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Similarities and differences in hydrous pyrolysis of biomass and source rocks
Organic Geochemistry 30 (1999) 1495±1507
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Similarities and dierences in hydrous pyrolysis of biomass and source rocks Tanja Barth* Department of Chemistry, University of Bergen, AlleÂgaten 41, N-5007 Bergen, Norway
Abstract In laboratory simulation of oil generation, products from closed systems pyrolysis of immature source rocks in the presence of water gives the closest match to petroleum compositions observed in nature. Fresh biomass can also be converted to ¯uids by pyrolysis, but in the absence of the sedimentary diagenetic transformations, the initially much higher oxygen content gives high yields of oxygen containing products. In this work, the reactions that occur during hydrous pyrolysis of a Kimmeridge source rock, a brown coal and two polymeric waste materials from alginate production are compared in terms of quantities of the main products and kinetic models of the reaction systems. The biomass pyrolysis and the simulated maturation are described in similar reaction networks. Conversion of biomass to ¯uids occurred with reaction networks and activation energy distributions comparable to the brown coal, while for the Kimmeridge source rock reactions a simpler reaction network could be used. The biomass samples gave a high degree of conversion to ¯uid products, and higher yields of bitumen than the coal. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Hydrous pyrolysis; Biomass; Source rock; Reaction network models; Activation energy; Petroleum ¯uids
1. Introduction Natural petroleum generation is generally understood to be part of the continuous transformation of sedimentary biomass with burial (e.g. Hunt, 1996). The process is often divided into three stages: (i)
diagenesis, where the reactive components of the biomass are degraded primarily by microbial processes, and the remaining material develops into the insoluble polymeric material termed ``kerogen''; (ii) catagenesis, where thermal processes ``crack'' the kerogen into ¯uid products (oil and gas) and residual solid material; and (iii) metagenesis, where further temperature increase results in hydrocarbon gas generation and leaves a coke-like carbon residue incapable of further reaction.
* Tel.: +47-5558-3483; fax: +47-5558-9490. E-mail address: [email protected] (T. Barth).
The diculty of observing such reaction sequences in nature, where the duration of the process is in the tensto hundreds of million year range, and no comparison of starting material and products is possible, has led to the development of laboratory pyrolysis processes simulating the conversion on a time scale of days at considerably increased temperatures. Mostly, immature source rocks or coals are used as starting materials, where the term immature implies that no catagenesis has occurred and ¯uids have not yet been generated. The main objectives of simulated oil generation are: . to quantitatively and compositionally reproduce natural petroleum generation; . to provide numerical parameters for reaction modelling; . to enable extrapolation of results from laboratory to natural time/temperature scales for use in evaluation of prospects for exploration. As the cost of oil exploration increases, for example in oshore oil provinces, modelling of the temperature history, source rock potentials and extent of oil the
0146-6380/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0146-6380(99)00121-7
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T. Barth / Organic Geochemistry 30 (1999) 1495±1507
generation is increasingly used in the evaluation of new prospects. Data from simulated maturation of possible source rocks in the area provide part of the necessary information basis for such basin models. Pyrolysis is also a widely used approach for the conversion of fresh biomass to liquids and gases. A considerable eort has gone into developing biomass pyrolysis processes for direct conversion of solid biomass into liquid fuels. At present, the preferred technology is fast (¯ash) pyrolysis at high temperatures with very short residence times (Elliott et al., 1991; Bridgewater et al., 1999). The liquids produced have a very high content of oxygen, a low hydrocarbon content and are incompatible with petroleum products. The pyrolysate must be upgraded to give stable products (Maggi and Delmon, 1994, 1997; Demirbras, 1998). In a dierent approach, pressurised aqueous pyrolysis with longer residence times has been tested both for oil and gas production from biomass (Elliott et al., 1993a,b; Minowa et al., 1997), resulting in more oil-compatible products. Pyrolysis in a hydrogen atmosphere (hydropyrolysis) is also considered to have a potential application in the conversion of biomass to liquids enriched in hydrocarbons (Rocha et al., 1997). For simulated maturation as used in petroleum geochemistry, a reaction environment with excess water, high pressure and a residence time of several days (hydrous pyrolysis) has been considered a necessary requirement to produce ¯uid compositions that correspond to natural petroleum (Winters et al. 1983; Lewan, 1985, 1997; Barth et al., 1989; Andresen et al., 1993). However, comparable experimental results have been obtained using con®ned pyrolysis in closed, high-pressure systems (compressible gold tubes) without added water (Monthioux et al., 1985; Behar et al., 1992; Michels et al., 1995). Hydropyrolysis also gives ¯uids with a composition resembling petroleum (Rocha et al., 1997). Fast, low-pressure pyrolysis processes do not give quantitatively or compositionally equivalent products (Andresen et al., 1993). The critical factor is suggested to be the hydrogen donating ability of the reaction medium, combined with sucient time for exchange reactions to occur (Michels et al., 1996). In the search for pyrolysis processes suitable for the conversion of biomass directly to petroleum compatible liquid fuels, the experience acquired in geochemical applications of pyrolysis can provide useful knowledge. In this work, hydrous pyrolysis, as developed for simulated oil generation, is used to compare the reaction pathways of biomass and sedimentary organic matter. The conversion of two non-degraded biomass samples is compared to that of an immature source rock for oil (Kimmeridge clay) and an immature coal (brown coal from the Rhine Graben). Since the biomass samples have not gone through the sedimentary diagenesis stage they are compositionally dierent from immature source rocks, and in particular the oxygen-to-carbon
ratio is high. The biomass used was mixed polymeric material from marine macro-algae, produced in large amounts as a residue during the extraction of alginate for the food industry. The polymers were precipitated from aqueous solution either with acid or calcium ions. From a geochemical perspective, this material represents a possible model for biopolymer input into the early diagenesis stage in the natural conversion process in a marine environment. Comparison of the results from simulated maturation of pure biomass and rocks containing immature sedimentary organic matter is scarcely reported in the literature. Rocha et al. (1997) reported hydropyrolysis of both geochemical and biomass samples, otherwise the more recent pyrolysis development seems to be restricted to either one or other type of starting material. Comparisons can be relevant both in the organic geochemical perspective for determining the importance of reduced oxygen content in the organic matter after diagenesis for the subsequent hydrocarbon generation reactions, as well as in biomass conversion for evaluation of the potential of hydrous pyrolysis processes as a route for conversion of biomass to petroleum compatible liquid fuels. The hydrous pyrolysis experiments for these series were run isothermally at temperatures between 240 and 400 C, with a residence time of 24±72 h. The yields of ¯uid products (oil phase, gas phase and aqueous organic compounds) were quanti®ed, and the amount of residual reactive carbon and generated coke in the solid residue determined. To make it possible to compare the reaction systems, the same set of product fractions was used in all series, and the conversion of the starting material to reactive intermediate products and stable end products was modelled in quantitative reaction networks. The network describes the reaction pathways from the starting materials through the intermediates to the stable end products in terms of kinetic parameters for each step, and stochiometric factors for each product distribution. Such models are frequently used to describe petroleum generation (JuÈntgen and Klein, 1975; Braun and Burnham, 1987; Ungerer and Pelet, 1987), though the validity of the extrapolation to geological time scales can be questioned (Snowdon, 1979). Inverse curve ®tting procedures were used to ®nd the best-®t activation energies and stochiometric factors (Barth and Nielsen, 1993; Nielsen et al, 1996). The resulting reaction models for the dierent starting materials were compared, and the degree of similarity between the fresh and sedimentary biomass types evaluated. 2. Experimental Pyrolysis experiments and data treatment were performed as described in Barth et al. (1989) and Barth and
T. Barth / Organic Geochemistry 30 (1999) 1495±1507
Nielsen (1993). A summary of the procedure is given here: 2.1. Starting material Dry, ®nely powdered and homogenised coal and source rock samples were used as starting materials. The Kimmeridge source rock was from an outcrop in Dorset, UK (Barth et al., 1989). The brown coal is a very immature sample from a lignite opencast mine in the Rhine area, DE, and had a vitrinite re¯ectance (Ro) of 0.26. The biomass samples were supplied by Pronova, NO, as a dry, homogenous powder consisting of mixed biopolymers precipitated from aqueous solution in process water by addition of acid (H-biomass) or calcium ions (Ca-biomass). Typically, the major compound classes in the calcium-precipitated polymers are 30±35% cellulose, 10±20% carbohydrates, 7±14% proteins, 13± 15% water and 9±10% calcium (weight). The total organic carbon content, TOC (in%), was measured for all the starting materials, and for the biomass elemental analysis of hydrogen and oxygen was also made. For the Kimmeridge source rock and the coal sample, RockEval determination of the petroleum potential (S2) and Tmax to indicate initial maturity was performed. The values are given in Table 1. 2.2. Pyrolysis About 5 g of the starting material and sucient water to ensure at least 5 ml liquid water at subcritical pyrolysis temperatures were placed in a 50 or 71 ml stainless steel autoclave which was put in an oven preheated to the chosen temperature. The reactor is then kept isothermally at this temperature for the selected time period (24 h for the biomass, 72 h for the source rock and coals), and then rapidly cooled. The shorter pyrolysis time for the biomass samples was used after initial experiments showing only small dierences in the yields after 24 and 72 h. 2.3. Sampling and analysis Sampling and analysis After cooling, the gas phase was collected quantitatively in a gas sampling bag and Table 1 Initial properties and pyrolysis data for the source rocks and biomasses Origin
TOC S2 Tmax %H %O (%) (mg/g) ( C) (weight) (weight)
Outcrop sample, Dorset Brown coal Biomass polymer, H-form Biomass polymer, Ca.-form
12.6 71.3 38.2 35.06
a
nm: Not measured.
64.8 58.9 nm nm
422 407 nm nm
nma nm 5.24 4.42
nm nm 42.65 41.55
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the excess gas volume measured. The gas phase composition was analysed by GC. The reactor was opened and the free water phase was removed using a Pasteur pipette. Any expelled oil present was combined with the bitumen extract (see below). Expelled oil was only observed in some of the higher temperature range experiments with the Kimmeridge source rock pyrolysis. The water phase was analysed for content of short-chain carboxylic acids by isotachophoresis (Barth et al., 1989). The remaining ``sludge'' of coke, bitumen (i.e. oil phase not separated from the solid residues) and water was transferred to a Soxhlet thimble. All reactor surfaces were washed with a dichloromethane/methanol constant boiling mixture (DCM: MeOH 93:7v/v), and the washing solvent containing traces of the solid phase products was added to the thimble, which had been placed in the Soxhlet extractor. A water separator over the extractor was used to remove moisture and ensure complete contact between the solvent and sample. The thimble was extracted with the solvent mixture until no water had separated out for more than 8 h (usually a total extraction period of 48 h). The organic extracted was concentrated to 2±8 ml on a rotary evaporator and the C12+ content of the extract quanti®ed by gravimetric determination of non-volatile components on a microscale. The volume of the solution of the extract was calculated from the weight and solvent density. The extracted Soxhlet thimble with residue was dried in an oven at 80 C, cooled and weighed. 2.4. Product fractions and quanti®cation The product fractions used in the description of the reaction processes are bitumen, aqueous organic acids, carbon dioxide, hydrocarbon gases and coke, and a composite group including the volatile hydrocarbons (C5±C12) that are lost during sample work-up, the unspeci®ed losses and analytical inaccuracies is calculated by dierence to conserve the mass balance in the system. Bitumen is the non-volatile liquid product, and corresponds to crude oil within a source rock. This is the major product from the source rock pyrolysis. Bitumen contains more high molecular weight polar compounds than reservoir crude oil, but this is not critical in the present application. To limit the complexity of the models, sub-fractions of bitumen (e.g. asphaltenes and de-asphalted oil) are not used here, although this makes the models less precise. The experimental uncertainty is not large, and bitumen yields are normally within 10% in parallel experiments. The hydrocarbon gases also represent petroleum type products and the yields are determined with a similar precision. The aqueous short-chain organic acids and carbon dioxide/carbonate (gas, aqueous and solid phase) are the major oxygen-containing fractions. They are minor products in source rock pyrolysis and are often not
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included in hydrocarbon generation models, but the yields increase signi®cantly for the coal and biomass samples where they represent a major part of the carbon in the system. There is a risk that the amount of organic acids produced is systematically underestimated, since a constant volume of water during the pyrolysis is assumed. Carbon dioxide yields are dicult to reproduce experimentally. The reason is not known at present, but a considerable, unsystematic variation in the yields was observed. In the solid phase, the organic phases are de®ned as the initial kerogen or biomass, which for modelling purposes is divided into fractions with increasing activation energies (Barth and Nielsen, 1993), and coke produced in the pyrolysis. Both the reactive carbon and the coke was quantitated from TOC measurements, as described below. The uncertainty in the quanti®cation is estimated to 10%. In hydrous pyrolysis, a mass balance on the weight of the starting material relative to the organic phase products cannot be assumed, since the amount of water produced or consumed in the reactions cannot be quanti®ed. Hydrogen and oxygen from water is probably also incorporated in the organic products. The recoveries and product distributions are therefore calculated on a carbon basis, even if this involves a degree of uncertainty in the estimates of the carbon content for some of the fractions. The following conversion factors were used based on the quanti®ed milligrams of product per gram stating material: The total amount of reactive carbon in the biomass was calculated from the maximum decrease in measured TOC (TOC) corrected for the fraction of solid converted to ¯uids: Reactive carbon (as 10ÿ3 mol C/g)= [TOC(%)10 (mg/%)recovery (g/g)/(12 mg/mmol C)]; unreacted kerogen/biomass was calculated in the same way from the measured TOC values corrected for unreactive carbon; bitumen and S2 (as 10ÿ3 mol C/g)= [yield (mg/g)0.8 (weight of C-atoms per mg)/12(mg/ 10ÿ3 mol carbon)]; CO2 and hydrocarbon gases: calculated precisely from concentration and volume data; organic acids: calculated from the total acid concentration in the aqueous phase and the starting volume of water assuming an average of three carbon atoms per molecule; coke: calculated as the increase of TOC at higher temperature relative to the minimum value; volatiles and loss: calculated by dierence to give the mass balance at a constant level of reactive carbon. To simplify the comparison between dierent starting materials with dierent carbon contents (12.6% for the Kimmeridge source rock, 71.3% for the brown coal), all yields are normalised to the initial carbon content and given as 10ÿ3 mol carbon per gram carbon initially in the material (mM C/g TOC). If all the initial carbon is reactive, the carbon mass balance should be at 83.3 mM C/g TOC. However, there is a fraction of unreactive
carbon in all the materials used, resulting in a lower, but variable level of reactive carbon, as discussed below. 2.5. Reaction network modelling The thermal conversion processes of biomass and sedimentary organic material are complex, and cannot be described in a single chemical reaction. The model of the reaction system assumes that the starting material consists of fractions with dierent reactivity, that intermediate products that react further are produced, and that interconversion from one type of product to another will occur. Since precise chemical models on a molecular basis become much too complex to be possible to establish and calibrate for real data, models describing conversion of fractions of the starting material into product groups are used. The kerogen or biomass is divided into reactions with dierent reactivity, and product fractions are de®ned as reactive intermediates or stable products, and the total system is established as a reaction network. A simple example of a network consists of a kerogen with two reactive fractions, bitumen as an intermediate and coke and gas as stable end products, with three ®rst-order reactions describing the reactions of the two kerogen fractions to the major products for each step: 1. kerogen fraction 1!kerogen fraction 2 and bitumen, 2. kerogen fraction 2!bitumen and gas, 3. bitumen ! gas and coke each with an activation energy and pre-exponential factor to describe the dependence of the reaction rate on temperature, and with a stochiometric factor giving the distribution of products from the reaction. These reactions can be represented as a reaction network in the form of a half matrix, where ±1 refers to the reactant and S to the stochiometric factors giving the fraction converted to each product: Kerl ÿ1 # Ker2 S1 + Bitumen 1ÿS1+ Kerl
Gas Coke
Ker2 Bitument Gas ÿ1 # S2! + 1-S2
ÿ1 # S3! + 1-S3
Coke
stable !stable
When the objective is to estimate values for both the kinetic parameters and stochiometric factors, a large number of parameters need to be determined by ®tting
T. Barth / Organic Geochemistry 30 (1999) 1495±1507
the network to the experimental data, and the number of unknown parameters increase with the complexity of the network. In all cases, the number of unknown parameters is too large to be for each parameter to be precisely calculated from experimental data values, so statistically based curve ®tting procedures are applied. Iterative least-square inverse parameter ®tting procedure as described in Nielsen et al. (1996). This gives a optimal solution with the levels of uncertainties in the experimental data taken into account. The mathematical requirement for a successful curve ®tting is that the iteration converges to a best-®t value, which was the case for all the models presented here. The ®tted variables comprise the activation energies and the stochiometric factors. The pre-exponential factor A of the Arrhenius equation is pre-de®ned and kept at 11013 sÿ1 for all the models. The uncertainty in the parameter values are not given for each parameter, but can be illustrated in Monte-Carlo models based on the best ®t model, and show a considerable range of uncertainty. Such models are not considered part of this presentation, but it should be kept in mind that the models below are a simpli®ed representation of complex processes with inherent uncertainties. The purpose of the models, as presented here, is to enable comparisons and not to perform high precision extrapolations. For the data presented here, the reaction networks were set up with the minimum number of reactions that result in converging models in the inverse ®tting procedure. All reactions are de®ned as ®rst order. The kerogen/biomass reactive potential was divided into ®ve levels (four for the Kimmeridge source rock), corresponding to a discrete distribution of activation energies (Juntgen and Klein, 1975; Braun and Burnham, 1987). Only the best-®t models achieved after extensive testing of alternative reaction networks are shown here. For the coal and the biomass, the network included pathways for internal rearrangement of the starting material during pyrolysis. This gives a continuous conversion from low to high activation energies in the reactant (for example representing increasing aromatisation), which was necessary to make the models converge to a best-®t set of parameters. For the Kimmeridge source rock, a simpler model with an initial distribution of the kerogen in four groups with speci®c activation energies was sucient to describe the data. Bitumen and organic acids were de®ned as intermediates which reacted in a single reaction to give the stable end products carbon dioxide, hydrocarbon gases and coke. The volatiles/loss group was also considered stable. 3. Results For all types of starting material, hydrous pyrolysis converts carbon from a solid phase form into ¯uid pro-
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ducts, as shown by the reduction in the solid phase carbon contents with increasing pyrolysis temperatures (Table 2). The apparent increase in carbon content at low pyrolysis temperatures compared to the initial values (Table 1) is accounted for by dissolution of solids into the aqueous phase during the pyrolysis. As shown in Table 2, only a proportion of the total carbon is converted, so the carbon content of the residues does not approach zero. This is well known from source rock evaluation, where the non-reactive fraction is termed ``dead'' carbon. The product distribution for each pyrolysis series is given in Tables 3±6. The yields are mass balanced by calculating the product group of volatiles/loss by dierence. Though the amount of carbon assigned to this group can be a signi®cant proportion of the total amount, the reaction networks still function well in the overall description of reaction pathways. The mass balance is based on the reactive carbon and varies from 24 mM C/g TOC for the brown coal to 73.2 mM C/g TOC for the acid form biomass, which is consistently lower than the 83.3 mM C/g TOC of a totally reactive starting material. The dierence between the reactive carbon used in the mass balance and the total carbon content corresponds to the ``dead'' carbon observed in the TOC measurements of the residues. Including ``dead'' carbon in the reaction networks as coke produced directly from kerogen is feasible, but is not included here since it does not improve the ®t of models and increases the number
Table 2 Measured carbon content of the solid residues after pyrolysis Kimmeridge Brown coal Biomass H Biomass Ca Pyrolysis time (72 h) (72 h) (24 h) (24 h) Pyrolysis TOC TOC TOC TOC temperature ( C) 240 250 260 270 280 290 300 305 310 320(1) 320(2) 330 340 350 360 365 380 400 a b
14.89 ± 14.11 13.65 12.05 11.22 9.33 ± 8.02 5.81 6.62 7.29 7.24 7.85 ± ± ± ±
±a 70.66 ± 69.25 ± 69.43 ± 70.03 ± 69.35 69.40 68.72 68.76 nmb ± 67.45 ± ±
51.69 ± ± 47.23 ± ± 35.16 ± ± 28.08 ± 20.21 ± ± 21.34 ± 18.20 24.05
±: no experiment at this temperature. nm: not measured.
70.58 ± ± 57.95 ± ± 52.63 ± 52.85 ± ± 49.51 ± 49.28 ± 47.12 45.06
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T. Barth / Organic Geochemistry 30 (1999) 1495±1507
Table 3 Pyrolysis data for the Kimmeridge source rock, mass balanced to 53.3 mM C/g TOC Pyrolysis temperature ( C)
Kerogen
Bitumen
Organic acids
CO2
Vol/lossa
HC-gasesb
Cokec
250 260 270 280 290 300 310 320 330 340 350
39.30 36.10 35.30 29.30 20.80 15.24 8.90 7.44 5.90 5.00 2.90
4.86 5.94 9.39 13.56 18.74 24.44 29.47 33.61 31.10 26.16 22.00
0.54 0.56 0.60 0.64 0.71 0.76 0.83 0.92 0.99 0.96 1.09
1.50 2.46 3.17 3.02 2.86 4.05 4.92 5.75 6.50 7.70 8.75
7.08 8.19 4.52 6.16 9.98 8.37 8.58 4.75 7.38 7.98 6.58
0.02 0.05 0.32 0.62 0.17 0.44 0.60 0.83 1.43 1.50 1.98
0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 4.00 10.00
a b c
Volatiles and loss calculated by dierence. C1ÐC5 hydrocarbon gases. Non-zero value required as input to the modelling.
Table 4 Pyrolysis data for the brown coal, mass balanced to 24 mM C/g TOC Pyrolysis temperature ( C)
Kerogen
Bitumen
Organic acids
CO2
Vol/
HC-gases
250 270 290 305 320 320 330 340 350 365
15.71 14.22 10.40 11.78 9.47 10.33 6.44 8.08 6.00 2.86
1.36 1.53 2.55 3.99 4.75 5.11 6.03 6.86 5.98 5.35
1.15 1.23 1.83 1.41 1.98 1.98 2.12 2.00 1.94 1.88
3.22 3.49 4.55 2.77 2.76 2.81 3.25 2.20 3.86 4.48
2.48 3.38 4.37 3.76 4.59 3.55 5.34 3.99 4.68 7.20
0.08 0.15 0.30 0.29 0.45 0.22 0.82 0.87 1.54 2.23
Table 5 Pyrolysis data for the acid precipitated biomass polymer, mass balanced to 73.2 mM C/g TOC Pyrolysis temperature ( C)
Kerogen
Bitumen
Organic acids
CO2
Vol/loss
HC-gases
Coke
240 270 300 320 340 360 380 400
33.08 29.36 19.30 13.37 6.84 7.79 5.17 5.04
3.56 11.69 15.09 19.00 21.93 22.18 15.46 5.46
11.90 11.15 8.75 7.04 7.14 7.32 6.57 10.73
12.25 13.80 14.82 14.73 15.22 21.53 20.89 20.90
12.44 7.13 14.95 17.43 20.73 11.87 20.13 18.52
0.07 0.17 0.39 1.73 1.44 2.61 4.98 7.65
0.01 0.01 0.01 0.01 0.01 0.01 0.11 5.01
of parameter values that have to be estimated in the curve ®tting. On a carbon basis, the Kimmeridge source rock yields most bitumen, with a maximum of 33.6 mM C/g TOC.
The biomass samples give much higher yields than the brown coals, at 22.2 mmol C/g TOC for the H-biomass and 21.4 mmol C/g TOC for the Ca-biomass, compared with only 6.9 mmol C/g TOC for the brown coal. The
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Table 6 Pyrolysis data for the Ca2+ precipitated biomass polymer, mass balanced to 65 mM C/g TOC Pyrolysis temperature ( C)
Kerogen
Bitumen
Organic acids
CO2
Vol/loss
HC-gases
240 270 300 320 340 360 380 400
28.29 18.28 13.86 14.04 11.26 11.07 9.27 7.55
6.74 13.87 21.44 20.74 20.26 18.42 12.47 7.93
10.00 9.38 7.80 8.20 8.66 7.50 6.64 6.03
16.23 23.20 19.90 18.62 21.69 20.41 20.68 24.10
3.67 0.10 1.50 2.34 1.55 5.96 10.98 11.88
0.07 0.17 0.50 1.06 1.58 1.64 4.96 7.51
biomass samples are characterised by very high yields of carbon dioxide in the gas phase and short-chain carboxylic acids in the aqueous phase, re¯ecting their high initial oxygen content. The two biomass forms gave dierent temperature dependencies in their reaction patterns, which shows that the incorporation of acid or calcium ions in¯uences the reaction pathway in the conversion process, even though the organic composition is the same. Precipitation of calcium carbonate (CaCO3) in the residue was observed after pyrolysis of the calcium-containing biomass, while the levels of carbonate in the aqueous phase were lower than for the acid precipitated biomass. At the highest temperatures, a signi®cant increase in the organic carbon content is observed for the solid residue from the Kimmeridge source rock and the Hbiomass. This shows that there is a quantitatively signi®cant generation of coke in these series (Tables 3 and 5). To keep the reaction networks as simple as possible, no coke product has been included in the series where the carbon content of the residues decline steadily, though the formation of coke may be masked by concurrent reduction of reactive carbon. Table 7 shows the reaction network for the Kimmeridge source rock, and Fig. 1 shows the ®tted model (continuous lines) and the measured data (points). The amount of reactive carbon or generation potential (53.3 mM C/g TOC) is equivalent to a Rock-Eval S2 (hydrocarbon potential) value of 100.7 mg/g source rock, and considerably higher than the measured value of 64.8 mg/g (Table 1). Four kerogen fractions are the minimum number needed to ®t the experimental data, and they are distributed with ®xed fractions of 0.13 of the total potential at an activation energy of (Ea) 176 kJ/mol, 0.30 at Ea=191 kJ/mol, 0.38 at Ea=201 kJ/mol and 0.19 at Ea=221 kJ/mol. The resulting model for the kerogen degradation corresponds well with the experimental data. All kerogen fractions generate bitumen in substantial amounts, re¯ecting that this is a high-quality oil-prone source rock. The bitumen is quite temperature stable, with an Ea=231 kJ/mol. The bitumen generation and degradation in the model goes through a wider
maximum than the measured values, which can be taken as an indication the single bitumen fraction is an over simpli®cation and that including separate generation and conversion of the heavy products (asphaltenes, resins) and the light hydrocarbon oil would improve the precision of the model. CO2 and aqueous organic acids (SCA) are generated both from the intermediate kerogen fractions and the bitumen, while hydrocarbon gases and coke are only generated at high degrees of conversion. The contribution from the volatile/loss group is small, re¯ecting low experimental uncertainties. Overall, the experimental data are well ®tted. The model is extrapolated to 400 C to enable comparison with the other series, even though no measured data points constrain the model in the highest temperature range. Table 8 shows the reaction network for the brown coal. The total reaction potential is again much higher than indicated by the Rock-Eval S2 (Table 1), corresponding to a S2 value of 257 mg/g. Dividing the coal into ®ve initial fractions and including internal rearrangement within the kerogen/bitumen fractions was necessary to establish a well-®tted model. This results in a much wider temperature range of reaction compared to the Kimmeridge source rock, with activation energies ranging from 180 to 240 kJ/mol. Fig. 2 shows the correspondingly wide range of temperatures for the coal reactions, as opposed to the steep curve of the kerogen degradation for the Kimmeridge source rock (Fig. 1). The ¯uid yields are low, and no bitumen production is assigned to the lowest and highest activation energy fraction. The proportion of oxygen containing products (CO2, SCA) is high, which explains some of the increase in reaction potential relative to S2. The volatiles/loss product group is quantitatively the second largest, indicating an increased uncertainty in the quantitative model. Overall, this coal has a minimal potential as a hydrocarbon source rock, as expected from general petroleum geochemical evaluations of coals as a source for oil. The reaction network for the acid precipitated biomass polymer sample is given in Table 9. The network is similar to the brown coal but with an even wider range
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T. Barth / Organic Geochemistry 30 (1999) 1495±1507
Table 7 Reaction network for the Kimmeridge source rock Pre-exponential factor A=constant=1013 sÿ1 K/B1a (176) Ea (kJ/mol) Initial fraction 0.13 Kero/bio 1a Kero/bio 2
ÿ1 #
Kero/bio 3
K/B2
K/B3
K/B4
Bit.
SCAb
(191) 0.30
(201) 0.38
(221) 0.19
(231)
(239)
ÿ1 #
ÿ1 #
Kero/bio 4 Bitumen SCA
0.40 +
CO2 Vol./loss HC-gasesd Coke a b c d
0.60
+ 0.65 + 0.08 + 0.27
+ 0.75 + +
ÿ1 # + 0.26 +
+ 0.18 + 0.07
+
0.40 +
+ 0.34
! ÿ1 # 0.18 + + 0.10 + 0.20 + 0.29 + + 0.22
! ÿ1 # + 1.50 + + 0.50
CO2
V/Lc
HCgd
Coke
!stable !stable !stable !stable
K/B and Ker/bio: reactant fractions in kerogen or biomass. SCA: short chain carboxylic acids. V/L and Vol/loss: volatiles and loss calculated by dierence. HC-gases and HCg: C1ÐC5 hydrocarbon gases.
Fig. 1. Data points and ®tted curves based on the reaction network for the Kimmeridge source rock.
of activation energies (167±237 kJ/mol) and the addition of coke as a product group. Bitumen is again generated only in the intermediate activation energy reactions, but the amounts are much higher than from the coal, as
illustrated in the ®tted curves in Fig. 3. Carbon dioxide is modelled as generated early in the process and remaining at a stable level. However, the measured CO2 yields show considerable unsystematic variation, so the
T. Barth / Organic Geochemistry 30 (1999) 1495±1507
1503
Table 8 Reaction network for the brown coal Pre-exponetial factor A=constant=1013 sÿ1 K/B1
K/B2
K/B3
K/B4
K/B5
Bit.
SCA
(180) Ea (kJ/mol) Initial fraction 0.98
(190) 0.004
(205) 0.004
(220) 0.004
(240) 0.004
(234)
(240)
Kero/bio 1 Kero/bio 2 Kero/bio 3
ÿ1 # 0.76 +
Kero/bio 4
! ÿ1 # 0.75 +
Kero/bio 5 Bitumen SCA CO2 Vol/loss HC-gases
0.11 + 0.13
+
0.12 + 0.10 +
+ 0.03
! ÿ1 # 0.59 + + 0.24 + +
+ +
0.07 + 0.10
! ÿ1 # 0.43 + + 0.23 + 0.09 + + + 0.25
! ÿ1 # +
0.50 + + 0.50
! ÿ1 # 0.10 + 0.20 + + 0.41 + 0.29
! ÿ1 # + 0.05 +
CO2
V/L
HCg
!stable
+ 0.50
!stable !stable
Fig. 2. Data points and ®tted curves based on the reaction network for the brown coal.
curve ®tting is uncertain both for this product and for the volatiles/loss product group. The bitumen is generated over a very wide temperature range and the maximum amount is again underestimated, indicating that a more detailed representation of the liquid products could improve the model. The volatiles/loss fraction is considerable, and may re¯ect a systematic underestimation of aqueous products yields as discussed above.
Table 10 shows the reaction network for the calcium precipitated biomass and Fig. 4 shows the ®tted model. It is similar to the coal model in that no coke generation is observed. The activation energy range is slightly wider (163±238 kJ/mol) than for the acid precipitated biomass model. Carbon dioxide is the major product in the model, in more than equivalent amounts to bitumen. About half of the measured yield is found as solid calcium carbonate. This trapping of carbon dioxide gas
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T. Barth / Organic Geochemistry 30 (1999) 1495±1507
Table 9 Reaction network for the H-form biomass Pre-exponential factor A=constant=1013 sÿ1 K/B1
K/B2
K/B3
K/B4
K/B5
Bit.
SCA
(167) Ea (kJ/mol) Initial fraction 0.98
(182) 0.004
(202) 0.004
(222) 0.004
(237) 0.004
(237)
(237)
Kero/bio 1 Kero.bio 2 Kero/bio 3
ÿ1 # 0.67 +
Kero/bio 4
! ÿ1 # 0.59 +
Kero/bio 5 Bitumen SCA CO2 Vol./loss HC-gases Coke
0.11 + 0.22
+
0.25 +
+ 0.16
! ÿ1 # 0.40 + + 0.22 + 0.06 + + 0.32
! ÿ1 # 0.35 + + 0.26 + +
+
! ÿ1 # 0.31 +
0.20 +
+
0.19
+ 0.59 + 0.10
! ÿ1 # + 0.10 + 0.10 + 0.21 + + 0.39 + + 0.20
! ÿ1 # + 0.51 + + 0.49
CO2
V/L
HCg
Coke
!stable !stable !stable !stable
Fig. 3. Data points and ®tted curves based on the reaction network for the H+-precipitated biomass polymer.
probably induces increased generation and can explain why the yields are much higher than in the previous series. Bitumen generation is again slightly underestimated in the model relative to the experimental values. Although the level of the volatiles/loss group is low and
indicates a reasonably good experimental precision, the correspondence between the data and the model is not as good as in the previous series. However, the main trends are reproduced suciently well for use in the comparison of the series.
T. Barth / Organic Geochemistry 30 (1999) 1495±1507
1505
Table 10 Reaction network for the Ca-form biomass Pre-exponential factor A=constant=1013 sÿ1 K/B1
K/B2
K/B3
K/B4
K/B5
Bit.
SCA
(163) Ea (kJ/mol) Initial fraction 0.98
(183) 0.005
(203) 0.005
(223) 0.005
(238) 0.005
(237)
(242)
Kero/bio 1 Kero/bio 2 Kero/bio 3
ÿ1 # 0.58 +
! ÿ1 # 0.54 +
Kero/bio 4 Kero/bio 5 Bitumen SCA
0.13 + 0.20
CO2
+
0.38 +
+
Vol./loss
0.06 + 0.02
HC-gases
! ÿ1 # 0.62 + + 0.23 + 0.07 + 0.08 + +
! ÿ1 # 0.59 + + 0.15 + +
! ÿ1 # + 0.20 +
+ 0.08 + 0.18
+ 0.42 + + 0.38
! ÿ1 # 0.10 + 0.19 + + 0.46 + + 0.25
! ÿ1 # + 0.51 +
CO2
V/L
HCg
!stable !stable
+ 0.49
!stable
Fig. 4. Data points and ®tted curves based on the reaction network for the Ca2+-precipitated biomass polymer. Table 11 Comparison of parameters from the reaction network modelsa Sample
Reactive carbon (%)
Ea range K/B (kJ/mol)
Ea Bit. (kJ/mol)
Major products
Kimmeridge Brown coal H-biomass Ca-biomass
63 29 88 78
176±221 180±240 167±237 163±238
231 234 237 237
Bitumen, CO2, coke Bitumen, CO2. HC-gases Bitumen, CO2. SCA CO2, bitumen, SCA
a
Volatiles/loss not included in the yield evaluation.
1506
T. Barth / Organic Geochemistry 30 (1999) 1495±1507
4. Discussion Table 11 gives a comparison of the models for the four series of experiments. The Kimmeridge source rock is distinguished by its high level of reactive carbon, simple reaction network and narrow range of activation energies for the kerogen degradation, and the high quantities of bitumen in the products. This can be understood as the natural diagenesis having removed the reactant groups that generate the oxygen-containing products, leave a more uniform, hydrocarbon-producing organic solid as kerogen. The remaining input materials have not been through such a ``pre-processing'', and a higher proportion of the carbon is used to generate carbon dioxide and aqueous organic acids from oxygen contained in the organic solid. The brown coal has the lowest level of reactive carbon, re¯ecting a low hydrogen to carbon ratio and tendency towards aromatisation reactions in the solid phase rather than ¯uid generation, and the amount of bitumen produced is quite low even if it is the major product. The biomasses have a very high proportion of reactive carbon, which to some degree compensates for the high yield of oxygen containing products and results in a considerable potential for bitumen generation. The range of activation energies is even wider than for the coals, and though the temperature range of the hydrous pyrolysis is expanded into the supercritical region (>374 C), the initial reactions have not gone to completion. In this, the biomass reactions re¯ect a more varied initial composition than the sedimentary materials. However, the correspondence in the reaction systems is also considerable. The biomasses are ``better source rocks'' than the brown coal, but the reaction networks resemble the coal more than the oil-prone source rock. The system of fractions used in the networks are suitable for all the staring materials, and the patterns in the reaction systems are similar though there are quantitative dierences. The activation energy ranges for kerogen/biomass degradation and the bitumen reactions are also similar. Experience acquired and models developed for organic geochemistry and coal processing thus can provide a very useful frame of reference for further development of processes for conversion of biomass into petroleum-compatible fuels, especially if the aim is to produce petroleum compatible liquids. The potential for such conversion is clearly considerable, although the yields of hydrocarbon-rich ``oil'' from biomass will necessarily be limited by the lower carbon content and higher oxygen content in the biomass. Because of this, oxygen-containing by-products like carbon dioxide and short-chain carboxylic acids can consume a considerable part of the initial carbon. Adjustment of pyrolysis conditions to minimise the amounts of these products is
required if the objective is a high yield of hydrocarbonrich ¯uids. Alternatively, pre-treatment of the biomass corresponding to the natural diagenesis of source rock organic matter should be considered. Acknowledgements The author thanks Reidun Myklebust for data on the brown coal pyrolysis and éystein Skeidsvoll for data on pyrolysis of marine algael biomass. Brendan Keely and Richard Patience are thanked for constructive and useful reviews.
References Andresen, B., Barth, T., Irwin, H., 1993. Yields and carbon isotopic composition of pyrolysis products from arti®cial maturation processes. Chemical Geology 106, 103±114. Barth, T., Nielsen, S.B., 1993. Estimating kinetic parameters for generation of petroleum and single components from hydrous pyrolysis of source rocks. Energy & Fuels 7, 100±110. Barth, T., Borgund, A.E., Hopland, A.L., 1989. Generation of organic compounds by hydrous pyrolysis of Kimmeridge oil shale- Bulk results and activation energy calculations. Organic Geochemistry 14, 69±76. Behar, F., Kressmann, S., Rudikiewicz, J.L., Vandenbrouke, M., 1992. Experimental simulation in a con®ned system and kinetic modelling of kerogen and oil cracking. Organic Geochemistry 19, 173±189. Braun, R.L., Burnham, A.K., 1987. Analysis of chemical reactions using a distribution of activation energies and simpler models. Energy & Fuels 1, 153±161. Bridgewater, A.V., Meier, D. & Radlein, D., in press. An overview of fast pyrolysis of biomass. Organic Geochemistry. Demirbas, A., 1998. Yields of oil products from thermochemical biomass conversion processes. Energy Conversion Management 39, 685±690. Elliott, D.C., Beckman, D., Bridgewater, A.V., Diebold, J.P., Gevert, S.B., Solantausta, Y., 1991. Developments in direct thermochemical liquefaction of biomass: 1983±1990. Energy & Fuels 5, 399±410. Elliott, D.C., Sealock Jr., L.J., Baker, E.G., 1993a. Chemical processing in high-pressure aqueous environments. 1. Historical perspectives and continuing developments. Industrial and Engineering Chemistry Research 32, 1535±1541. Elliott, D.C., Sealock Jr., L.J., Baker, E.G., 1993b. Chemical processing in high-pressure aqueous environments. 2. Development of catalysts for gasi®cation. Industrial and Engineering Chemistry Research 32, 1542±1548. Hunt, J.M., 1996. Petroleum Goechemistry and Geology. W.H. Freeman and Company, New York. JuÈntgen, H., Klein, J., 1975. Entstehung von Erdgas aus Kohligen Sedimenten. ErdoÈl, Kohle, Erdgas, Petrochimie 28, 65± 73. Lewan, M.D., 1997. Experiments on the role of water in petroleum formation. Geochimica et Cosmochimica Acta 61, 3691±3723.
T. Barth / Organic Geochemistry 30 (1999) 1495±1507 Lewan, M.D., 1985. Evaluation of petroleum generation by hydrous pyrolysis. Philosophical Transanctions of the Royal Society London Series A 315, 123±134. Maggi, R., Delmon, B., 1997. A review of catalytic hydrotreating processes for the upgrading of liquids produced by ¯ash pyrolysis. In: Fromet, G.F., Delmon, B., Grange, P. (Eds.), Hydrotreating an Hydrocracking of Oil Fractions. Elsevier, Oxford, pp. 99±113. Maggi, R., Delmon, B., 1994. Comparison between ``slow'' and ``¯ash'' pyrolysis oils from biomass. Fuel 73, 671±677. Michels, R., Langlois, E., Ruau, O., Mansuy, L., Elie, M., Landais, P., 1996. Evolution of asphaltenes during arti®cial maturation: A record of the chemical processes. Energy & Fuels 10, 39±48. Michels, R., Landais, P., Torkelson, B.E., Philp, R.P., 1995. Eects of euents and water-pressure on oil generation during con®ned pyrolysis and high-pressure hydrous pyrolysis. Geochimica et Cosmochimica Acta 59, 1589±1604. Minowa, T., Zhen, F., Ogi, T., VaÂrhegyi, G., 1997. Liquefaction of cellulose in hot compressed water using sodium carbonate: product distribution at dierent reaction temperatures. Journal of Chemical Engineering of Japan 30, 186±190.
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Monthioux, M., Landais, P., Monin, J.C., 1985. Comparison between natural and arti®cial maturation series of humic coals from the Makaham delta, Indonesia. Organic Geochemistry 8, 275±292. Nielsen, S.B., Barth, T., Dahl, L.C., 1996. Inverse estimation of parameters in petroleum reaction networks. In: Jacobsen, B.H., Moosegard, K., Sibani, P. (Eds.), Inverse Methods; Interdisciplinary Elements of Methodology, Computation, and Applications. Springer, Berlin, p. 167±177. Rocha, J.D., Brown, S.D., Love, G.D., Snape, C.E., 1997. Hydropyrolysis: a versatile technique for solid fuel liquefaction, sulphur speciation and biomarker release. Journal of Analytical and Applied Pyrolysis 40, 91±103. Snowdon, L.R., 1979. Errors in extrapolation of kinetic parameters to organic geochemical systems. American Assiciation of Petroleum Geology Bulletin 63, 1128±1138. Ungerer, Ph., Pelet, R., 1987. Extrapolation of the kinetics of oil and gas formation from labororatory experiments to sedimentory basins. Nature 327, 52±54. Winters, J.C., Williams, J.A., Lewan, M.D., 1983. A laboratory study of petroleum generation by hydrous pyrolysis. In: Bjorùy, M. et al. (Ed.), Advances in Organic Geochemistry 1981. Wiley, Chichester, pp. 524±533.
www.elsevier.nl/locate/orggeochem
Similarities and dierences in hydrous pyrolysis of biomass and source rocks Tanja Barth* Department of Chemistry, University of Bergen, AlleÂgaten 41, N-5007 Bergen, Norway
Abstract In laboratory simulation of oil generation, products from closed systems pyrolysis of immature source rocks in the presence of water gives the closest match to petroleum compositions observed in nature. Fresh biomass can also be converted to ¯uids by pyrolysis, but in the absence of the sedimentary diagenetic transformations, the initially much higher oxygen content gives high yields of oxygen containing products. In this work, the reactions that occur during hydrous pyrolysis of a Kimmeridge source rock, a brown coal and two polymeric waste materials from alginate production are compared in terms of quantities of the main products and kinetic models of the reaction systems. The biomass pyrolysis and the simulated maturation are described in similar reaction networks. Conversion of biomass to ¯uids occurred with reaction networks and activation energy distributions comparable to the brown coal, while for the Kimmeridge source rock reactions a simpler reaction network could be used. The biomass samples gave a high degree of conversion to ¯uid products, and higher yields of bitumen than the coal. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Hydrous pyrolysis; Biomass; Source rock; Reaction network models; Activation energy; Petroleum ¯uids
1. Introduction Natural petroleum generation is generally understood to be part of the continuous transformation of sedimentary biomass with burial (e.g. Hunt, 1996). The process is often divided into three stages: (i)
diagenesis, where the reactive components of the biomass are degraded primarily by microbial processes, and the remaining material develops into the insoluble polymeric material termed ``kerogen''; (ii) catagenesis, where thermal processes ``crack'' the kerogen into ¯uid products (oil and gas) and residual solid material; and (iii) metagenesis, where further temperature increase results in hydrocarbon gas generation and leaves a coke-like carbon residue incapable of further reaction.
* Tel.: +47-5558-3483; fax: +47-5558-9490. E-mail address: [email protected] (T. Barth).
The diculty of observing such reaction sequences in nature, where the duration of the process is in the tensto hundreds of million year range, and no comparison of starting material and products is possible, has led to the development of laboratory pyrolysis processes simulating the conversion on a time scale of days at considerably increased temperatures. Mostly, immature source rocks or coals are used as starting materials, where the term immature implies that no catagenesis has occurred and ¯uids have not yet been generated. The main objectives of simulated oil generation are: . to quantitatively and compositionally reproduce natural petroleum generation; . to provide numerical parameters for reaction modelling; . to enable extrapolation of results from laboratory to natural time/temperature scales for use in evaluation of prospects for exploration. As the cost of oil exploration increases, for example in oshore oil provinces, modelling of the temperature history, source rock potentials and extent of oil the
0146-6380/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0146-6380(99)00121-7
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generation is increasingly used in the evaluation of new prospects. Data from simulated maturation of possible source rocks in the area provide part of the necessary information basis for such basin models. Pyrolysis is also a widely used approach for the conversion of fresh biomass to liquids and gases. A considerable eort has gone into developing biomass pyrolysis processes for direct conversion of solid biomass into liquid fuels. At present, the preferred technology is fast (¯ash) pyrolysis at high temperatures with very short residence times (Elliott et al., 1991; Bridgewater et al., 1999). The liquids produced have a very high content of oxygen, a low hydrocarbon content and are incompatible with petroleum products. The pyrolysate must be upgraded to give stable products (Maggi and Delmon, 1994, 1997; Demirbras, 1998). In a dierent approach, pressurised aqueous pyrolysis with longer residence times has been tested both for oil and gas production from biomass (Elliott et al., 1993a,b; Minowa et al., 1997), resulting in more oil-compatible products. Pyrolysis in a hydrogen atmosphere (hydropyrolysis) is also considered to have a potential application in the conversion of biomass to liquids enriched in hydrocarbons (Rocha et al., 1997). For simulated maturation as used in petroleum geochemistry, a reaction environment with excess water, high pressure and a residence time of several days (hydrous pyrolysis) has been considered a necessary requirement to produce ¯uid compositions that correspond to natural petroleum (Winters et al. 1983; Lewan, 1985, 1997; Barth et al., 1989; Andresen et al., 1993). However, comparable experimental results have been obtained using con®ned pyrolysis in closed, high-pressure systems (compressible gold tubes) without added water (Monthioux et al., 1985; Behar et al., 1992; Michels et al., 1995). Hydropyrolysis also gives ¯uids with a composition resembling petroleum (Rocha et al., 1997). Fast, low-pressure pyrolysis processes do not give quantitatively or compositionally equivalent products (Andresen et al., 1993). The critical factor is suggested to be the hydrogen donating ability of the reaction medium, combined with sucient time for exchange reactions to occur (Michels et al., 1996). In the search for pyrolysis processes suitable for the conversion of biomass directly to petroleum compatible liquid fuels, the experience acquired in geochemical applications of pyrolysis can provide useful knowledge. In this work, hydrous pyrolysis, as developed for simulated oil generation, is used to compare the reaction pathways of biomass and sedimentary organic matter. The conversion of two non-degraded biomass samples is compared to that of an immature source rock for oil (Kimmeridge clay) and an immature coal (brown coal from the Rhine Graben). Since the biomass samples have not gone through the sedimentary diagenesis stage they are compositionally dierent from immature source rocks, and in particular the oxygen-to-carbon
ratio is high. The biomass used was mixed polymeric material from marine macro-algae, produced in large amounts as a residue during the extraction of alginate for the food industry. The polymers were precipitated from aqueous solution either with acid or calcium ions. From a geochemical perspective, this material represents a possible model for biopolymer input into the early diagenesis stage in the natural conversion process in a marine environment. Comparison of the results from simulated maturation of pure biomass and rocks containing immature sedimentary organic matter is scarcely reported in the literature. Rocha et al. (1997) reported hydropyrolysis of both geochemical and biomass samples, otherwise the more recent pyrolysis development seems to be restricted to either one or other type of starting material. Comparisons can be relevant both in the organic geochemical perspective for determining the importance of reduced oxygen content in the organic matter after diagenesis for the subsequent hydrocarbon generation reactions, as well as in biomass conversion for evaluation of the potential of hydrous pyrolysis processes as a route for conversion of biomass to petroleum compatible liquid fuels. The hydrous pyrolysis experiments for these series were run isothermally at temperatures between 240 and 400 C, with a residence time of 24±72 h. The yields of ¯uid products (oil phase, gas phase and aqueous organic compounds) were quanti®ed, and the amount of residual reactive carbon and generated coke in the solid residue determined. To make it possible to compare the reaction systems, the same set of product fractions was used in all series, and the conversion of the starting material to reactive intermediate products and stable end products was modelled in quantitative reaction networks. The network describes the reaction pathways from the starting materials through the intermediates to the stable end products in terms of kinetic parameters for each step, and stochiometric factors for each product distribution. Such models are frequently used to describe petroleum generation (JuÈntgen and Klein, 1975; Braun and Burnham, 1987; Ungerer and Pelet, 1987), though the validity of the extrapolation to geological time scales can be questioned (Snowdon, 1979). Inverse curve ®tting procedures were used to ®nd the best-®t activation energies and stochiometric factors (Barth and Nielsen, 1993; Nielsen et al, 1996). The resulting reaction models for the dierent starting materials were compared, and the degree of similarity between the fresh and sedimentary biomass types evaluated. 2. Experimental Pyrolysis experiments and data treatment were performed as described in Barth et al. (1989) and Barth and
T. Barth / Organic Geochemistry 30 (1999) 1495±1507
Nielsen (1993). A summary of the procedure is given here: 2.1. Starting material Dry, ®nely powdered and homogenised coal and source rock samples were used as starting materials. The Kimmeridge source rock was from an outcrop in Dorset, UK (Barth et al., 1989). The brown coal is a very immature sample from a lignite opencast mine in the Rhine area, DE, and had a vitrinite re¯ectance (Ro) of 0.26. The biomass samples were supplied by Pronova, NO, as a dry, homogenous powder consisting of mixed biopolymers precipitated from aqueous solution in process water by addition of acid (H-biomass) or calcium ions (Ca-biomass). Typically, the major compound classes in the calcium-precipitated polymers are 30±35% cellulose, 10±20% carbohydrates, 7±14% proteins, 13± 15% water and 9±10% calcium (weight). The total organic carbon content, TOC (in%), was measured for all the starting materials, and for the biomass elemental analysis of hydrogen and oxygen was also made. For the Kimmeridge source rock and the coal sample, RockEval determination of the petroleum potential (S2) and Tmax to indicate initial maturity was performed. The values are given in Table 1. 2.2. Pyrolysis About 5 g of the starting material and sucient water to ensure at least 5 ml liquid water at subcritical pyrolysis temperatures were placed in a 50 or 71 ml stainless steel autoclave which was put in an oven preheated to the chosen temperature. The reactor is then kept isothermally at this temperature for the selected time period (24 h for the biomass, 72 h for the source rock and coals), and then rapidly cooled. The shorter pyrolysis time for the biomass samples was used after initial experiments showing only small dierences in the yields after 24 and 72 h. 2.3. Sampling and analysis Sampling and analysis After cooling, the gas phase was collected quantitatively in a gas sampling bag and Table 1 Initial properties and pyrolysis data for the source rocks and biomasses Origin
TOC S2 Tmax %H %O (%) (mg/g) ( C) (weight) (weight)
Outcrop sample, Dorset Brown coal Biomass polymer, H-form Biomass polymer, Ca.-form
12.6 71.3 38.2 35.06
a
nm: Not measured.
64.8 58.9 nm nm
422 407 nm nm
nma nm 5.24 4.42
nm nm 42.65 41.55
1497
the excess gas volume measured. The gas phase composition was analysed by GC. The reactor was opened and the free water phase was removed using a Pasteur pipette. Any expelled oil present was combined with the bitumen extract (see below). Expelled oil was only observed in some of the higher temperature range experiments with the Kimmeridge source rock pyrolysis. The water phase was analysed for content of short-chain carboxylic acids by isotachophoresis (Barth et al., 1989). The remaining ``sludge'' of coke, bitumen (i.e. oil phase not separated from the solid residues) and water was transferred to a Soxhlet thimble. All reactor surfaces were washed with a dichloromethane/methanol constant boiling mixture (DCM: MeOH 93:7v/v), and the washing solvent containing traces of the solid phase products was added to the thimble, which had been placed in the Soxhlet extractor. A water separator over the extractor was used to remove moisture and ensure complete contact between the solvent and sample. The thimble was extracted with the solvent mixture until no water had separated out for more than 8 h (usually a total extraction period of 48 h). The organic extracted was concentrated to 2±8 ml on a rotary evaporator and the C12+ content of the extract quanti®ed by gravimetric determination of non-volatile components on a microscale. The volume of the solution of the extract was calculated from the weight and solvent density. The extracted Soxhlet thimble with residue was dried in an oven at 80 C, cooled and weighed. 2.4. Product fractions and quanti®cation The product fractions used in the description of the reaction processes are bitumen, aqueous organic acids, carbon dioxide, hydrocarbon gases and coke, and a composite group including the volatile hydrocarbons (C5±C12) that are lost during sample work-up, the unspeci®ed losses and analytical inaccuracies is calculated by dierence to conserve the mass balance in the system. Bitumen is the non-volatile liquid product, and corresponds to crude oil within a source rock. This is the major product from the source rock pyrolysis. Bitumen contains more high molecular weight polar compounds than reservoir crude oil, but this is not critical in the present application. To limit the complexity of the models, sub-fractions of bitumen (e.g. asphaltenes and de-asphalted oil) are not used here, although this makes the models less precise. The experimental uncertainty is not large, and bitumen yields are normally within 10% in parallel experiments. The hydrocarbon gases also represent petroleum type products and the yields are determined with a similar precision. The aqueous short-chain organic acids and carbon dioxide/carbonate (gas, aqueous and solid phase) are the major oxygen-containing fractions. They are minor products in source rock pyrolysis and are often not
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included in hydrocarbon generation models, but the yields increase signi®cantly for the coal and biomass samples where they represent a major part of the carbon in the system. There is a risk that the amount of organic acids produced is systematically underestimated, since a constant volume of water during the pyrolysis is assumed. Carbon dioxide yields are dicult to reproduce experimentally. The reason is not known at present, but a considerable, unsystematic variation in the yields was observed. In the solid phase, the organic phases are de®ned as the initial kerogen or biomass, which for modelling purposes is divided into fractions with increasing activation energies (Barth and Nielsen, 1993), and coke produced in the pyrolysis. Both the reactive carbon and the coke was quantitated from TOC measurements, as described below. The uncertainty in the quanti®cation is estimated to 10%. In hydrous pyrolysis, a mass balance on the weight of the starting material relative to the organic phase products cannot be assumed, since the amount of water produced or consumed in the reactions cannot be quanti®ed. Hydrogen and oxygen from water is probably also incorporated in the organic products. The recoveries and product distributions are therefore calculated on a carbon basis, even if this involves a degree of uncertainty in the estimates of the carbon content for some of the fractions. The following conversion factors were used based on the quanti®ed milligrams of product per gram stating material: The total amount of reactive carbon in the biomass was calculated from the maximum decrease in measured TOC (TOC) corrected for the fraction of solid converted to ¯uids: Reactive carbon (as 10ÿ3 mol C/g)= [TOC(%)10 (mg/%)recovery (g/g)/(12 mg/mmol C)]; unreacted kerogen/biomass was calculated in the same way from the measured TOC values corrected for unreactive carbon; bitumen and S2 (as 10ÿ3 mol C/g)= [yield (mg/g)0.8 (weight of C-atoms per mg)/12(mg/ 10ÿ3 mol carbon)]; CO2 and hydrocarbon gases: calculated precisely from concentration and volume data; organic acids: calculated from the total acid concentration in the aqueous phase and the starting volume of water assuming an average of three carbon atoms per molecule; coke: calculated as the increase of TOC at higher temperature relative to the minimum value; volatiles and loss: calculated by dierence to give the mass balance at a constant level of reactive carbon. To simplify the comparison between dierent starting materials with dierent carbon contents (12.6% for the Kimmeridge source rock, 71.3% for the brown coal), all yields are normalised to the initial carbon content and given as 10ÿ3 mol carbon per gram carbon initially in the material (mM C/g TOC). If all the initial carbon is reactive, the carbon mass balance should be at 83.3 mM C/g TOC. However, there is a fraction of unreactive
carbon in all the materials used, resulting in a lower, but variable level of reactive carbon, as discussed below. 2.5. Reaction network modelling The thermal conversion processes of biomass and sedimentary organic material are complex, and cannot be described in a single chemical reaction. The model of the reaction system assumes that the starting material consists of fractions with dierent reactivity, that intermediate products that react further are produced, and that interconversion from one type of product to another will occur. Since precise chemical models on a molecular basis become much too complex to be possible to establish and calibrate for real data, models describing conversion of fractions of the starting material into product groups are used. The kerogen or biomass is divided into reactions with dierent reactivity, and product fractions are de®ned as reactive intermediates or stable products, and the total system is established as a reaction network. A simple example of a network consists of a kerogen with two reactive fractions, bitumen as an intermediate and coke and gas as stable end products, with three ®rst-order reactions describing the reactions of the two kerogen fractions to the major products for each step: 1. kerogen fraction 1!kerogen fraction 2 and bitumen, 2. kerogen fraction 2!bitumen and gas, 3. bitumen ! gas and coke each with an activation energy and pre-exponential factor to describe the dependence of the reaction rate on temperature, and with a stochiometric factor giving the distribution of products from the reaction. These reactions can be represented as a reaction network in the form of a half matrix, where ±1 refers to the reactant and S to the stochiometric factors giving the fraction converted to each product: Kerl ÿ1 # Ker2 S1 + Bitumen 1ÿS1+ Kerl
Gas Coke
Ker2 Bitument Gas ÿ1 # S2! + 1-S2
ÿ1 # S3! + 1-S3
Coke
stable !stable
When the objective is to estimate values for both the kinetic parameters and stochiometric factors, a large number of parameters need to be determined by ®tting
T. Barth / Organic Geochemistry 30 (1999) 1495±1507
the network to the experimental data, and the number of unknown parameters increase with the complexity of the network. In all cases, the number of unknown parameters is too large to be for each parameter to be precisely calculated from experimental data values, so statistically based curve ®tting procedures are applied. Iterative least-square inverse parameter ®tting procedure as described in Nielsen et al. (1996). This gives a optimal solution with the levels of uncertainties in the experimental data taken into account. The mathematical requirement for a successful curve ®tting is that the iteration converges to a best-®t value, which was the case for all the models presented here. The ®tted variables comprise the activation energies and the stochiometric factors. The pre-exponential factor A of the Arrhenius equation is pre-de®ned and kept at 11013 sÿ1 for all the models. The uncertainty in the parameter values are not given for each parameter, but can be illustrated in Monte-Carlo models based on the best ®t model, and show a considerable range of uncertainty. Such models are not considered part of this presentation, but it should be kept in mind that the models below are a simpli®ed representation of complex processes with inherent uncertainties. The purpose of the models, as presented here, is to enable comparisons and not to perform high precision extrapolations. For the data presented here, the reaction networks were set up with the minimum number of reactions that result in converging models in the inverse ®tting procedure. All reactions are de®ned as ®rst order. The kerogen/biomass reactive potential was divided into ®ve levels (four for the Kimmeridge source rock), corresponding to a discrete distribution of activation energies (Juntgen and Klein, 1975; Braun and Burnham, 1987). Only the best-®t models achieved after extensive testing of alternative reaction networks are shown here. For the coal and the biomass, the network included pathways for internal rearrangement of the starting material during pyrolysis. This gives a continuous conversion from low to high activation energies in the reactant (for example representing increasing aromatisation), which was necessary to make the models converge to a best-®t set of parameters. For the Kimmeridge source rock, a simpler model with an initial distribution of the kerogen in four groups with speci®c activation energies was sucient to describe the data. Bitumen and organic acids were de®ned as intermediates which reacted in a single reaction to give the stable end products carbon dioxide, hydrocarbon gases and coke. The volatiles/loss group was also considered stable. 3. Results For all types of starting material, hydrous pyrolysis converts carbon from a solid phase form into ¯uid pro-
1499
ducts, as shown by the reduction in the solid phase carbon contents with increasing pyrolysis temperatures (Table 2). The apparent increase in carbon content at low pyrolysis temperatures compared to the initial values (Table 1) is accounted for by dissolution of solids into the aqueous phase during the pyrolysis. As shown in Table 2, only a proportion of the total carbon is converted, so the carbon content of the residues does not approach zero. This is well known from source rock evaluation, where the non-reactive fraction is termed ``dead'' carbon. The product distribution for each pyrolysis series is given in Tables 3±6. The yields are mass balanced by calculating the product group of volatiles/loss by dierence. Though the amount of carbon assigned to this group can be a signi®cant proportion of the total amount, the reaction networks still function well in the overall description of reaction pathways. The mass balance is based on the reactive carbon and varies from 24 mM C/g TOC for the brown coal to 73.2 mM C/g TOC for the acid form biomass, which is consistently lower than the 83.3 mM C/g TOC of a totally reactive starting material. The dierence between the reactive carbon used in the mass balance and the total carbon content corresponds to the ``dead'' carbon observed in the TOC measurements of the residues. Including ``dead'' carbon in the reaction networks as coke produced directly from kerogen is feasible, but is not included here since it does not improve the ®t of models and increases the number
Table 2 Measured carbon content of the solid residues after pyrolysis Kimmeridge Brown coal Biomass H Biomass Ca Pyrolysis time (72 h) (72 h) (24 h) (24 h) Pyrolysis TOC TOC TOC TOC temperature ( C) 240 250 260 270 280 290 300 305 310 320(1) 320(2) 330 340 350 360 365 380 400 a b
14.89 ± 14.11 13.65 12.05 11.22 9.33 ± 8.02 5.81 6.62 7.29 7.24 7.85 ± ± ± ±
±a 70.66 ± 69.25 ± 69.43 ± 70.03 ± 69.35 69.40 68.72 68.76 nmb ± 67.45 ± ±
51.69 ± ± 47.23 ± ± 35.16 ± ± 28.08 ± 20.21 ± ± 21.34 ± 18.20 24.05
±: no experiment at this temperature. nm: not measured.
70.58 ± ± 57.95 ± ± 52.63 ± 52.85 ± ± 49.51 ± 49.28 ± 47.12 45.06
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T. Barth / Organic Geochemistry 30 (1999) 1495±1507
Table 3 Pyrolysis data for the Kimmeridge source rock, mass balanced to 53.3 mM C/g TOC Pyrolysis temperature ( C)
Kerogen
Bitumen
Organic acids
CO2
Vol/lossa
HC-gasesb
Cokec
250 260 270 280 290 300 310 320 330 340 350
39.30 36.10 35.30 29.30 20.80 15.24 8.90 7.44 5.90 5.00 2.90
4.86 5.94 9.39 13.56 18.74 24.44 29.47 33.61 31.10 26.16 22.00
0.54 0.56 0.60 0.64 0.71 0.76 0.83 0.92 0.99 0.96 1.09
1.50 2.46 3.17 3.02 2.86 4.05 4.92 5.75 6.50 7.70 8.75
7.08 8.19 4.52 6.16 9.98 8.37 8.58 4.75 7.38 7.98 6.58
0.02 0.05 0.32 0.62 0.17 0.44 0.60 0.83 1.43 1.50 1.98
0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 4.00 10.00
a b c
Volatiles and loss calculated by dierence. C1ÐC5 hydrocarbon gases. Non-zero value required as input to the modelling.
Table 4 Pyrolysis data for the brown coal, mass balanced to 24 mM C/g TOC Pyrolysis temperature ( C)
Kerogen
Bitumen
Organic acids
CO2
Vol/
HC-gases
250 270 290 305 320 320 330 340 350 365
15.71 14.22 10.40 11.78 9.47 10.33 6.44 8.08 6.00 2.86
1.36 1.53 2.55 3.99 4.75 5.11 6.03 6.86 5.98 5.35
1.15 1.23 1.83 1.41 1.98 1.98 2.12 2.00 1.94 1.88
3.22 3.49 4.55 2.77 2.76 2.81 3.25 2.20 3.86 4.48
2.48 3.38 4.37 3.76 4.59 3.55 5.34 3.99 4.68 7.20
0.08 0.15 0.30 0.29 0.45 0.22 0.82 0.87 1.54 2.23
Table 5 Pyrolysis data for the acid precipitated biomass polymer, mass balanced to 73.2 mM C/g TOC Pyrolysis temperature ( C)
Kerogen
Bitumen
Organic acids
CO2
Vol/loss
HC-gases
Coke
240 270 300 320 340 360 380 400
33.08 29.36 19.30 13.37 6.84 7.79 5.17 5.04
3.56 11.69 15.09 19.00 21.93 22.18 15.46 5.46
11.90 11.15 8.75 7.04 7.14 7.32 6.57 10.73
12.25 13.80 14.82 14.73 15.22 21.53 20.89 20.90
12.44 7.13 14.95 17.43 20.73 11.87 20.13 18.52
0.07 0.17 0.39 1.73 1.44 2.61 4.98 7.65
0.01 0.01 0.01 0.01 0.01 0.01 0.11 5.01
of parameter values that have to be estimated in the curve ®tting. On a carbon basis, the Kimmeridge source rock yields most bitumen, with a maximum of 33.6 mM C/g TOC.
The biomass samples give much higher yields than the brown coals, at 22.2 mmol C/g TOC for the H-biomass and 21.4 mmol C/g TOC for the Ca-biomass, compared with only 6.9 mmol C/g TOC for the brown coal. The
T. Barth / Organic Geochemistry 30 (1999) 1495±1507
1501
Table 6 Pyrolysis data for the Ca2+ precipitated biomass polymer, mass balanced to 65 mM C/g TOC Pyrolysis temperature ( C)
Kerogen
Bitumen
Organic acids
CO2
Vol/loss
HC-gases
240 270 300 320 340 360 380 400
28.29 18.28 13.86 14.04 11.26 11.07 9.27 7.55
6.74 13.87 21.44 20.74 20.26 18.42 12.47 7.93
10.00 9.38 7.80 8.20 8.66 7.50 6.64 6.03
16.23 23.20 19.90 18.62 21.69 20.41 20.68 24.10
3.67 0.10 1.50 2.34 1.55 5.96 10.98 11.88
0.07 0.17 0.50 1.06 1.58 1.64 4.96 7.51
biomass samples are characterised by very high yields of carbon dioxide in the gas phase and short-chain carboxylic acids in the aqueous phase, re¯ecting their high initial oxygen content. The two biomass forms gave dierent temperature dependencies in their reaction patterns, which shows that the incorporation of acid or calcium ions in¯uences the reaction pathway in the conversion process, even though the organic composition is the same. Precipitation of calcium carbonate (CaCO3) in the residue was observed after pyrolysis of the calcium-containing biomass, while the levels of carbonate in the aqueous phase were lower than for the acid precipitated biomass. At the highest temperatures, a signi®cant increase in the organic carbon content is observed for the solid residue from the Kimmeridge source rock and the Hbiomass. This shows that there is a quantitatively signi®cant generation of coke in these series (Tables 3 and 5). To keep the reaction networks as simple as possible, no coke product has been included in the series where the carbon content of the residues decline steadily, though the formation of coke may be masked by concurrent reduction of reactive carbon. Table 7 shows the reaction network for the Kimmeridge source rock, and Fig. 1 shows the ®tted model (continuous lines) and the measured data (points). The amount of reactive carbon or generation potential (53.3 mM C/g TOC) is equivalent to a Rock-Eval S2 (hydrocarbon potential) value of 100.7 mg/g source rock, and considerably higher than the measured value of 64.8 mg/g (Table 1). Four kerogen fractions are the minimum number needed to ®t the experimental data, and they are distributed with ®xed fractions of 0.13 of the total potential at an activation energy of (Ea) 176 kJ/mol, 0.30 at Ea=191 kJ/mol, 0.38 at Ea=201 kJ/mol and 0.19 at Ea=221 kJ/mol. The resulting model for the kerogen degradation corresponds well with the experimental data. All kerogen fractions generate bitumen in substantial amounts, re¯ecting that this is a high-quality oil-prone source rock. The bitumen is quite temperature stable, with an Ea=231 kJ/mol. The bitumen generation and degradation in the model goes through a wider
maximum than the measured values, which can be taken as an indication the single bitumen fraction is an over simpli®cation and that including separate generation and conversion of the heavy products (asphaltenes, resins) and the light hydrocarbon oil would improve the precision of the model. CO2 and aqueous organic acids (SCA) are generated both from the intermediate kerogen fractions and the bitumen, while hydrocarbon gases and coke are only generated at high degrees of conversion. The contribution from the volatile/loss group is small, re¯ecting low experimental uncertainties. Overall, the experimental data are well ®tted. The model is extrapolated to 400 C to enable comparison with the other series, even though no measured data points constrain the model in the highest temperature range. Table 8 shows the reaction network for the brown coal. The total reaction potential is again much higher than indicated by the Rock-Eval S2 (Table 1), corresponding to a S2 value of 257 mg/g. Dividing the coal into ®ve initial fractions and including internal rearrangement within the kerogen/bitumen fractions was necessary to establish a well-®tted model. This results in a much wider temperature range of reaction compared to the Kimmeridge source rock, with activation energies ranging from 180 to 240 kJ/mol. Fig. 2 shows the correspondingly wide range of temperatures for the coal reactions, as opposed to the steep curve of the kerogen degradation for the Kimmeridge source rock (Fig. 1). The ¯uid yields are low, and no bitumen production is assigned to the lowest and highest activation energy fraction. The proportion of oxygen containing products (CO2, SCA) is high, which explains some of the increase in reaction potential relative to S2. The volatiles/loss product group is quantitatively the second largest, indicating an increased uncertainty in the quantitative model. Overall, this coal has a minimal potential as a hydrocarbon source rock, as expected from general petroleum geochemical evaluations of coals as a source for oil. The reaction network for the acid precipitated biomass polymer sample is given in Table 9. The network is similar to the brown coal but with an even wider range
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T. Barth / Organic Geochemistry 30 (1999) 1495±1507
Table 7 Reaction network for the Kimmeridge source rock Pre-exponential factor A=constant=1013 sÿ1 K/B1a (176) Ea (kJ/mol) Initial fraction 0.13 Kero/bio 1a Kero/bio 2
ÿ1 #
Kero/bio 3
K/B2
K/B3
K/B4
Bit.
SCAb
(191) 0.30
(201) 0.38
(221) 0.19
(231)
(239)
ÿ1 #
ÿ1 #
Kero/bio 4 Bitumen SCA
0.40 +
CO2 Vol./loss HC-gasesd Coke a b c d
0.60
+ 0.65 + 0.08 + 0.27
+ 0.75 + +
ÿ1 # + 0.26 +
+ 0.18 + 0.07
+
0.40 +
+ 0.34
! ÿ1 # 0.18 + + 0.10 + 0.20 + 0.29 + + 0.22
! ÿ1 # + 1.50 + + 0.50
CO2
V/Lc
HCgd
Coke
!stable !stable !stable !stable
K/B and Ker/bio: reactant fractions in kerogen or biomass. SCA: short chain carboxylic acids. V/L and Vol/loss: volatiles and loss calculated by dierence. HC-gases and HCg: C1ÐC5 hydrocarbon gases.
Fig. 1. Data points and ®tted curves based on the reaction network for the Kimmeridge source rock.
of activation energies (167±237 kJ/mol) and the addition of coke as a product group. Bitumen is again generated only in the intermediate activation energy reactions, but the amounts are much higher than from the coal, as
illustrated in the ®tted curves in Fig. 3. Carbon dioxide is modelled as generated early in the process and remaining at a stable level. However, the measured CO2 yields show considerable unsystematic variation, so the
T. Barth / Organic Geochemistry 30 (1999) 1495±1507
1503
Table 8 Reaction network for the brown coal Pre-exponetial factor A=constant=1013 sÿ1 K/B1
K/B2
K/B3
K/B4
K/B5
Bit.
SCA
(180) Ea (kJ/mol) Initial fraction 0.98
(190) 0.004
(205) 0.004
(220) 0.004
(240) 0.004
(234)
(240)
Kero/bio 1 Kero/bio 2 Kero/bio 3
ÿ1 # 0.76 +
Kero/bio 4
! ÿ1 # 0.75 +
Kero/bio 5 Bitumen SCA CO2 Vol/loss HC-gases
0.11 + 0.13
+
0.12 + 0.10 +
+ 0.03
! ÿ1 # 0.59 + + 0.24 + +
+ +
0.07 + 0.10
! ÿ1 # 0.43 + + 0.23 + 0.09 + + + 0.25
! ÿ1 # +
0.50 + + 0.50
! ÿ1 # 0.10 + 0.20 + + 0.41 + 0.29
! ÿ1 # + 0.05 +
CO2
V/L
HCg
!stable
+ 0.50
!stable !stable
Fig. 2. Data points and ®tted curves based on the reaction network for the brown coal.
curve ®tting is uncertain both for this product and for the volatiles/loss product group. The bitumen is generated over a very wide temperature range and the maximum amount is again underestimated, indicating that a more detailed representation of the liquid products could improve the model. The volatiles/loss fraction is considerable, and may re¯ect a systematic underestimation of aqueous products yields as discussed above.
Table 10 shows the reaction network for the calcium precipitated biomass and Fig. 4 shows the ®tted model. It is similar to the coal model in that no coke generation is observed. The activation energy range is slightly wider (163±238 kJ/mol) than for the acid precipitated biomass model. Carbon dioxide is the major product in the model, in more than equivalent amounts to bitumen. About half of the measured yield is found as solid calcium carbonate. This trapping of carbon dioxide gas
1504
T. Barth / Organic Geochemistry 30 (1999) 1495±1507
Table 9 Reaction network for the H-form biomass Pre-exponential factor A=constant=1013 sÿ1 K/B1
K/B2
K/B3
K/B4
K/B5
Bit.
SCA
(167) Ea (kJ/mol) Initial fraction 0.98
(182) 0.004
(202) 0.004
(222) 0.004
(237) 0.004
(237)
(237)
Kero/bio 1 Kero.bio 2 Kero/bio 3
ÿ1 # 0.67 +
Kero/bio 4
! ÿ1 # 0.59 +
Kero/bio 5 Bitumen SCA CO2 Vol./loss HC-gases Coke
0.11 + 0.22
+
0.25 +
+ 0.16
! ÿ1 # 0.40 + + 0.22 + 0.06 + + 0.32
! ÿ1 # 0.35 + + 0.26 + +
+
! ÿ1 # 0.31 +
0.20 +
+
0.19
+ 0.59 + 0.10
! ÿ1 # + 0.10 + 0.10 + 0.21 + + 0.39 + + 0.20
! ÿ1 # + 0.51 + + 0.49
CO2
V/L
HCg
Coke
!stable !stable !stable !stable
Fig. 3. Data points and ®tted curves based on the reaction network for the H+-precipitated biomass polymer.
probably induces increased generation and can explain why the yields are much higher than in the previous series. Bitumen generation is again slightly underestimated in the model relative to the experimental values. Although the level of the volatiles/loss group is low and
indicates a reasonably good experimental precision, the correspondence between the data and the model is not as good as in the previous series. However, the main trends are reproduced suciently well for use in the comparison of the series.
T. Barth / Organic Geochemistry 30 (1999) 1495±1507
1505
Table 10 Reaction network for the Ca-form biomass Pre-exponential factor A=constant=1013 sÿ1 K/B1
K/B2
K/B3
K/B4
K/B5
Bit.
SCA
(163) Ea (kJ/mol) Initial fraction 0.98
(183) 0.005
(203) 0.005
(223) 0.005
(238) 0.005
(237)
(242)
Kero/bio 1 Kero/bio 2 Kero/bio 3
ÿ1 # 0.58 +
! ÿ1 # 0.54 +
Kero/bio 4 Kero/bio 5 Bitumen SCA
0.13 + 0.20
CO2
+
0.38 +
+
Vol./loss
0.06 + 0.02
HC-gases
! ÿ1 # 0.62 + + 0.23 + 0.07 + 0.08 + +
! ÿ1 # 0.59 + + 0.15 + +
! ÿ1 # + 0.20 +
+ 0.08 + 0.18
+ 0.42 + + 0.38
! ÿ1 # 0.10 + 0.19 + + 0.46 + + 0.25
! ÿ1 # + 0.51 +
CO2
V/L
HCg
!stable !stable
+ 0.49
!stable
Fig. 4. Data points and ®tted curves based on the reaction network for the Ca2+-precipitated biomass polymer. Table 11 Comparison of parameters from the reaction network modelsa Sample
Reactive carbon (%)
Ea range K/B (kJ/mol)
Ea Bit. (kJ/mol)
Major products
Kimmeridge Brown coal H-biomass Ca-biomass
63 29 88 78
176±221 180±240 167±237 163±238
231 234 237 237
Bitumen, CO2, coke Bitumen, CO2. HC-gases Bitumen, CO2. SCA CO2, bitumen, SCA
a
Volatiles/loss not included in the yield evaluation.
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T. Barth / Organic Geochemistry 30 (1999) 1495±1507
4. Discussion Table 11 gives a comparison of the models for the four series of experiments. The Kimmeridge source rock is distinguished by its high level of reactive carbon, simple reaction network and narrow range of activation energies for the kerogen degradation, and the high quantities of bitumen in the products. This can be understood as the natural diagenesis having removed the reactant groups that generate the oxygen-containing products, leave a more uniform, hydrocarbon-producing organic solid as kerogen. The remaining input materials have not been through such a ``pre-processing'', and a higher proportion of the carbon is used to generate carbon dioxide and aqueous organic acids from oxygen contained in the organic solid. The brown coal has the lowest level of reactive carbon, re¯ecting a low hydrogen to carbon ratio and tendency towards aromatisation reactions in the solid phase rather than ¯uid generation, and the amount of bitumen produced is quite low even if it is the major product. The biomasses have a very high proportion of reactive carbon, which to some degree compensates for the high yield of oxygen containing products and results in a considerable potential for bitumen generation. The range of activation energies is even wider than for the coals, and though the temperature range of the hydrous pyrolysis is expanded into the supercritical region (>374 C), the initial reactions have not gone to completion. In this, the biomass reactions re¯ect a more varied initial composition than the sedimentary materials. However, the correspondence in the reaction systems is also considerable. The biomasses are ``better source rocks'' than the brown coal, but the reaction networks resemble the coal more than the oil-prone source rock. The system of fractions used in the networks are suitable for all the staring materials, and the patterns in the reaction systems are similar though there are quantitative dierences. The activation energy ranges for kerogen/biomass degradation and the bitumen reactions are also similar. Experience acquired and models developed for organic geochemistry and coal processing thus can provide a very useful frame of reference for further development of processes for conversion of biomass into petroleum-compatible fuels, especially if the aim is to produce petroleum compatible liquids. The potential for such conversion is clearly considerable, although the yields of hydrocarbon-rich ``oil'' from biomass will necessarily be limited by the lower carbon content and higher oxygen content in the biomass. Because of this, oxygen-containing by-products like carbon dioxide and short-chain carboxylic acids can consume a considerable part of the initial carbon. Adjustment of pyrolysis conditions to minimise the amounts of these products is
required if the objective is a high yield of hydrocarbonrich ¯uids. Alternatively, pre-treatment of the biomass corresponding to the natural diagenesis of source rock organic matter should be considered. Acknowledgements The author thanks Reidun Myklebust for data on the brown coal pyrolysis and éystein Skeidsvoll for data on pyrolysis of marine algael biomass. Brendan Keely and Richard Patience are thanked for constructive and useful reviews.
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