a given experiment was compared with the total gas velocity inside the reactor under the relevant conditions. The gas flowrate was selected such that the gas velocity was equal to or greater than the terminal velocity of the maximum sized particle [12,13], therefore minimising the slip velocity and corresponding to the flow conditions in industrial gasifiers. Samples were further considered to be entrained for the entirety of each experiment based on the decrease in density and particle size which occurs as either pyrolysis or gasification proceed. The residence time for a particle of average size was 6 s. Table 1 Properties of prepared Morwell coal. Chemical analysis Moisture (% a.s.) Ash (% d.b.) C (% d.a.f.) H (% d.a.f.) N (% d.a.f.) S (% d.a.f.) O (by difference)
14.92 2.04 61.68 4.69 1.57 0.87 31.19
Physical analysis Surface area (m2/g)
196.66
a.s. = as sampled; d.b. = dry basis; d.a.f. = dry, ash free basis.
Fig. 1. Entrained flow reactor schematic.
Pyrolysis experiments were conducted under an atmosphere of 100% N2. The atmosphere was varied from 5 vol% to 20 vol% CO2 in N2 for the char-CO2 gasification experiments, all of which were performed at 1000 °C. The solid residue from each experiment was collected and the final char conversion determined by the ash tracer method by comparison of the char ash content before and after gasification experiments [4,14]. The char samples from pyrolysis experiments were stored under dry conditions at less than 4 °C to retard surface oxidation prior to gasification and further analysis. The solid residues from gasification experiments were also preserved in this manner. The surface area of the solid samples was determined by CO2 gas adsorption at 0 °C using a Micromeretics Accelerated Surface Area and Porosimetry (ASAP) analyser 2020 instrument. Tar species were collected after the reactor using a series of dry impingers submerged in ice-water at 0 °C. The traps were washed out with dichloromethane (DCM) and tar analysis was performed offline using a Perkin Elmer Clarus 600 Gas Chromatograph coupled with a Clarus 600S Mass Spectrometer (GC–MS). The gas evolved from the pyrolysis and gasification experiments was analysed online using a Varian 490-GC Micro-GC equipped with Molsieve-5A and PoraPlot Q columns and a thermal conductivity detector. The gas volume generated was determined by normalisation with the known reactant gas flowrate. 3. Results and discussion Pyrolysis and gasification of Morwell coal and char samples were carried out to validate the process of gasification of Victorian brown coals under entrained flow conditions at temperatures up to 1000 °C. Representative subsamples of the coal were
J. Tanner et al. / Fuel 154 (2015) 107–113
pyrolysed at 800 °C, 900 °C and 1000 °C to determine the effect of temperature on the composition and yield of tar and major fuel gas products from devolatilisation. The variations in physical properties of the resultant char were also analysed. Subsequent gasification of char prepared at 1000 °C was then carried out under varying concentrations of CO2 in N2 at 800 °C, 900 °C and 1000 °C to determine the effect of temperature and gasification reagent partial pressures on char conversion, evolution of the char structure, and composition and yield of gaseous products. 3.1. Effect of pyrolysis temperature on solid, tar and gaseous products Pyrolysis experiments were conducted using pulverised Morwell coal at 800 °C, 900 °C and 1000 °C. Solid char was collected and analysed offline, and the chemical composition is presented in Table 2.
Table 2 Properties of Morwell coal char generated under entrained flow pyrolysis conditions at 800 °C, 900 °C and 1000 °C. Reaction
Pyrolysis
Pyrolysis
Pyrolysis
Experimental conditions Atmosphere Temperature (°C) Conversion (%)
100% N2 800 52.67
100% N2 900 50.00
100% N2 1000 53.64
Chemical analysis Moisture (% a.s.) Ash (% d.b.) C (% d.a.f.) H (% d.a.f.) N (% d.a.f.) S (% d.a.f.) O (by difference)
– 4.31 86.06 2.75 1.64 0.46 9.09
– 4.08 86.00 1.89 0.95 0.20 10.96
– 4.40 92.18 0.85 0.21 0.00 6.76
Physical analysis Surface area (m2/g)
417.13
433.23
503.52
a.s. = as sampled; d.b. = dry basis; d.a.f. = dry, ash free basis.
109
Tar mass yields were calculated by difference and decreased from 0.15 g/g coal at 800 °C to 0.05 g/g coal at 1000 °C. The compounds dissolved in the DCM solution were analysed by GC–MS to determine major tar species present at the three pyrolysis temperatures. The scaled GC–MS spectra with the structures of various species of interest shown are presented in Fig. 2. The major identified compounds are presented in Table 3 as a ratio of the relative amount generated at 800 °C. The mass of tar was not measured directly due to the low quantities collected; therefore the results are presented as a ratio of the amount of tars evolved relative to those at 800 °C. The tar analysis represents the final products of both pyrolysis and subsequent gas phase reactions. The structure of a generalised coal molecule at successive stages of pyrolysis has been discussed by Solomon et al. [15]. The pyrolysis process is broadly split into primary and secondary pyrolysis stages where the division between the two is designated by the availability of donatable hydrogen in the coal structure. Primary pyrolysis involves the cleavage of aliphatic bridges and decomposition of functional groups within the coal structure, resulting in the production of permanent gases (CO, H2, CO2, CH4, and H2O) and hydrocarbon fragments. Crosslinking of larger hydrocarbon fragments also occurs during this phase of pyrolysis, as some fragments are recombined into the matrix via crosslinking reactions. Secondary pyrolysis proceeds once the available hydrogen has been consumed by crosslinking or tar-release reactions. Additional gas formation occurs with methane evolution from methyl groups, HCN from ring nitrogen compounds and CO from ether links. Finally, high temperature ring condensation occurs in the macromolecule, releasing H2 and establishing the final char structure [15]. Fig. 2 shows that pyrolysis of Morwell coal at 800 °C produced a range of tar compounds. The majority of the detected species are functionalised polyaromatic hydrocarbons (PAH). As pyrolysis temperature increases, there may be a corresponding increase in the rate of crosslinking reactions between hydrocarbon fragments generated by cleavage and decomposition reactions, thus the yield
Fig. 2. GC–MS spectra showing major components detected in tar samples collected from pyrolysis experiments at 800 °C, 900 °C and 1000 °C.
110
J. Tanner et al. / Fuel 154 (2015) 107–113
Table 3 Major components in tar evolved from pyrolysis experiments at 800 °C, 900 °C and 1000 °C. Results are presented as amount relative to components detected at 800 °C. Relative amounts generated at each pyrolysis temperature are not available due to the semi-quantitative nature of the measurements. Name
*
Formula
MW
Pyrolysis temperature 800 °C
900 °C
1000 °C
Aromatic compounds Benzene Toluene Phenylethyne Styrene Xylene Indene Naphthalene Methylnaphthalene Biphenylene Biphenyl Acenapthene Dimethylnaphthalene Fluorene Phenalene Diphenylmethane Phenanthrene Diphenylethyne Methylfluorene Methylphenanthrene Methylenephenanthrene Methylanthracene Pyrene Fluranthene Phenylnaphthalene Methylpyrene Benzofluorene Benzofluoranthene Triphenylene Chrysene Naphthacene Dihydronaphthacene Methyl Chrysene Benzopyrene
C6H6 C7H8 C8H6 C8H8 C8H10 C9H8 C10H8 C11H10 C12H8 C12H10 C12H10 C12H12 C13H10 C13H10 C13H12 C14H10 C14H10 C14H12 C15H12 C15H10 C15H12 C16H10 C16H10 C16H12 C17H12 C17H12 C18H10 C18H12 C18H12 C18H12 C18H14 C19H14 C20H12
78 92 102 104 106 116 128 142 152 154 154 156 166 166 168 178 178 180 180 190 192 202 202 204 216 216 226 228 228 228 230 242 252
Trace Trace 1 1 1 1 1 1 1 1 1 Trace 1 1 Trace 1 Trace Trace 1 1 Trace 1 1 1 1 1 1 Trace Trace 1 1 1 1
nd* nd nd nd nd Trace 0.02 nd 0.35 Trace Trace nd 0.12 Trace nd 0.37 nd Trace Trace 0.13 nd 0.31 0.23 Trace Trace 0.08 nd nd Trace 0.6 nd Trace Trace
nd nd nd nd nd nd 0.08 nd 0.31 Trace Trace nd nd Trace nd 0.21 nd nd Trace nd nd 0.4 nd Trace nd nd nd nd nd 0.56 nd Trace Trace
Oxygen containing species Phenol Cresol Benzofuran Methylbenzofuran Dibenzofuran Fluorenone Fluorenol Anthracenemethanol Hydroxypyrene
C6H6O C7H8O C8H8O C9H8O C12H8O C13H8O C13H10O C15H12O C16H10O
94 108 118 132 168 180 182 208 218
1 Trace 1 Trace 1 1 1 Trace 1
nd nd nd nd 0.16 0.12 nd nd Trace
nd nd nd nd nd nd nd nd nd
Nitrogen containing species Benzonitrile C7H5 N Indole C8H7 N Quinoline C9H7 N Carbazole C12H9 N Phenanthridine C13H9 N Nitropyrene C16H9NO2
103 117 129 167 179 247
1 1 Trace 1 Trace 1
nd nd Trace Trace nd nd
nd nd Trace nd nd nd
Sulfur containing species Benzothiofuran Dibenzothiophene
134 184
Trace Trace
Trace Trace
Trace Trace
C8H6S C12H8S
nd: not detected.
and range of tar species at 900 °C and 1000 °C decreased. The functionalised and heterocyclic polyaromatic species which were detected at 800 °C but markedly absent at 900 °C and 1000 °C may have decomposed in the gas phase, producing a small permanent gas molecule from the functional group and reverting to their base homocyclic molecules. They may also have been recombined into the solid matrix due to their relative instability in comparison to their remaining non-functionalised and homocyclic counterparts. A good example of this is the cluster of methyl-, dimethyl-, nitro- and hydroxy pyrenes detected at around 22–24 min in the
Fig. 3. Comparison of component and total syngas yields from pyrolysis of Morwell coal at 800 °C, 900 °C and 1000 °C.
800 °C spectrum in Fig. 2, the majority of which are not present in the 900 °C and 1000 °C spectra. The tar analysis is consistent with the online gas analysis (Fig. 3) and composition of the char samples (Table 2). Fig. 3 shows that the total yield of gaseous products increased with temperature as a result of increased devolatilisation and decomposition of tar products. The changes in yield of the evolved pyrolysis gas components with temperature correspond to increased decomposition in the coal structure and gas phase cracking at higher temperatures. As the pyrolysis temperature was increased, the volume of hydrogen evolved in the gas phase increased due to increased structural decomposition and solid- and gas-phase ring condensation at higher temperatures. This is also apparent from the results of the chemical analysis of the resultant chars where a corresponding decrease in hydrogen was observed. The yield of CO increased only slightly with increasing temperature, possibly due to the increased decomposition of oxygen-containing heterocyclic compounds, carbonyl groups and ether links [15]. The yields of CH4, C2H6 and CO2 showed a slight increasing trend with temperature as the solid structure was further decomposed, and the functionalised fragments reincorporated into the solid structure by the increased rate of crosslinking. Non-functionalised, predominantly homocyclic compounds were still detected at higher temperatures as they are relatively inert in comparison to fragments with reactive functional groups, hence less prone to reincorporation by crosslinking reactions. A generally decreasing trend in the relative abundance of larger homocyclic molecules in the higher temperature tars was nevertheless observed, possibly due to increased gas-phase cracking at higher temperature. Several of the polyaromatic compounds were detected in slightly higher relative amounts at 1000 °C than at 900 °C, namely naphthalene and pyrene. The contrary trend for these two species may also be attributed to gas-phase reactions, including increased cracking, decomposition of functionalised species, and combinatorial reactions between lighter hydrocarbons. In addition to the dominant PAH compounds, a number of heterocyclic compounds and species with O-, S- and N-containing functional groups were detected at 800 °C. Of the six nitrogencontaining species detected at 800 °C, only two were detected at 900 °C and one at 1000 °C. This decreasing tar-N trend is attributed to formation of gaseous species such as NH3 from functional groups and HCN during ring condensation [16]; however, these species could not be detected in the gas phase due to low sensitivity of the analysis instrument to these gases. The majority of the nine oxygen containing species detected in the tar collected from pyrolysis at 800 °C were phenolics, aromatic
J. Tanner et al. / Fuel 154 (2015) 107–113
alcohols and heterocyclic compounds. Of these, only three were detected in tar collected from pyrolysis at 900 °C while no oxygen containing species were detected in the 1000 °C pyrolysis tars. The majority of these were likely recombined via crosslinking reactions or decomposed in the solid or gas phase to their corresponding base homocyclic compounds at higher temperatures. Two sulphur containing species were detected in trace amount for tar samples collected at 800 °C, 900 °C and 1000 °C and their relative abundance is reflected in the char chemical analysis results. It was not possible to detect the corresponding sulphur-containing gas-phase decomposition products from these species with the gas analyser used. This part of the study reveals several important trends. Relatively few tar compounds are expected to be generated during entrained flow pyrolysis or gasification of Victorian brown coal above 1000 °C. Therefore, further increasing the temperature of the overall gasification process, of which pyrolysis is an integral part, should result in a decrease in the hydrocarbon contaminants in the raw syngas. Various aspects of the pyrolysis process will also assist in increasing the rate and efficiency of the char gasification process. The reincorporation of heavier tar fragments back into the coal matrix will render carbon which was removed from the system as tar species at lower temperature available to participate in gasification reactions, increasing the process efficiency for higher temperature gasification. Thermal decomposition, in particular the loss of oxygen containing functional groups, also increases particle porosity and total surface area (Table 2). Finally, the demonstrated increase in the char condensation process will influence reactivity of the char toward gasification or combustion, since edge carbon atoms are thought to be among the active sites [17]. 3.2. Entrained flow gasification Morwell char generated at 1000 °C was reacted at 1000 °C under 5%, 10% and 20% concentrations of CO2 by volume to determine the effect of reagent partial pressure, and at 800 °C and 900 °C under 20% CO2 to determine the effect of temperature on char conversion and resultant syngas concentration from the Boudouard reaction:
C ðsÞ þ CO2ðgÞ $ 2COðgÞ This reaction can be considered the most important in a gasification process as it is directly responsible for the conversion of the solid carbon in the char into the desirable gaseous product CO. The yield of CO and H2 from CO2 gasification under varying reactant concentrations at 1000 °C is presented in Fig. 4. Analogous experiments were carried out under 20% CO2 at 800 °C and 900 °C, and some conversion of the char was evident from the ash tracer analysis (Fig. 4). However, the syngas components of interest were below detectable levels in the entraining gas at these temperatures due to the low feed rate and high flowrate of carrier gas required for entrainment. At 1000 °C, increased reactant concentration resulted in an increase in the yield of CO from the Boudouard reaction. H2 yield also increased with increasing CO2 concentration. As H2 is not a direct product of the Boudouard reaction, its evolution is attributed to the liberation of residual hydrogen in the coal structure. As the carbon is oxidised to CO, the hydrogen atoms stabilising the coal structure are released as hydrogen gas, hence the H2 trend tracks that of CO evolution, but is two orders of magnitude lower as no additional hydrogen is added to the system. To achieve the highest efficiency in any gasifier, the conversion of char to desirable gaseous products should be maximised. The effects of temperature and variable CO2 concentration on char
111
Fig. 4. Variation in char conversion and syngas component yield from CO2 gasification under varying reactant concentrations and temperatures.
conversion are presented in Fig. 4. As CO2 concentration increased from 5 vol% to 20 vol% at 1000 °C, char conversion increased from 40% to 55%. Over the temperature range 800–1000 °C, at 20% CO2, conversion increased from 10% to 55%. This indicates that temperature has a much greater influence on char-CO2 gasification reactivity than reagent concentration over these ranges. The gasification results are supported by the chemical analysis, presented in Table 4, of the char before and after gasification under various conditions. The data has been normalised for ash content, and shows that gasification under increasing CO2 concentration at 1000 °C results in little variation in the composition of the remaining carbonaceous material. It is therefore only the conversion which appears to be affected by altering the reagent concentration. Gasification at 800 °C, 900 °C and 1000 °C under 20% CO2 also results in similar normalised composition of the char residue, indicating that the Boudouard reaction appears to proceed at increasing rates with increasing temperature, but has little impact on the remaining nitrogen and sulphur in the char. Entrained flow gasifiers operate with particle sizes in the range of hundreds of microns to maximise throughput and minimise the gas flowrate required for particle entrainment. To compensate for the consequently low residence times, they therefore require higher operating temperatures, typically 1200–1600 °C, than fixed and fluidised bed technologies which operate with larger particle sizes. From the gasification experiments reported here, it is clear that the most favourable operating conditions used in this study, being 1000 °C, 20% CO2 and a 6 s residence time, are insufficient for complete conversion of Morwell char. This investigation should therefore be extended to higher temperatures and longer residence times under laboratory conditions in order to establish the maximum possible char conversion and corresponding process parameters.
3.3. Evolution of particle structure during pyrolysis and gasification The surface area of the parent coal and char samples following treatment under various pyrolysis and gasification conditions was measured by CO2 adsorption at 0 °C and the results are presented in Table 4 and Fig. 5. It is well known that more open, porous char structures and higher surface area lead to higher gasification reactivity due to the greater accessibility of reactive gas molecules to active carbon sites in the char [18,19]. From an initial coal surface area of 196.66 m2/g, pyrolysis at increasing temperatures resulted in a 2- to 2.5-fold increase in surface area due to opening and fragmentation of the pore structure during devolatilisation.
112
J. Tanner et al. / Fuel 154 (2015) 107–113
Table 4 Properties of Morwell coal char produced at 1000 °C after entrained flow gasification at various CO2 concentrations and temperatures. Reaction
Gasification
Gasification
Gasification
Gasification
Gasification
Experimental conditions Atmosphere Temperature (°C) Conversion (%)
5% CO2/N2 1000 39.31
10% CO2/N2 1000 47.74
20% CO2/N2 1000 55.60
20% CO2/N2 900 21.15
20% CO2/N2 800 9.84
Chemical analysis Moisture (% a.s.) Ash (% d.b.) C (% d.a.f.) H (% d.a.f.) N (% d.a.f.) S (% d.a.f.) O (by difference)
– 7.25 88.05 1.18 0.93 0.25 9.59
– 8.42 91.17 1.15 0.90 0.21 6.58
– 9.91 88.06 1.05 1.04 0.85 9.00
– 5.58 90.88 1.06 0.80 0.16 7.10
– 4.88 91.37 1.07 0.78 0.15 6.62
Physical analysis Surface area (m2/g)
580.30
620.82
620.19
548.79
529.36
a.s. = as sampled; d.b. = dry basis; d.a.f. = dry, ash free basis.
of steam lead to even greater expansion of the char structure, and a maximum char conversion of approximately 80% was achieved. As for the CO2 gasification chars, the increase in surface area appears to be independent of gasification temperature. The increase in char surface area upon gasification under steam and CO2 conditions is attributed to the fracturing and removal of solid carbon from the structure. As gasification proceeds by either of the two heterogeneous char gasification reactions, the remaining solid is depleted. The steam gasification reaction results in higher conversion and higher surface area of the remaining char due to the smaller size of the water molecule, enabling it to enter smaller pores in the char particles and access active carbon sites which are only available to CO2 once the surrounding material has been consumed. Furthermore, the rate of gasification reactivity of char to steam in comparison to CO2 is known to be higher by a factor of approximately 3 [23], which may contribute to the higher surface area of the steam-gasified char. Fig. 5. Variation in solid surface area between (a) raw Morwell coal, (b) char from pyrolysis at 800–1000 °C and (c) char from gasification under 5–20% CO2 at 800– 1000 °C.
Additionally, the experiments yielded chars with surface areas comparable to chars from Victorian brown coals produced under fixed bed and fluidised bed conditions [20,21]. Therefore, it appears that entrained flow treatment of this coal is effective at creating chars with high surface areas for gasification. Upon gasification of the 1000 °C char at 800 °C, 900 °C and 1000 °C, the CO2 surface area increased by a further 20%, and the results for the three residual samples were slightly increasing with increasing temperature. This supports the results of Feng and Bhatia [22] who showed that the development of the pore structure during gasification was similar for heat treated subbituminous coal chars at different temperatures. The char conversion ranged from approximately 40% at 800 °C to 55% at 1000 °C by mass. It therefore appears to be the pyrolysis conditions which have the greatest influence on char gasification reactivity in terms of surface area. Furthermore, if the slight increase in surface area following gasification is not dependent on temperature, it is only the increase in reaction rate at higher gasification temperatures which leads to the increase in char conversion. Steam gasification of the 1000 °C char was also carried out at varying concentrations. Unfortunately, it was not possible to accurately set the steam concentration to which the char was exposed due to equipment limitations; however gasification of the char was evident none the less. The results of the surface area analysis of the three steam gasification chars shown in Fig. 5 indicate that the use
4. Conclusions Entrained flow pyrolysis of a Victorian brown coal and gasification of the resultant char were investigated at various temperatures and concentrations of the gasifying reagents CO2 and steam. From the results of this study, the following conclusions may be drawn: 1. Few tar compounds, which can be detrimental to the operability and efficiency of the entrained flow gasification process, are expected to be generated during entrained flow pyrolysis or gasification of Victorian brown coal above 1000 °C due to decomposition of functionalised groups to permanent gases and subsequent crosslinking of the majority of remaining hydrocarbon fragments prior to volatilisation and release. 2. As gasification temperature increases, the yield of hydrocarbon contaminants in the product syngas will be reduced, increasing conversion efficiency and resulting in a cleaner syngas product requiring less conditioning prior to subsequent use. 3. Entrained flow pyrolysis process is successful in generating high surface area, reactive char particles from this coal for subsequent gasification. 4. The most favourable reaction conditions used in the reported gasification experiments, 1000 °C, 20% CO2 and a 6 s residence time, are insufficient for complete conversion of Morwell char. Based on the results of this proof-of-concept study, it is therefore surmised that entrained flow gasification of Victorian brown coals may represent a feasible, practical route to syngas generation
J. Tanner et al. / Fuel 154 (2015) 107–113
from these fuels. However, the temperatures (up to 1000 °C) and residence times (up to 6 s) employed in this study were insufficient to achieve the high conversions which are typical of an industrial entrained flow gasification system. Therefore, further investigation should be undertaken at higher temperatures and residence times. Acknowledgements The authors would like to acknowledge Brown Coal Innovation Australia (BCIA), the Australian China Strategic Research Fund (ACSRF), Go8-DAAD funding scheme and the Australian Research Council’s Linkage Infrastructure, Equipment and Facilities (LIEF 120100141) for providing the funding for this study. References [1] Higman C, van der Burgt M. The kinetics of gasification and reactor theory. In: Gasification. Burlington: Gulf Professional Publishing; 2008 [Chapter 3]. [2] Weil KS. Coal gasification and IGCC technology: a brief primer. Proc ICE Energy 2010;163:7–16. [3] Higman C, Tam S. Advances in coal gasification, hydrogenation, and gas treating for the production of chemicals and fuels. Chem Rev 2013;114:1673–708. [4] Tremel A, Haselsteiner T, Kunze C, Spliethoff H. Experimental investigation of high temperature and high pressure coal gasification. Appl Energy 2012;92:279–85. [5] Brown BW. Effect of coal type on entrained flow gasification, in PhD Thesis 1985, Brigham Young University. [6] Lee JG, Kim JH, Lee HJ, Park TJ, Kim SD. Characteristics of entrained flow coal gasification in a drop tube reactor. Fuel 1996;75:1035–42. [7] Bhattacharya SP. Gasification performance of Australian lignites in a pressurized fluidized bed gasifier process development unit under air and oxygen-enriched air blown conditions. Process Saf Environ Prot 2006;84:453–60.
113
[8] Johnson T. A competitive gasification technology for brown coal. In: Proceddings of the EPRI-VGB conference on lignites and low-rank coals. Weisbaden, Germany; 2001. [9] Kajitani S, Suzuki N, Ashizawa M, Hara S. Co2 gasification rate analysis of coal char in entrained flow coal gasifier. Fuel 2006;85:163–9. [10] Kajitani S, Hara S, Matsuda H. Gasification rate analysis of coal char with a pressurized drop tube furnace. Fuel 2002;81:539–46. [11] Zhang L, Binner E, Qiao Y, Li C-Z. High-speed camera observation of coal combustion in air and O2/CO2 mixtures and measurement of burning coal particle velocity. Energy Fuels 2009;24:29–37. [12] Flagan RC, Seinfeld JH. Fundamentals of air pollution engineering. Englewood Cliffs. New Jersey: Prentice Hall; 1988. [13] Lehto J. Development and characterization of test reactor with results of its application to pyrolysis kinetics of peat and biomass fuels. PhD Thesis, Tampere University of Technology; 2007. [14] Marney D. Investigation of the pyrolysis of two low-rank coals at elevated pressures. PhD Thesis, Swinburne University of Technology; 2002. [15] Solomon PR, Hamblen DG, Carangelo RM, Serio MA, Deshpande GV. General model of coal devolatilization. Energy Fuels 1988;2:405–22. [16] Hayashi Ji, Takahashi H, Iwatsuki M, Essaki K, Tsutsumi A, Chiba T. Rapid conversion of tar and char from pyrolysis of a brown coal by reactions with steam in a drop-tube reactor. Fuel 2000;79:439–47. [17] Laurendeau NM. Heterogeneous kinetics of coal char gasification and combustion. Prog Energy Combust Sci 1978;4:221–70. [18] Molina A, Mondragón F. Reactivity of coal gasification with steam and co2. Fuel 1998;77:1831–9. [19] Miura K, Mae K, Murata A, Sato A, Sakurada K, Hashimoto K. Flash pyrolysis of coal in solvent vapor for controlling product distribution. Energy Fuels 1992;6:179–84. [20] Harris DJ, Smith IW. Intrinsic reactivity of petroleum coke and brown coal char to carbon dioxide, steam and oxygen. Symp (Int) Combust 1991;23:1185–90. [21] Cameron A, Stacy WO. The pore structure of chars and cokes. Aust J Appl Sci 1959;10:449. [22] Feng B, Bhatia SK. Variation of the pore structure of coal chars during gasification. Carbon 2003;41:507–23. [23] Walker Jr PL, Rusinko Jr F, Austin LG. Gas reactions of carbon. In: Eley DD, Selwood PW, Weisz PB, editors. Advances in catalysts, xi. New York: Academic Press; 1959.
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Low temperature entrained flow pyrolysis and gasification of a Victorian brown coal Joanne Tanner a, Kazi Bayzid Kabir a,1, Michael Müller b, Sankar Bhattacharya a,⇑ a b
Monash University, Department of Chemical Engineering, Wellington Rd, Clayton 3800, Australia Institute for Energy and Climate Research (IEK-2), Leo-Brandt-Str. 1, 52425 Jülich, Germany
h i g h l i g h t s The entrained flow pyrolysis and gasification of a Victorian brown coal was studied. Entrained flow processing of this fuel was proven to be technically viable. Pyrolysis up to 1000 °C resulted in high surface area char suitable for gasification. Tars and syngas contaminants decreased with increasing pyrolysis temperature. Char conversion and syngas yield increased with increasing gasification temperature.
a r t i c l e
i n f o
Article history: Received 26 November 2014 Received in revised form 25 February 2015 Accepted 25 March 2015 Available online 4 April 2015 Keywords: Brown coal Pyrolysis Entrained flow gasification
a b s t r a c t The entrained flow gasification of Victorian brown coals is of interest for its potential use as a syngas feedstock in established commercial processes. To determine the applicability of entrained flow gasification to a Victorian brown coal, and to establish the scope for further investigation, coal pyrolysis and char gasification trials were conducted using a vertical entrained flow reactor. Pyrolysis between 800 and 1000 °C resulted in high surface area char, and tar production decreased with increasing temperature. Char conversion and syngas yield increased with increasing gasification temperature. Pyrolysis tars and contaminants in the syngas were also evaluated to determine their potential impact on the industrial process. The concept of entrained flow gasification of Victorian brown coals was shown to be operationally sound, with minimal gas and liquid phase contaminants. However, the temperature and residence times chosen for this study were insufficient to achieve complete conversion of the fuel. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Entrained flow (EF) technologies are currently state of the art for coal-to-products processes, as they can achieve higher carbon conversion and a cleaner syngas product than the previously more prolific moving and fluidised bed technologies [1]. In an EF gasifier, pulverised coal is entrained in a stream of gas, and a series of pyrolysis, gasification and gas-phase reactions result in a syngas product composed predominantly of H2 and CO with fuel-dependent concentrations of CO2, CH4, H2O, and H2S. The syngas can be cleaned and utilised directly for power generation, as in the IGCC process [2], or further converted into liquid fuels or chemical products by various commercial processes [3]. Some by-products of the coal gasification process include tars and gas- and solid-phase ⇑ Corresponding author. Tel.: +61 3 9905 9623; fax: +61 3 9905 5686. E-mail address: [email protected] (S. Bhattacharya). Present address: Department of Chemical Engineering, BUET, Dhaka 1000, Bangladesh. 1
http://dx.doi.org/10.1016/j.fuel.2015.03.069 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.
inorganic species. Any evolved tar species represent a reduction in process efficiency in terms of overall conversion to syngas as carbon and hydrogen are bound in the tars and therefore unavailable for conversion to the desirable products – H2 and CO. The behaviour, efficiency and by-products of Victorian brown coals (VBC) treated under entrained flow gasification conditions is of interest to determine the potential to convert this abundant, low quality resource into high quality, value added products for domestic and international markets. Data for various gasification processes is available for similar European [4] and US coals [5]; however it is difficult to apply data obtained using these fuels to predict the behaviour of VBC due to the differences in coal properties such as coal rank and composition, which affect the complex suite of reactions which occur [6]. Additionally, while a considerable amount of work has been done assessing fluidised bed gasification of Victorian brown coals at both ambient and high pressure [7,8], data for process design and optimisation concerning the entrained flow gasification of these fuels is not available.
108
J. Tanner et al. / Fuel 154 (2015) 107–113
The purpose of this study was therefore to conduct a proof-ofconcept investigation to determine the potential applicability of entrained flow gasification to a Victorian brown coal. By undertaking low temperature (800–1000 °C) coal pyrolysis and char gasification trials using a drop tube furnace (DTF) operated under entrained flow conditions, a baseline and scope for further investigation has been established. Numerous studies have been performed for other coals using DTF apparatus as entrained flow reactors (EFR) to simulate various aspects of industrial gasification processes [6,9,10] as they can be operated with sufficiently short residence times under isothermal conditions. In order to gain a clear understanding of the potential effect of various parameters on the process operability and products from the entrained flow gasification of Victorian brown coal, and to provide a baseline for further investigations, the two main process steps – coal pyrolysis and subsequent char gasification – were studied separately, and the implications of the results with respect to the industrial process considered.
2. Materials and methods 2.1. Raw materials Morwell brown coal was used in this study. The sample was airdried, then pulverised and sieved to obtain particles within the size range 90–106 lm, representative of those commonly used in industrial EF gasifiers. The coal properties are presented in Table 1.
2.2. Coal pyrolysis and gasification Pyrolysis and gasification experiments were conducted using an electrically heated vertical entrained flow reactor (EFR), as described in [11], at furnace temperatures of 800 °C, 900 °C and 1000 °C. The samples were oven-dried at 60 °C for over 1 h (final moisture content 2–3%) prior to feeding to the reactor. The particles were entrained under a total of 5.0 L/min of gas with 10% of the flow being diverted to purge the vibratory feed system, and the balance delivered directly to the reactor. A schematic of the apparatus is presented in Fig. 1. To determine the gas flowrate required to achieve entrainment of the coal or char particles, the terminal velocity for a particle of the maximum size employed for a given experiment was compared with the total gas velocity inside the reactor under the relevant conditions. The gas flowrate was selected such that the gas velocity was equal to or greater than the terminal velocity of the maximum sized particle [12,13], therefore minimising the slip velocity and corresponding to the flow conditions in industrial gasifiers. Samples were further considered to be entrained for the entirety of each experiment based on the decrease in density and particle size which occurs as either pyrolysis or gasification proceed. The residence time for a particle of average size was 6 s. Table 1 Properties of prepared Morwell coal. Chemical analysis Moisture (% a.s.) Ash (% d.b.) C (% d.a.f.) H (% d.a.f.) N (% d.a.f.) S (% d.a.f.) O (by difference)
14.92 2.04 61.68 4.69 1.57 0.87 31.19
Physical analysis Surface area (m2/g)
196.66
a.s. = as sampled; d.b. = dry basis; d.a.f. = dry, ash free basis.
Fig. 1. Entrained flow reactor schematic.
Pyrolysis experiments were conducted under an atmosphere of 100% N2. The atmosphere was varied from 5 vol% to 20 vol% CO2 in N2 for the char-CO2 gasification experiments, all of which were performed at 1000 °C. The solid residue from each experiment was collected and the final char conversion determined by the ash tracer method by comparison of the char ash content before and after gasification experiments [4,14]. The char samples from pyrolysis experiments were stored under dry conditions at less than 4 °C to retard surface oxidation prior to gasification and further analysis. The solid residues from gasification experiments were also preserved in this manner. The surface area of the solid samples was determined by CO2 gas adsorption at 0 °C using a Micromeretics Accelerated Surface Area and Porosimetry (ASAP) analyser 2020 instrument. Tar species were collected after the reactor using a series of dry impingers submerged in ice-water at 0 °C. The traps were washed out with dichloromethane (DCM) and tar analysis was performed offline using a Perkin Elmer Clarus 600 Gas Chromatograph coupled with a Clarus 600S Mass Spectrometer (GC–MS). The gas evolved from the pyrolysis and gasification experiments was analysed online using a Varian 490-GC Micro-GC equipped with Molsieve-5A and PoraPlot Q columns and a thermal conductivity detector. The gas volume generated was determined by normalisation with the known reactant gas flowrate. 3. Results and discussion Pyrolysis and gasification of Morwell coal and char samples were carried out to validate the process of gasification of Victorian brown coals under entrained flow conditions at temperatures up to 1000 °C. Representative subsamples of the coal were
J. Tanner et al. / Fuel 154 (2015) 107–113
pyrolysed at 800 °C, 900 °C and 1000 °C to determine the effect of temperature on the composition and yield of tar and major fuel gas products from devolatilisation. The variations in physical properties of the resultant char were also analysed. Subsequent gasification of char prepared at 1000 °C was then carried out under varying concentrations of CO2 in N2 at 800 °C, 900 °C and 1000 °C to determine the effect of temperature and gasification reagent partial pressures on char conversion, evolution of the char structure, and composition and yield of gaseous products. 3.1. Effect of pyrolysis temperature on solid, tar and gaseous products Pyrolysis experiments were conducted using pulverised Morwell coal at 800 °C, 900 °C and 1000 °C. Solid char was collected and analysed offline, and the chemical composition is presented in Table 2.
Table 2 Properties of Morwell coal char generated under entrained flow pyrolysis conditions at 800 °C, 900 °C and 1000 °C. Reaction
Pyrolysis
Pyrolysis
Pyrolysis
Experimental conditions Atmosphere Temperature (°C) Conversion (%)
100% N2 800 52.67
100% N2 900 50.00
100% N2 1000 53.64
Chemical analysis Moisture (% a.s.) Ash (% d.b.) C (% d.a.f.) H (% d.a.f.) N (% d.a.f.) S (% d.a.f.) O (by difference)
– 4.31 86.06 2.75 1.64 0.46 9.09
– 4.08 86.00 1.89 0.95 0.20 10.96
– 4.40 92.18 0.85 0.21 0.00 6.76
Physical analysis Surface area (m2/g)
417.13
433.23
503.52
a.s. = as sampled; d.b. = dry basis; d.a.f. = dry, ash free basis.
109
Tar mass yields were calculated by difference and decreased from 0.15 g/g coal at 800 °C to 0.05 g/g coal at 1000 °C. The compounds dissolved in the DCM solution were analysed by GC–MS to determine major tar species present at the three pyrolysis temperatures. The scaled GC–MS spectra with the structures of various species of interest shown are presented in Fig. 2. The major identified compounds are presented in Table 3 as a ratio of the relative amount generated at 800 °C. The mass of tar was not measured directly due to the low quantities collected; therefore the results are presented as a ratio of the amount of tars evolved relative to those at 800 °C. The tar analysis represents the final products of both pyrolysis and subsequent gas phase reactions. The structure of a generalised coal molecule at successive stages of pyrolysis has been discussed by Solomon et al. [15]. The pyrolysis process is broadly split into primary and secondary pyrolysis stages where the division between the two is designated by the availability of donatable hydrogen in the coal structure. Primary pyrolysis involves the cleavage of aliphatic bridges and decomposition of functional groups within the coal structure, resulting in the production of permanent gases (CO, H2, CO2, CH4, and H2O) and hydrocarbon fragments. Crosslinking of larger hydrocarbon fragments also occurs during this phase of pyrolysis, as some fragments are recombined into the matrix via crosslinking reactions. Secondary pyrolysis proceeds once the available hydrogen has been consumed by crosslinking or tar-release reactions. Additional gas formation occurs with methane evolution from methyl groups, HCN from ring nitrogen compounds and CO from ether links. Finally, high temperature ring condensation occurs in the macromolecule, releasing H2 and establishing the final char structure [15]. Fig. 2 shows that pyrolysis of Morwell coal at 800 °C produced a range of tar compounds. The majority of the detected species are functionalised polyaromatic hydrocarbons (PAH). As pyrolysis temperature increases, there may be a corresponding increase in the rate of crosslinking reactions between hydrocarbon fragments generated by cleavage and decomposition reactions, thus the yield
Fig. 2. GC–MS spectra showing major components detected in tar samples collected from pyrolysis experiments at 800 °C, 900 °C and 1000 °C.
110
J. Tanner et al. / Fuel 154 (2015) 107–113
Table 3 Major components in tar evolved from pyrolysis experiments at 800 °C, 900 °C and 1000 °C. Results are presented as amount relative to components detected at 800 °C. Relative amounts generated at each pyrolysis temperature are not available due to the semi-quantitative nature of the measurements. Name
*
Formula
MW
Pyrolysis temperature 800 °C
900 °C
1000 °C
Aromatic compounds Benzene Toluene Phenylethyne Styrene Xylene Indene Naphthalene Methylnaphthalene Biphenylene Biphenyl Acenapthene Dimethylnaphthalene Fluorene Phenalene Diphenylmethane Phenanthrene Diphenylethyne Methylfluorene Methylphenanthrene Methylenephenanthrene Methylanthracene Pyrene Fluranthene Phenylnaphthalene Methylpyrene Benzofluorene Benzofluoranthene Triphenylene Chrysene Naphthacene Dihydronaphthacene Methyl Chrysene Benzopyrene
C6H6 C7H8 C8H6 C8H8 C8H10 C9H8 C10H8 C11H10 C12H8 C12H10 C12H10 C12H12 C13H10 C13H10 C13H12 C14H10 C14H10 C14H12 C15H12 C15H10 C15H12 C16H10 C16H10 C16H12 C17H12 C17H12 C18H10 C18H12 C18H12 C18H12 C18H14 C19H14 C20H12
78 92 102 104 106 116 128 142 152 154 154 156 166 166 168 178 178 180 180 190 192 202 202 204 216 216 226 228 228 228 230 242 252
Trace Trace 1 1 1 1 1 1 1 1 1 Trace 1 1 Trace 1 Trace Trace 1 1 Trace 1 1 1 1 1 1 Trace Trace 1 1 1 1
nd* nd nd nd nd Trace 0.02 nd 0.35 Trace Trace nd 0.12 Trace nd 0.37 nd Trace Trace 0.13 nd 0.31 0.23 Trace Trace 0.08 nd nd Trace 0.6 nd Trace Trace
nd nd nd nd nd nd 0.08 nd 0.31 Trace Trace nd nd Trace nd 0.21 nd nd Trace nd nd 0.4 nd Trace nd nd nd nd nd 0.56 nd Trace Trace
Oxygen containing species Phenol Cresol Benzofuran Methylbenzofuran Dibenzofuran Fluorenone Fluorenol Anthracenemethanol Hydroxypyrene
C6H6O C7H8O C8H8O C9H8O C12H8O C13H8O C13H10O C15H12O C16H10O
94 108 118 132 168 180 182 208 218
1 Trace 1 Trace 1 1 1 Trace 1
nd nd nd nd 0.16 0.12 nd nd Trace
nd nd nd nd nd nd nd nd nd
Nitrogen containing species Benzonitrile C7H5 N Indole C8H7 N Quinoline C9H7 N Carbazole C12H9 N Phenanthridine C13H9 N Nitropyrene C16H9NO2
103 117 129 167 179 247
1 1 Trace 1 Trace 1
nd nd Trace Trace nd nd
nd nd Trace nd nd nd
Sulfur containing species Benzothiofuran Dibenzothiophene
134 184
Trace Trace
Trace Trace
Trace Trace
C8H6S C12H8S
nd: not detected.
and range of tar species at 900 °C and 1000 °C decreased. The functionalised and heterocyclic polyaromatic species which were detected at 800 °C but markedly absent at 900 °C and 1000 °C may have decomposed in the gas phase, producing a small permanent gas molecule from the functional group and reverting to their base homocyclic molecules. They may also have been recombined into the solid matrix due to their relative instability in comparison to their remaining non-functionalised and homocyclic counterparts. A good example of this is the cluster of methyl-, dimethyl-, nitro- and hydroxy pyrenes detected at around 22–24 min in the
Fig. 3. Comparison of component and total syngas yields from pyrolysis of Morwell coal at 800 °C, 900 °C and 1000 °C.
800 °C spectrum in Fig. 2, the majority of which are not present in the 900 °C and 1000 °C spectra. The tar analysis is consistent with the online gas analysis (Fig. 3) and composition of the char samples (Table 2). Fig. 3 shows that the total yield of gaseous products increased with temperature as a result of increased devolatilisation and decomposition of tar products. The changes in yield of the evolved pyrolysis gas components with temperature correspond to increased decomposition in the coal structure and gas phase cracking at higher temperatures. As the pyrolysis temperature was increased, the volume of hydrogen evolved in the gas phase increased due to increased structural decomposition and solid- and gas-phase ring condensation at higher temperatures. This is also apparent from the results of the chemical analysis of the resultant chars where a corresponding decrease in hydrogen was observed. The yield of CO increased only slightly with increasing temperature, possibly due to the increased decomposition of oxygen-containing heterocyclic compounds, carbonyl groups and ether links [15]. The yields of CH4, C2H6 and CO2 showed a slight increasing trend with temperature as the solid structure was further decomposed, and the functionalised fragments reincorporated into the solid structure by the increased rate of crosslinking. Non-functionalised, predominantly homocyclic compounds were still detected at higher temperatures as they are relatively inert in comparison to fragments with reactive functional groups, hence less prone to reincorporation by crosslinking reactions. A generally decreasing trend in the relative abundance of larger homocyclic molecules in the higher temperature tars was nevertheless observed, possibly due to increased gas-phase cracking at higher temperature. Several of the polyaromatic compounds were detected in slightly higher relative amounts at 1000 °C than at 900 °C, namely naphthalene and pyrene. The contrary trend for these two species may also be attributed to gas-phase reactions, including increased cracking, decomposition of functionalised species, and combinatorial reactions between lighter hydrocarbons. In addition to the dominant PAH compounds, a number of heterocyclic compounds and species with O-, S- and N-containing functional groups were detected at 800 °C. Of the six nitrogencontaining species detected at 800 °C, only two were detected at 900 °C and one at 1000 °C. This decreasing tar-N trend is attributed to formation of gaseous species such as NH3 from functional groups and HCN during ring condensation [16]; however, these species could not be detected in the gas phase due to low sensitivity of the analysis instrument to these gases. The majority of the nine oxygen containing species detected in the tar collected from pyrolysis at 800 °C were phenolics, aromatic
J. Tanner et al. / Fuel 154 (2015) 107–113
alcohols and heterocyclic compounds. Of these, only three were detected in tar collected from pyrolysis at 900 °C while no oxygen containing species were detected in the 1000 °C pyrolysis tars. The majority of these were likely recombined via crosslinking reactions or decomposed in the solid or gas phase to their corresponding base homocyclic compounds at higher temperatures. Two sulphur containing species were detected in trace amount for tar samples collected at 800 °C, 900 °C and 1000 °C and their relative abundance is reflected in the char chemical analysis results. It was not possible to detect the corresponding sulphur-containing gas-phase decomposition products from these species with the gas analyser used. This part of the study reveals several important trends. Relatively few tar compounds are expected to be generated during entrained flow pyrolysis or gasification of Victorian brown coal above 1000 °C. Therefore, further increasing the temperature of the overall gasification process, of which pyrolysis is an integral part, should result in a decrease in the hydrocarbon contaminants in the raw syngas. Various aspects of the pyrolysis process will also assist in increasing the rate and efficiency of the char gasification process. The reincorporation of heavier tar fragments back into the coal matrix will render carbon which was removed from the system as tar species at lower temperature available to participate in gasification reactions, increasing the process efficiency for higher temperature gasification. Thermal decomposition, in particular the loss of oxygen containing functional groups, also increases particle porosity and total surface area (Table 2). Finally, the demonstrated increase in the char condensation process will influence reactivity of the char toward gasification or combustion, since edge carbon atoms are thought to be among the active sites [17]. 3.2. Entrained flow gasification Morwell char generated at 1000 °C was reacted at 1000 °C under 5%, 10% and 20% concentrations of CO2 by volume to determine the effect of reagent partial pressure, and at 800 °C and 900 °C under 20% CO2 to determine the effect of temperature on char conversion and resultant syngas concentration from the Boudouard reaction:
C ðsÞ þ CO2ðgÞ $ 2COðgÞ This reaction can be considered the most important in a gasification process as it is directly responsible for the conversion of the solid carbon in the char into the desirable gaseous product CO. The yield of CO and H2 from CO2 gasification under varying reactant concentrations at 1000 °C is presented in Fig. 4. Analogous experiments were carried out under 20% CO2 at 800 °C and 900 °C, and some conversion of the char was evident from the ash tracer analysis (Fig. 4). However, the syngas components of interest were below detectable levels in the entraining gas at these temperatures due to the low feed rate and high flowrate of carrier gas required for entrainment. At 1000 °C, increased reactant concentration resulted in an increase in the yield of CO from the Boudouard reaction. H2 yield also increased with increasing CO2 concentration. As H2 is not a direct product of the Boudouard reaction, its evolution is attributed to the liberation of residual hydrogen in the coal structure. As the carbon is oxidised to CO, the hydrogen atoms stabilising the coal structure are released as hydrogen gas, hence the H2 trend tracks that of CO evolution, but is two orders of magnitude lower as no additional hydrogen is added to the system. To achieve the highest efficiency in any gasifier, the conversion of char to desirable gaseous products should be maximised. The effects of temperature and variable CO2 concentration on char
111
Fig. 4. Variation in char conversion and syngas component yield from CO2 gasification under varying reactant concentrations and temperatures.
conversion are presented in Fig. 4. As CO2 concentration increased from 5 vol% to 20 vol% at 1000 °C, char conversion increased from 40% to 55%. Over the temperature range 800–1000 °C, at 20% CO2, conversion increased from 10% to 55%. This indicates that temperature has a much greater influence on char-CO2 gasification reactivity than reagent concentration over these ranges. The gasification results are supported by the chemical analysis, presented in Table 4, of the char before and after gasification under various conditions. The data has been normalised for ash content, and shows that gasification under increasing CO2 concentration at 1000 °C results in little variation in the composition of the remaining carbonaceous material. It is therefore only the conversion which appears to be affected by altering the reagent concentration. Gasification at 800 °C, 900 °C and 1000 °C under 20% CO2 also results in similar normalised composition of the char residue, indicating that the Boudouard reaction appears to proceed at increasing rates with increasing temperature, but has little impact on the remaining nitrogen and sulphur in the char. Entrained flow gasifiers operate with particle sizes in the range of hundreds of microns to maximise throughput and minimise the gas flowrate required for particle entrainment. To compensate for the consequently low residence times, they therefore require higher operating temperatures, typically 1200–1600 °C, than fixed and fluidised bed technologies which operate with larger particle sizes. From the gasification experiments reported here, it is clear that the most favourable operating conditions used in this study, being 1000 °C, 20% CO2 and a 6 s residence time, are insufficient for complete conversion of Morwell char. This investigation should therefore be extended to higher temperatures and longer residence times under laboratory conditions in order to establish the maximum possible char conversion and corresponding process parameters.
3.3. Evolution of particle structure during pyrolysis and gasification The surface area of the parent coal and char samples following treatment under various pyrolysis and gasification conditions was measured by CO2 adsorption at 0 °C and the results are presented in Table 4 and Fig. 5. It is well known that more open, porous char structures and higher surface area lead to higher gasification reactivity due to the greater accessibility of reactive gas molecules to active carbon sites in the char [18,19]. From an initial coal surface area of 196.66 m2/g, pyrolysis at increasing temperatures resulted in a 2- to 2.5-fold increase in surface area due to opening and fragmentation of the pore structure during devolatilisation.
112
J. Tanner et al. / Fuel 154 (2015) 107–113
Table 4 Properties of Morwell coal char produced at 1000 °C after entrained flow gasification at various CO2 concentrations and temperatures. Reaction
Gasification
Gasification
Gasification
Gasification
Gasification
Experimental conditions Atmosphere Temperature (°C) Conversion (%)
5% CO2/N2 1000 39.31
10% CO2/N2 1000 47.74
20% CO2/N2 1000 55.60
20% CO2/N2 900 21.15
20% CO2/N2 800 9.84
Chemical analysis Moisture (% a.s.) Ash (% d.b.) C (% d.a.f.) H (% d.a.f.) N (% d.a.f.) S (% d.a.f.) O (by difference)
– 7.25 88.05 1.18 0.93 0.25 9.59
– 8.42 91.17 1.15 0.90 0.21 6.58
– 9.91 88.06 1.05 1.04 0.85 9.00
– 5.58 90.88 1.06 0.80 0.16 7.10
– 4.88 91.37 1.07 0.78 0.15 6.62
Physical analysis Surface area (m2/g)
580.30
620.82
620.19
548.79
529.36
a.s. = as sampled; d.b. = dry basis; d.a.f. = dry, ash free basis.
of steam lead to even greater expansion of the char structure, and a maximum char conversion of approximately 80% was achieved. As for the CO2 gasification chars, the increase in surface area appears to be independent of gasification temperature. The increase in char surface area upon gasification under steam and CO2 conditions is attributed to the fracturing and removal of solid carbon from the structure. As gasification proceeds by either of the two heterogeneous char gasification reactions, the remaining solid is depleted. The steam gasification reaction results in higher conversion and higher surface area of the remaining char due to the smaller size of the water molecule, enabling it to enter smaller pores in the char particles and access active carbon sites which are only available to CO2 once the surrounding material has been consumed. Furthermore, the rate of gasification reactivity of char to steam in comparison to CO2 is known to be higher by a factor of approximately 3 [23], which may contribute to the higher surface area of the steam-gasified char. Fig. 5. Variation in solid surface area between (a) raw Morwell coal, (b) char from pyrolysis at 800–1000 °C and (c) char from gasification under 5–20% CO2 at 800– 1000 °C.
Additionally, the experiments yielded chars with surface areas comparable to chars from Victorian brown coals produced under fixed bed and fluidised bed conditions [20,21]. Therefore, it appears that entrained flow treatment of this coal is effective at creating chars with high surface areas for gasification. Upon gasification of the 1000 °C char at 800 °C, 900 °C and 1000 °C, the CO2 surface area increased by a further 20%, and the results for the three residual samples were slightly increasing with increasing temperature. This supports the results of Feng and Bhatia [22] who showed that the development of the pore structure during gasification was similar for heat treated subbituminous coal chars at different temperatures. The char conversion ranged from approximately 40% at 800 °C to 55% at 1000 °C by mass. It therefore appears to be the pyrolysis conditions which have the greatest influence on char gasification reactivity in terms of surface area. Furthermore, if the slight increase in surface area following gasification is not dependent on temperature, it is only the increase in reaction rate at higher gasification temperatures which leads to the increase in char conversion. Steam gasification of the 1000 °C char was also carried out at varying concentrations. Unfortunately, it was not possible to accurately set the steam concentration to which the char was exposed due to equipment limitations; however gasification of the char was evident none the less. The results of the surface area analysis of the three steam gasification chars shown in Fig. 5 indicate that the use
4. Conclusions Entrained flow pyrolysis of a Victorian brown coal and gasification of the resultant char were investigated at various temperatures and concentrations of the gasifying reagents CO2 and steam. From the results of this study, the following conclusions may be drawn: 1. Few tar compounds, which can be detrimental to the operability and efficiency of the entrained flow gasification process, are expected to be generated during entrained flow pyrolysis or gasification of Victorian brown coal above 1000 °C due to decomposition of functionalised groups to permanent gases and subsequent crosslinking of the majority of remaining hydrocarbon fragments prior to volatilisation and release. 2. As gasification temperature increases, the yield of hydrocarbon contaminants in the product syngas will be reduced, increasing conversion efficiency and resulting in a cleaner syngas product requiring less conditioning prior to subsequent use. 3. Entrained flow pyrolysis process is successful in generating high surface area, reactive char particles from this coal for subsequent gasification. 4. The most favourable reaction conditions used in the reported gasification experiments, 1000 °C, 20% CO2 and a 6 s residence time, are insufficient for complete conversion of Morwell char. Based on the results of this proof-of-concept study, it is therefore surmised that entrained flow gasification of Victorian brown coals may represent a feasible, practical route to syngas generation
J. Tanner et al. / Fuel 154 (2015) 107–113
from these fuels. However, the temperatures (up to 1000 °C) and residence times (up to 6 s) employed in this study were insufficient to achieve the high conversions which are typical of an industrial entrained flow gasification system. Therefore, further investigation should be undertaken at higher temperatures and residence times. Acknowledgements The authors would like to acknowledge Brown Coal Innovation Australia (BCIA), the Australian China Strategic Research Fund (ACSRF), Go8-DAAD funding scheme and the Australian Research Council’s Linkage Infrastructure, Equipment and Facilities (LIEF 120100141) for providing the funding for this study. References [1] Higman C, van der Burgt M. The kinetics of gasification and reactor theory. In: Gasification. Burlington: Gulf Professional Publishing; 2008 [Chapter 3]. [2] Weil KS. Coal gasification and IGCC technology: a brief primer. Proc ICE Energy 2010;163:7–16. [3] Higman C, Tam S. Advances in coal gasification, hydrogenation, and gas treating for the production of chemicals and fuels. Chem Rev 2013;114:1673–708. [4] Tremel A, Haselsteiner T, Kunze C, Spliethoff H. Experimental investigation of high temperature and high pressure coal gasification. Appl Energy 2012;92:279–85. [5] Brown BW. Effect of coal type on entrained flow gasification, in PhD Thesis 1985, Brigham Young University. [6] Lee JG, Kim JH, Lee HJ, Park TJ, Kim SD. Characteristics of entrained flow coal gasification in a drop tube reactor. Fuel 1996;75:1035–42. [7] Bhattacharya SP. Gasification performance of Australian lignites in a pressurized fluidized bed gasifier process development unit under air and oxygen-enriched air blown conditions. Process Saf Environ Prot 2006;84:453–60.
113
[8] Johnson T. A competitive gasification technology for brown coal. In: Proceddings of the EPRI-VGB conference on lignites and low-rank coals. Weisbaden, Germany; 2001. [9] Kajitani S, Suzuki N, Ashizawa M, Hara S. Co2 gasification rate analysis of coal char in entrained flow coal gasifier. Fuel 2006;85:163–9. [10] Kajitani S, Hara S, Matsuda H. Gasification rate analysis of coal char with a pressurized drop tube furnace. Fuel 2002;81:539–46. [11] Zhang L, Binner E, Qiao Y, Li C-Z. High-speed camera observation of coal combustion in air and O2/CO2 mixtures and measurement of burning coal particle velocity. Energy Fuels 2009;24:29–37. [12] Flagan RC, Seinfeld JH. Fundamentals of air pollution engineering. Englewood Cliffs. New Jersey: Prentice Hall; 1988. [13] Lehto J. Development and characterization of test reactor with results of its application to pyrolysis kinetics of peat and biomass fuels. PhD Thesis, Tampere University of Technology; 2007. [14] Marney D. Investigation of the pyrolysis of two low-rank coals at elevated pressures. PhD Thesis, Swinburne University of Technology; 2002. [15] Solomon PR, Hamblen DG, Carangelo RM, Serio MA, Deshpande GV. General model of coal devolatilization. Energy Fuels 1988;2:405–22. [16] Hayashi Ji, Takahashi H, Iwatsuki M, Essaki K, Tsutsumi A, Chiba T. Rapid conversion of tar and char from pyrolysis of a brown coal by reactions with steam in a drop-tube reactor. Fuel 2000;79:439–47. [17] Laurendeau NM. Heterogeneous kinetics of coal char gasification and combustion. Prog Energy Combust Sci 1978;4:221–70. [18] Molina A, Mondragón F. Reactivity of coal gasification with steam and co2. Fuel 1998;77:1831–9. [19] Miura K, Mae K, Murata A, Sato A, Sakurada K, Hashimoto K. Flash pyrolysis of coal in solvent vapor for controlling product distribution. Energy Fuels 1992;6:179–84. [20] Harris DJ, Smith IW. Intrinsic reactivity of petroleum coke and brown coal char to carbon dioxide, steam and oxygen. Symp (Int) Combust 1991;23:1185–90. [21] Cameron A, Stacy WO. The pore structure of chars and cokes. Aust J Appl Sci 1959;10:449. [22] Feng B, Bhatia SK. Variation of the pore structure of coal chars during gasification. Carbon 2003;41:507–23. [23] Walker Jr PL, Rusinko Jr F, Austin LG. Gas reactions of carbon. In: Eley DD, Selwood PW, Weisz PB, editors. Advances in catalysts, xi. New York: Academic Press; 1959.