Co-gasification of petroleum coke and biomass

Fuel 117 (2014) 870–875

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Co-gasification of petroleum coke and biomass Vera Nemanova ⇑, Araz Abedini, Truls Liliedahl, Klas Engvall Department of Chemical Engineering and Technology, Chemical Technology, KTH Royal University of Technology, 100 44 Stockholm, Sweden

h i g h l i g h t s  Co-gasification of biomass and petroleum coke in an atmospheric bubbling fluidised bed and thermogravimetric analyzer.  Synergetic effect between biomass and petroleum coke was observed.  The activation energy for pure petcoke is 121.5 kJ/mol, for the 50/50 blend it is 96.3, and for the 20/80 blend – 83.5 kJ/mol.  Biomass ash has a catalytic effect on the reactivity of petcoke due to the alkali and alkaline earth metals present in ash.

a r t i c l e

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Article history: Received 4 July 2013 Received in revised form 5 September 2013 Accepted 17 September 2013 Available online 12 October 2013 Keywords: Activation energy Biomass Co-gasification Fluidised bed Petroleum coke

a b s t r a c t Gasification may be an attractive alternative for converting heavy oil residue – petroleum coke into valuable synthetic gas. Due to the low reactivity of petroleum coke, it is maybe preferable to convert it in combination with other fuels such as biomass. Co-gasification of petroleum coke and biomass was studied in an atmospheric bubbling fluidised bed reactor and a thermogravimetric analyser (TGA) at KTH Royal University of Technology. Biomass ash in the blends was found to have a catalytic effect on the reactivity of petroleum coke during co-gasification. Furthermore, this synergetic effect between biomass and petcoke was observed in the kinetics data. The activation energy Ea determined from the Arrhenius law for pure petcoke steam gasification in the TGA was 121.5 kJ/mol, whereas for the 50/50 mixture it was 96.3, and for the 20/80 blend – 83.5 kJ/mol. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Currently, the world production of petroleum coke is steadily expanding due to an increased consumption of oil and oil-related products [1–3]. The low price, high calorific value, low ash content and abundant availability of petcoke make it a very attractive fuel. Petroleum coke (petcoke) is a by-product produced from heavy crude oil residues by thermal cracking processes – delayed or fluid coking (Fig. 1). Bayram et al. [4] reported that from 1 tonne of crude oil approximately 31 kg of petcoke is produced. Petroleum coke is a black solid material consisting of polycyclic aromatic hydrocarbons with low hydrogen content. The typical elemental composition of petcoke depends on the crude oil origin, but usually it is composed of C: 91–99.5%, H: 0.035–4%, S: 0.5–8%, (N + O): 1.3–3.8% and the rest is metals [5]. The manufacture of carbon anodes for the aluminium smelting industry, graphite electrodes for steel production and also nonferrous industries are the main non-energy applications of petcoke [5]. In Russia petcoke usage is higher than supply due to the ⇑ Corresponding author. Tel.: +46 768447559. E-mail address: [email protected] (V. Nemanova). 0016-2361/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2013.09.050

demand in aluminium and electrode industries. In 2005 more than 650 thousand tonnes of petroleum coke were imported from China (65%), Turkmenistan, Japan, Kazakhstan, England and the United States [6]. According to Akpabio and Obot [7] around 75% of the world’s petroleum coke is used in the energy sector (mostly in North America). In the past, petroleum coke has often been burned as fuel in boilers for heat and power production. However, the current increasing use of natural gas, which is more convenient and cleaner to burn, has decreased the demand for petcoke [8]. Therefore, new alternatives for utilisation of petroleum coke are of interest. Gasification may be an effective option for converting petcoke into syngas [9]. Gasification is a thermo-chemical process in which feedstock undergoes partial oxidation reaction with an oxidising medium (oxygen, air, carbon dioxide or steam) to produce permanent gases. The process consists of several steps: drying, pyrolysis, gasification and combustion, occurring simultaneously inside a gasifier. The product composition, including solid, liquid and gas phases, depends on feedstock (type of fuel, moisture, composition) and gasification operating conditions (oxidising medium, gasifier type, temperature, pressure etc.) [10].

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V. Nemanova et al. / Fuel 117 (2014) 870–875 Table 1 Ultimate and proximate analysis of pine pellets and petroleum coke.

Fig. 1. Schematic view of petroleum coke production from crude oil.

Due to the high fixed carbon content in petroleum coke, it may be complicated to gasify separately [11,12]; therefore it is, perhaps, preferable to convert petcoke in combination with additional fuels such as coal or biomass. Fermoso et al. [13–15] investigated gasification of a mixture of coal, petcoke and biomass at atmospheric and high pressure in a fixed bed reactor. The results showed that binary (coal–biomass) and ternary blends (coal–petcoke–biomass) have some synergetic effects that enhance gas production. Zhan et al. [12] used a thermogravimetric system, TherMax 500, to gasify Chinese petroleum coke and lignite with carbon dioxide. They reported low reactivity of petcoke, due to its small BET surface area and carbon network structure similar to graphitic carbon. However, when petcoke was mixed with lignite the reactivity increased. There are also other studies where petcoke is gasified using different catalysts to increase its low reactivity. Zhou et al. [16] tested the catalytic activity of iron species for CO2 petcoke gasification. They found that the gasification reactivity of petroleum coke was enhanced with higher gasification temperature and increasing iron species concentration. Several researchers have carried through simulations and modelling in regard to petcoke conversion into permanent gases [17–19], and others have investigated co-gasification of petcoke and coal [1,11,20] and the ternary mixture (petcoke–coal–biomass) [13–15]. However, there are no studies on co-gasification of petcoke and biomass. The aim of the present work is to investigate the effect of biomass on the reactivity of petroleum coke by co-gasifying in a bubbling fluidised bed reactor. Several tests with different petcoke contents mixed with biomass have been carried out in the gasifier. The reactivities of petcoke, biomass and various petcoke–biomass blends were also determined by thermogravimetric analysis.

Pine pellets

Petroleum coke

Proximate analysis, as delivered Moisture, wt% Ash, wt% Volatiles, wt% Fixed carbon (by diff.), wt% Lower heating value (MJ/kg)

4.5 0.5 76.5 18.5 19.2

0.5 1.4 6.0 92.1 36.2

Ultimate analysis, dry basis Carbon, wt% Hydrogen, wt% Nitrogen, wt% Oxygen (by diff.), wt% Sulphur, wt%

47.7 6.3 0.16 45.3 <0.012

92.3 3.4 0.95 0.7 1.168

At first glance petroleum coke seems to have better properties compared to biomass, such as higher heating value and higher carbon content. However, petcoke also contains significantly more sulphur. Pinto et al. [21] have examined the influence of different catalytic materials on removal of sulphur and halogens compounds during co-gasification of coals with different types of wastes, such as pine, petcoke and polyethylene. They reported that the presence of dolomite in the bubbling fluidised bed led to the highest sulphur reduction. However, this paper focuses on the gasification process and does not touch upon sulphur contamination or the consequences thereof. 2.2. Gasification setup Two series of experiments have been carried out: gasification tests performed in 5 kW bubbling bed and thermogravimetric analyses of different mixtures of petroleum coke and biomass in the thermogravimetric analyser (TGA). The atmospheric fluidised bed gasification system includes a fuel feeder, a gas supply system, gas preheater, and a bubbling fluidised bed gasifier. A schematic view of the gasifier is shown in Fig. 2. The gasification reactor consists of a fluidised bed (an inner diameter of 0.05 m and a height of 0.30 m) and a freeboard (a

2. Materials and methods 2.1. Materials Pine pellets in the size range of 1.5–2 mm and ground and sieved petroleum coke (1–1.5 mm) provided by Statoil, Norway were used in the experiments. The results of ultimate and proximate analysis are listed in Table 1.

Fig. 2. Schematic view of the KTH atmospheric fluidised bed reactor.

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V. Nemanova et al. / Fuel 117 (2014) 870–875

diameter of 0.10 m and a height of 0.45 m). The total volume of the reactor is 5.1 L. The reactor is heated with external heaters and the maximum temperature is 950 °C. The bed material in the reactor is 350 g a-alumina with a particle size of 63–125 lm and density 3960 kg/m3. The fluidisation medium is nitrogen while the oxidising agent is pure oxygen. Before entering the reactor the fluidisation and oxidation media are pre-heated to 650 °C. When the system is heated up to the desired temperature the fuel is fed, with help of a screw feeder, directly into the fluidised bed near the gas distribution plate at the bottom. The fuel hopper is provided with nitrogen purge and a cooling system to prevent hot gases from entering the hopper and making the fuel sticky. After each experiment the remaining fuel and bed material were weighed in order to calculate the unconverted fuel content and as well as the feeding rate. The unconverted material was sieved and the fractions of unconverted petcoke and biomass were determined. The cool, dry, clean gas composition was analysed with a gas chromatograph (GC), equipped with a flame ionisation detector (FID) and a thermal conductivity detector (TCD). The tar sampling and analysis were accomplished using the solid phase adsorption (SPA) method. In this method, a 100 ml gas sample is manually drawn through a solid phase extraction (SPE) tube during 1 min. Later the SPE tube is eluted using two different solvent mixtures to obtain an aromatic fraction and a phenolic fraction. The eluates are afterwards analysed by gas chromatography. The details of the SPA method is described elsewhere [22]. Gas composition and tar samples were both taken every 10 min, after the system was in the steady-state condition. The data reported is an average of the 12–15 gas and tar values. 2.3. Thermogravimetric setup Thermogravimetric analysis was carried out using a Netzsch STA 449 F3 Jupiter apparatus, equipped with an electrical steam furnace with a maximum working temperature of 1250 °C. Steam was generated in a Bronkhorst High-Tech unit and led with a rate of 86 ml/min together with nitrogen of high purity (99.99%) (a rate: 67 ml/min) through a transfer line, heated up to 200 °C to avoid steam condensation. In the thermogravimetric analyser, the same nitrogen at a flow rate of 100 ml/min was used as protective gas. The original petcoke was milled and a 100 mg sample was heated to the desired 1200 °C at a rate of 10 °C/min, at total pressure of 1 bar. All experiments were performed two or three times in order to examine the reliability of the test data. The results for the same tests showed a good reproducibility (<3%). 3. Results and discussion 3.1. Gasification tests The gasification conditions for the five experiments are summarised in Table 2. Table 2 Gasification operating conditions. Gasification test

I

II

III

IV

V

Petcoke concentration, % Petcoke fed, g Biomass fed, g Total gasification time, min Bed temperature, °C Nitrogen flow rate, L/min Oxygen flow rate, L/min Oxygen concentration, %

0 0 569 185 800 8.45 1.08 11

20 117 467 166 800 8.45 1.08 11

50 292 292 150 800 8.45 1.08 11

20 117 467 158 900 7.90 1.00 11

20 117 467 159 900 7.64 1.24 14

Fig. 3. Carbon mass balance for experiments.

For all gasification experiments the carbon mass balance was calculated based on fuel input (carbon content) and output (gas composition measured by GC, tar content and measured unconverted char from biomass and petcoke) and the results are shown in Fig. 3. The found carbon mass balance is in the range of 92–98%. Light carbons CxHylight include CH4, C2H2, C2H4 and C2H6. Heavy carbons CxHyheavy (tar) are hydrocarbons with a molecular mass higher than benzene. One interesting observation is that the amount of unconverted biomass in the different blends seems to be almost constant in all experiments and does not depend on the gasification temperature, oxygen concentration or petcoke content in the blend. Petcoke conversion in the blends increases with higher gasification temperature (compare experiments II and IV) and higher oxygen content in the input gas (experiments IV, V). It can also be observed that the more petcoke in the mixture, the higher the content of unconverted petcoke (experiments II and III). The main results, such as unconverted matter in the bed, product gas composition, total tar concentration and benzene, fuel conversion and product gas yield during co-gasification, are listed in Table 3. Conversion of biomass, petcoke and overall process is calculated based on fuel input (Table 2) and output (Table 3), respectively. The lowest overall conversion was found in the 50/50 petcoke/ biomass blend. Using a 20/80 mixture it may be possible to enhance the overall conversion from 85.2% to 89.6% by increasing gasification temperature from 800 to 900 °C; by raising oxygen concentration in the fluidisation gas from 11% to 14% the overall conversion may be increased further up to 90.8%. Petcoke conversion in a 20/80 mixture increases by 19% when the gasification temperature is raised by 100 °C (experiments II and IV) and a further 8.6% when the concentration of oxygen in the input gas is increased by 3%. Biomass conversion four of the experiments is 94–95%, and only in the test with high petroleum coke content (experiment III), the conversion decreases to around 90%. The product gas yield decreases with higher petcoke content in the mixture due to a higher percentage of unconverted petcoke. More gas is obtained at higher gasification temperature and oxygen content due to higher fuel and tar conversation (tests IV and V). The gas composition at a gasification temperature of 800 °C for different blends can be considered constant: the CO2 concentration is the range of 23–24%, H2 29–33%, CH4 8–9% and CO content in the range of 33–37%. At the higher temperature, 900 °C, the main differences in the gas composition can be observed for CO2 and CO (experiments II and IV): the CO2 concentration is strongly

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V. Nemanova et al. / Fuel 117 (2014) 870–875 Table 3 Results of gasification tests with biomass and petcoke-biomass blends. Gasification test number

I

II

III

IV

V

Unconverted material, g Biomass Petcoke Total

30 – 30

25 61 86

28 161 189

22 39 61

29 25 54

Conversion, % Biomass Petcoke Overall Product gas yield, Nm3 dry/kg fuel Fuel residue, g/kg fuel Tar content, g/Nm3 dry (excluding benzene) Benzene, g/Nm3

94.9 – 94.9 1.18 52.7 1.9 1.5

94.6 47.6 85.2 1.02 140.6 1.2 1.1

90.4 44.8 67.6 0.87 346.2 1.1 1.1

95.3 66.7 89.6 1.18 107.2 0.9 1.5

94.6 75.3 90.8 1.21 77.3 0.8 1.6

24.1 29.3 8.1 37.0 1.5

23.2 29.5 9.2 36.5 1.6

23.3 31.4 9.1 34.8 1.4

7.3 30.8 6.4 54.9 0.6

7.2 28.4 7.2 55.9 0.8

Gas composition, vol.% dry gas, N2 Free CO2 H2 CH4 CO C2H2 C2H4

reduced from 23% to 7% and the CO content is increased from 36% to 55%. This effect can be explained by water–gas shift reaction:

CO þ H2 O $ CO2 þ H2 þ 41 kJ=mol The tar content in the product gas diminishes with higher gasification temperature and higher oxygen concentration (experiments IV and V); these factors that have been reported in the literature [23]. Tests I–III indicate that most of tar in the gasification gas originates from the biomass: the less biomass in the mixture, the lower total tar concentration in the product gas. Normally, benzene is not classified as a tar component. However, it is a precursor for carbon deposition in downstream catalytic processes. Benzene is also an important product and an intermediate in the complex tar reaction network of cracking heavier tars. As summarised in Table 3, benzene presence in the product gas approximately equals the sum of the tar components. In contrast to total tar content versus temperature dependence, the concentration of benzene in the product gas increases with higher gasification temperature and higher oxygen content in the fluidisation gas. This fact can be explained by thermal cracking of heavier tars into smaller molecules including benzene [24]; the detailed tar content is thus needed. Fractions of the different tar components in the product gas were measured and the concentration of each tar component in the total tar was calculated, the results are given in Table 4. The molecular mass of the chromatographically determined tar compounds were in the range between 78 and 202 g/mol.

The higher the gasification temperature (experiment IV), the amounts of toluene and indene are significantly lower compared to the experiment conducted at 800 °C (test II). According to Devi et al. [24] and Milne et al. [25], naphthalene is the main and most stable component of the tar compounds. As shown in Table 4 naphthalene concentration is in the range of 36–52% of the total tar content, independently of the gasification temperature. Toluene, indene, acenaphthylene and phenanthrene are present in the range of 23–39% of the total amount. 3.2. Thermogravimetric analysis (TGA) In order to investigate the co-gasification of petcoke and biomass in detail, thermogravimetric tests were performed. Steam gasification of various blends of petcoke and biomass was carried out in a thermobalance. The thermogravimetric curves (the mass losses), obtained for biomass and petroleum coke, are illustrated in Fig. 4. As shown in Fig. 4, biomass gasification occurs in two steps: fast pyrolysis (in the temperature range of 200–400 °C) and subsequent slow gasification of the char remaining from biomass in the temperature range of 400–900 °C. Petroleum coke has very low reactivity due to its aromatic nature and high heavy aromatic-to-aliphatic ratio [4]. It has previously been reported that aromatic carbon atoms are less reactive than aliphatic carbons [26]. Gasification of petcoke commences

Table 4 GC-detectable tar components in the product gas (% of total tar, excluding benzene).

Toluene m/p-Xylene Indan Indene Naphthalene 1-Methylnaphthalene Acenaphthylene Fluorene Phenanthrene Anthracene Fluorantene Pyrene

I

II

III

IV

V

18.4 1.1 5.4 10.4 44.7 1.3 8.3 2.0 4.4 1.2 1.4 1.3

12.9 2.1 1.4 10.0 42.3 1.0 8.6 3.2 8.6 2.7 3.6 3.5

12.1 4.5 2.3 10.9 36.4 2.3 9.2 3.2 9.0 2.5 4.0 3.7

2.3 5.1 3.1 5.1 44.2 2.2 14.0 0.9 9.9 1.2 5.9 6.1

2.8 5.6 3.0 5.6 51.3 2.2 9.8 0.9 7.2 0.7 5.2 5.9

Fig. 4. Steam gasification of petcoke and biomass using TGA.

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V. Nemanova et al. / Fuel 117 (2014) 870–875

at temperatures above 800 °C (Fig. 4), when the biomass is almost gasified. In other words, co-gasification of petroleum coke and biomass in the thermobalance does not occur simultaneously, but sequentially. Therefore, from the curves obtained during single component gasification, it is possible to calculate new numerical data for different petcoke–biomass blends based on their relative proportions in the mixtures using the following equation:

mm ¼ mp x þ mb y

ð1Þ

where mm is the mass losses for a given mixture (%), mb, mp is mass losses for biomass and petcoke at the same temperature, respectively, obtained experimentally (%), x, y is the petcoke and biomass concentrations in the given mixture (–). Using the previously mentioned equation new theoretical plots have been calculated for the mixtures 50/50 (x = 0.5, y = 0.5) and 20/80 (x = 0.2, y = 0.8). Steam co-gasification tests were performed in the TG for the same blends, and the calculated and experimental plots are shown in Fig. 5. Fig. 5A illustrates an excellent correlation between calculated and experimental data for the mixtures (50/50, 20/80) up to 900 °C (90 min). After 90 min a difference between the theoretical and experimental values may be noticed (Fig. 6B): gasification of the 50/50 blend ends 6 min earlier than calculated, for the 20/80 mixture the difference between experimental and theoretical gasification completion is 13 min. In Fig. 5 this is summarised, the more biomass in the mixture, the higher the difference between calculated and experimental values, the less time/lower temperature needed to complete the co-gasification process.

Fig. 6. Relation between reaction rate ln r (min1) and temperature 1000/T (K1).

In accordance with the results obtained during co-gasification, it can be concluded that there is a synergetic effect between petcoke and biomass in high-temperature steam co-gasification due to biomass residue – ash. In other words, the more biomass ash in the mixture with petcoke, the faster complete gasification is achieved. Biomass ash has early been found to have a good catalytic effect on tar reduction [27]. Detailed ash analysis is presented in Table 5. Biomass ash has more than 65% of alkali and alkaline earth metals (CaO, K2O, MgO and Na2O) that play a catalytic role in gasification process [28], whereas petcoke ash has a higher content of nickel and vanadium (55%). The synergetic effect between petcoke and biomass may be proven by kinetic calculations. The carbon conversion fraction (x) and the gasification reaction rate (r) were determined using the following equations: o m x ¼ mm0 m

ð2Þ

ash

r¼

dx 1  dt 1  x

ð3Þ

where m0 is the initial mass of the sample (mg), m is instantaneous sample mass at reaction time t (mg), mash is the ash mass (mg). The gasification conversion (x) and the gasification reaction rate (r) for experiments with petroleum coke and the blends (50/50 and 20/80) were calculated using Eqs. (1) and (2), respectively. The logarithmic reaction rate ln r (min1) versus the reciprocal gasification temperature 1000/T (K1) for the experiments with petcoke are shown in Fig. 6. Table 5 Biomass and petroleum coke ash analysis.

Fig. 5. Experimental and calculated data for 50/50 and 20/80 petcoke/biomass mixtures. (A) Temperature range 20–1200 °C. (B) Temperature range 900–1200 °C.

SiO2 Al2O3 CaO Fe2O3 K2O MgO MnO Na2O P2O5 Ba Cu Ni V

Biomass

Petcoke

12.8 1.0 33.0 1.7 23.2 5.4 3.7 1.7 5.3 2.5 0.5 0.0 0.0

12.8 15.4 1.5 2.9 0.6 0.6 0.0 1.9 0.4 2.4 1.2 27.5 28.4

V. Nemanova et al. / Fuel 117 (2014) 870–875 Table 6 Activation energy Ea and pre-exponential factor estimated from r at 850–950 °C.

100% Petcoke 50% Petcoke + 50% biomass, experimental 20% Petcoke + 80% biomass, experimental

Ea, kJ/mol

A0, min1

121.5 96.3 83.5

3856 597 299

As can be seen in Fig. 6 the logarithmic gasification reaction rate ln r exhibits a linear relationship with the temperature, 1/T, the same dependence was found in the literature [2,16,29–31]. Applying the Arrhenius Eq. (4), the activation energy Ea and the preexponential factor A0 were calculated; the results are presented in Table 5.

r ¼ A0 exp

  Ea RT

ð4Þ

According to the kinetic data listed in Table 6 there is a significant synergetic effect between petroleum coke and biomass during co-gasification: petroleum coke with high biomass ash content has remarkably higher reactivity with steam than pure petcoke. The catalytic effect of mineral content in the carbonaceous material on the petroleum coke reactivity has also been noticed in the work by Harris and Smith [31]. 4. Conclusions Co-gasification testes of petroleum coke and biomass were conducted at KTH Royal University of Technology. The first part of this study was performed in the 5 kW atmospheric fluidised bed reactor. Operating temperature in the bed was 800 and 900 °C, oxygen was used as oxidising medium. Petroleum coke/biomass blends were used in the proportions of 0/100, 20/80 and 50/50. Higher gasification temperature (900 °C and above) and higher oxygen concentration in the fluidising gas are favourable for high petcoke conversion. By raising temperature in the reactor by 100 °C and oxygen concentration by 3% it is possible to increase the petcoke conversion in a 20/80 mixture from 47.6% to 75.3% and the total conversion from 85.2% to 90.8%. Moreover, the tar concentration in the product gas decreases from 1.2 to 0.8 g/Nm3. The second part of the present work was dedicated to non-isothermal steam gasification tests in a thermogravimetric analyser. The same petroleum coke/biomass blends were heated up to 1200 °C at a rate of 10 °C/min, with a total pressure of 1 bar. Higher biomass content in the mixture leads to shorter gasification finishing time than calculated (6 min difference for the 50/50 mixture and 13 min for the 20/80). Alkali and alkaline earth metals from biomass ash increase the reactivity of petroleum coke during cogasification. This synergetic effect between biomass and petcoke was proven by kinetics data. The activation energy Ea, determined from the Arrhenius law, was 121.5 kJ/mol for pure petcoke steam gasification, whereas for the 50/50 mixture it was 96.3 and for the 20/80 blend – 83.5 kJ/mol. Acknowledgment The research is supported by the Swedish Energy Agency (Energimyndigheten). References [1] Liu X, Zhou Z, Hu Q, Dai Z, Wang F. Experimental study on co-gasification of coal liquefaction residue and petroleum coke. Energy Fuels 2011;25:3377–81.

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