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High energy syngas production by waste tyres steam gasification in a rotary kiln pilot plant. Experimental and numerical investigations
Fuel 89 (2010) 2721–2728
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High energy syngas production by waste tyres steam gasification in a rotary kiln pilot plant. Experimental and numerical investigations A. Donatelli a,*, P. Iovane a, A. Molino b a ENEA – Italian National Agency for New Technologies, Energy and Environment – Environment, Global Change and Sustainable Development Department (ACS) – STP Division – SS 106 Jonica km 419.500, 75026 Rotondella, MT, Italy b ENEA – Italian National Agency for New Technologies, Energy and Environment – Energy Technologies, Renewable Energy Sources and Energy Saving Department (TER) – SS 106 Jonica km 419.500, 75026 Rotondella, MT, Italy
a r t i c l e
i n f o
Article history: Received 3 December 2009 Received in revised form 10 March 2010 Accepted 23 March 2010 Available online 1 April 2010 Keywords: Waste tyres Steam gasification Rotary kiln Syngas
a b s t r a c t This paper presents experimental and numerical results on steam gasification of waste tyres in a rotary kiln pilot plant. Both the process performance and the gas features have been evaluated varying the feeding ratio (FR), defined as the steam/tyres mass ratio. First, several experimental tests have been performed. Then, the obtained experimental results have been used to verify the consistency of a numerical model developed with the aid of the commercial code ChemCADÒ. Once done, the effect of increasing the FR on the gas energy content has been evaluated. Numerical results showed that the gas energy content increases as the FR increases as well, achieving a maximum value for FR = 0.33 that produced a gas which composition N2 free is (H2 = 52.7%vol, CO = 18.1%vol, CO2 = 7.0%vol, CH4 = 22.2%vol) in correspondence of which the lower heating value (LHV) is equal to 29.5 MJ kggas1. Higher FR values do not produce a further increase of the gas energy content, rather require a greater amount of input energy for heating the steam from the atmospheric to the process temperature. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The rapid increase in number of road vehicles has been accompanied worldwide by a major generation of waste tyres. Almost 5 million tonnes per year of waste tyres are actually generated, 2% of the total annual solid wastes. In 2003 the United States (US) produced around 290 million of waste tyres, the European Union (EU) around 260 million (http://www.environment-agency.gov.uk/). In the past, about 40% of waste tyres produced in Europe were sent to landfill. After the implementation of the EU Directive on Waste Landfill in 2006, this practice could no longer be done. In spite of this, the 1999/31/EC Landfill Directive, the 2000/53/EC End-of-Life Vehicle Directive, and the 2000/76/EC Waste Incineration Directive guaranteed that the percentage of waste tyres sent to landfill was recycled by using alternative technologies (http:// www.environment-agency.gov.uk/). With the aim of recovering energy from waste tyres, many researchers studied different utilization techniques, such as pyrolysis [4,5,9,11], coal co-pyrolysis [16,19], incineration/combustion
* Corresponding author. Tel.: +39 0835974423; fax: +39 0835974284. E-mail address: [email protected] (A. Donatelli). 0016-2361/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.03.040
[2,8], gasification [10,17,15,18,20], and, more recently, tyre refreshment and rubber regeneration [7,13]. Gasification, in particular, is a thermal process that can convert carbonaceous materials, such as organic waste or biomass, into carbon monoxide and hydrogen with a controlled amount of oxygen or steam. The resulting gas mixture, also called synthesis gas or syngas, is a fuel able to power gas turbines or fuel cells [12]. Typically, a gasification plant consists of a gasifier unit, a gas cleaning system and an energy recovery system. Gasification reactors can be basically classified as fixed beds, fluidized beds, or entrained beds. This process has several advantages over traditional combustion of solid waste: a limited formation of dioxins and of nitrous and sulphur oxides; a strong reduction in the process gas volume; production of an energy carrier that can be integrated with combined cycle turbines or reciprocating engines; a reduced amount of secondary wastes; and the possibility to apply the process at a smaller scale [14,1]. On the other hand, during gasification, tars, heavy metals, halogens and alkaline compounds are released within the gas and can cause environmental and operational problems so it is important an improved syngas cleaning, able to meet defined specifications [6,3]. Only a few studies were conducted to investigate the gasification of waste tyres, either in laboratory-scale or pilot-scale. In a laboratory-scale fluidized-bed gasifier [15], granulated tyres were
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Nomenclature FR LHVi HHVi t h L B S
steam-tyres ratio (kgsteam kg1tyres) lower heating value (MJ kg1species) higher heating value (MJ kg1species) solid residence time (min) response angle of matrix (°) length of the kiln (m) correction factor slope of the kiln
D n DHr T Gi E
gasified with air in the temperatures range of 350–900 °C, giving a gaseous product having a heating value of about 6 MJ Nm3 and a yield of solid residue (char) of 24–37% and oil of 0–37% with an energy recovery of 15–38%. Xiao et al. [20] tested the low-temperature gasification of waste tyres in a lab-scale fluidized bed at 400–800 °C. The air equivalence ratio (ER), written as the air/waste tyres mass ratio, was in the range of 0.2–0.6. The syngas LHV (about 4–9 MJ Nm3) increased with increasing temperature or decreasing ER and its yield (about 1.8–3.7 Nm3 kg1) was linearly proportional to ER. Surface area and yield of carbon black were respectively in range of 20– 30 m2 g1 and 550–650 g kg1. Waste tyres steam gasification in a laboratory-scale rotary kiln was already studied [10]. Results showed that the obtained syngas had a higher hydrogen content (almost 45% v/v), a higher yield of methane, ethylene, and ethane, and finally a lower yield of oxygenates than that produced by gasification of poplar and refuse derived fuel (RDF) at the same process conditions. Waste tyres steam gasification in a pilot-scale fluidized-bed reactor [18] highlighted a gas yield decrease from 0.76 Nm3 kg1tyres DAF to 0.21 Nm3 kg1tyres DAF, where DAF is dry ash free, and a heating value increase from 22.2 MJ Nm3 to 39.6 MJ Nm3, when decreasing gasification temperature from 787 °C to 627 °C. Finally, another interesting study on a waste tyres air/steam gasification in a fluidized-bed reactor [17] provided a numerical
kiln diameter (m) angular velocity (min1) reaction enthalpy (kJ kg1species) temperature (°C) mass flow-rate of i-species (kgspecies h1) required energy for gasification process or energy production by combustion of gas (MJ)
scheme for predicting gas composition under different working conditions, including flow-rate, composition and temperature of feed materials as well as operating pressure and temperature. Within a range of small deviations, the model provided results similar to those of experiments. This work is in this area. Unlike previous works, it deals with experimental and numerical tests on waste tyres steam gasification in a rotary kiln pilot plant. Its aim is to evaluate both the process performance and the gas features when varying the feeding ratio (FR), written as the steam/tyres mass ratio. First, experiments have been carried out. Then, a numerical model, whose consistency has been tested on the basis of experimental results, has been considered to assess the effect of increasing the FR on the gas energy content. The numerical scheme has been designed in the framework of the commercial code ChemCAD 5.2Ò, a pretty intuitive suite of chemical process simulation software (http://www.chemstations.com/Why_ChemCAD).
2. Experimental work 2.1. Feedstock matrix The raw materials chosen for experiments have been rubber grain derived from waste tyres: the reinforced fibers and the steel belts have been extracted and removed then samples have been
Fig. 1. Thermogram of waste tyres.
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Fig. 2. Scheme of pilot plant.
shredded to 1–2 cm diameter particle size by a cold mechanical grinding. The materials have been stored under dry conditions before feeding the reactor via a screw feeder system. They have been characterized by an elementary analyzer EA 1110 CHNS-O (ThermoQuest), a thermo-balance 2950 (TA Instruments), and a calorimetric bomb IKA C5000 in agreement with the UNI TS norms. These norms are defined by UNI, an Italian private association founded in 1921 and appointed by the Italian Government and the European Union to develop, approve and publish technical standards in all economic sectors. The role of UNI is recognized by the 83/189/EEC European Directive, implemented in Italy by the national Law 317/86.
rotary kiln reactor. The 50 dm3 storage tank has an appropriate system to prevent bridge formation. The reactor is fed by the top.
2.2. Pilot plant and gasification process
T ¼ ð1; 77
The experiments have been performed in a continuous pilot plant, having a feeding capacity of 5 kg/h. The plant is basically a rotary kiln gasifier with an indirect electric heating system and its layout is sketched in Fig. 2; each component will be described in detail in the following sections.
The geometric features of the rotary kiln reactor are reported in Table 3.
2.3. Fuel system The feeding system consists of a closed tank for matrix storage and a screw conveyor which transports the matrix directly into the Table 1 Elementary analysis of waste tyres.
a
2.4. Rotary kiln reactor The reactor is an indirectly heated rotary kiln with an internal diameter of 400 mm and a length of 1000 mm and then has an overall volume of about 126 dm3. The speed rotation can be varied in the range of 0.5–3 (min1) and the temperature in the range 400–1100 °C. The residence time of the solid matrix can be varied changing appropriately the slope and the speed of the kiln. It can be analytically computed by Eq. (1) which reads [21]:
pffi
h B LÞ=ðS n DÞ
ð1Þ
2.5. Gas treatment system The produced gas treatment system consists of a gravity settler to collect the large char particles eventually present into syngas, a scrubber (quencher) to collect fine particles and tars and cool down the gas. On the scrubber water recirculation line, a water-heat exchanger cools down the water. 2.6. Procedure
Material
N
C
H
S
Ash
Oa
Tyres (w/w)
0.4
85.2
7.3
2.3
4.4
0.4
Determined by difference.
The raw materials (waste tyres) have been loaded into the feed hopper. The tank of the quencher has been loaded with a measured quantity of water. At the process temperature (reached by electrically heating the reactor), the raw materials and the gasifying agent (steam) have been fed to the rotary kiln. The transport of the raw materials from the hopper to the reactor has been realized
Table 2 Proximate analysis of waste tyres. Volatile (% w/wdry) Fixed carbon (% w/wdry) Ash (% w/wdry) Moisture (% w/w) HHV (MJ kg1)
61.3 33.5 4.4 0.8 37.1
Table 3 Geometric characteristics of the rotary kiln. N (min1) S (°) D (m)
0.7–0.5 3.75° 0.4
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Table 4 Process parameters for experimental tests. No. test
T (°C)
Gasifying agent
Tyres flow-rate (kg h1)
FR ðwH2 O =wtyres Þ
N2 flow-rate (Nm3 h1)
Solid residence time (min)
Gas residence time (s)
1 2 3 4 5
850 850 850 850 850
Steam Steam Steam Steam Steam
3 3 3 3 1.5
0.33 0.5 0.67 1 1.3
1 1 1 1 0.7
15 15 15 15 15
13 10.4 10.1 9 11
by a screw conveyor whose rotation has been controlled by an inverter in order to obtain the desired flow-rate. The steam has been first produced into a heat exchanger by the combustion of LPG and syngas and then it has sent to the reactor. The steam mass flow-rate has been controlled by a valve and a volumetric counter, directly posed on the water adduction circuit, for monitoring the set FR value. In order to analyze the product features, different tests have been carried out at different FR. The process parameters used in the experiments are shown in Table 4. For FR = 1.3 the tyres mass flow-rate and the N2 mass flow-rate have been reduced to 1.5 kg/h and 0.7 Nm3/h, respectively. This has been basically due to the need of avoiding high gas mass flow-rates and, consequently, process line occlusion for dragging of fine particles. The tyres as well as the char have been fed and discharged from the reactor in a continuous way. The char has been collected into a tank and then unloaded after the test. The produced gas crossed the quencher for cooling and condensing the excess steam, and the washing section for removing acid components eventually present in the gas. Then it crossed a hydraulic guard with 50 mm water column for assuring a small overpressure and avoiding undesired air entrance. Finally, it has been analyzed by using a gas chromatograph and quantified by using a volumetric counter. Once done, the gas has been sent to a torch for realizing the complete oxidation. A hydraulic emergency guard, placed on the line between the quencher and the reactor, assured the drainage of gas in the torch in case of an undesired overpressure. During the experiments, a fixed nitrogen mass flow-rate has been introduced in the plant in order to: create an inert atmosphere during the reactor heating starting phase (washing); remove produced gases from the reactor (carrier).
Fig. 3 illustrates it: the pyrolysis gas (circle 1) has been mixed with steam (circle 2) and then the mix has been heated till 850 °C (circle 3) and sent to the gasification section (circle 4). Reactions have been evaluated in both reactors (i.e. pyrolysis and steam gasification), according with the simplification assumptions at equilibrium. For that it concerns the pyrolysis section, only five components have been considered: H2, CO, CO2, CH4 and C so that five linearly independent equations have been introduced. Three of them describe the balance on the atomic species (C, H, and O) at equilibrium, considering the atomic species as invariant during the process. The remaining two describe the reactions involving the species at equilibrium: 1
Ca Hb Oc $ ða b=4 c=2ÞC þ b=4CH4 þ c=2CO2
DHr ¼ 218:9 kJ mol
ð2Þ 1
C þ CO2 $ 2CO DHr ¼ 172:5 kJ mol
ðBoudouard reactionÞ
For that it concerns the steam gasification section, the components considered at equilibrium have been six: H2, CH4, CO, CO2, C and H2O. Similarly, the linearly independent equations have been the atomic balance of the species at equilibrium (C, H, and O) and three reactions of equilibrium as follows:
C þ H2 O $ CO þ H2
DHr ¼ 131:3 kJ mol
CH4 þ H2 O $ CO þ 3H2
1
ð4Þ
DHr ¼ 205:8 kJ mol
1
ðmethane steam reformingÞ CO þ H2 O $ CO2 þ H2
ð5Þ 1
DHr ¼ 50:8 kJ mol
ðCO shift reactionÞ
ð6Þ
The experiments showed that the steam gasification of a matrix with organic components produces a gas containing at least two additional components: ethane (C2H6) and ethylene (C2H4). Moreover, their fractions are smaller than those of the other components.
Moreover, a secondary nitrogen mass flow-rate has been sent to the mechanical seals for preventing gas leakage from the reactor to the external. At the end of the experiments, a total mass balances have been done, unloaded and weighed residual tyres in the hopper.
3. Numerical model The influence of key parameters (e.g. temperature, feeding ratio) on waste tyres steam gasification process have been numerically evaluated with the aid of a simulation tool created by ChemCAD 5.2Ò. The numerical model is able to determine the gas composition at equilibrium by following the two subsequent operating steps: waste tyres pyrolysis at atmospheric pressure and 400 °C, under inert atmosphere conditions; pyrolysis gas steam gasification at atmospheric pressure and 850 °C.
ð3Þ
Fig. 3. Numerical model detailed scheme (ChemCad 5.2Ò).
A. Donatelli et al. / Fuel 89 (2010) 2721–2728
4. Results The waste tyres elementary analysis (ASTM D5373-08) and proximate analysis (ASTM D3175-77 and ASTM D2866-83) are shown in Tables 1 and 2, respectively. The corresponding higher heating value (ASTM D5865) is reported in Table 2. Fig. 1 shows the waste tyres thermo-gravimetric analysis (TGDTG) in a nitrogen atmosphere until 900 °C with a constant heating rate of 20 °C min1 followed by air combustion. The waste tyres de-volatilization starts at about 220 °C and is practically complete at 600 °C. At 900 °C the remaining carbon black is oxidized with the corresponding mass loss. The decomposition occurs through a series of peaks which represent the simultaneous degradation of the main components of the tyres, mainly natural rubber (NR), styrene–butadiene rubber (SBR), and butadiene rubber (BR). The maximum degradation rates are in the temperature range of 375 °C for NR, 445 °C for SBR, and 465 °C for BR. The last peak at 900 °C accounts for the mass loss due to air combustion [10]. Table 5 shows the volumetric gas nitrogen free composition in different experiments. For each species, the trend is reported in Fig. 4. Hydrogen, carbon monoxide, and carbon dioxide contents increase as the FR increases as well. Correspondingly, methane content decreases. This is basically due to the high steam quantity which makes possible the steam reforming reaction of methane (5), giving back hydrogen, carbon monoxide and carbon dioxide. The gas reported in graphs is nitrogen free, to make able a direct comparison with the composition of gas obtained from numerical simulation, when nitrogen is not present.
Table 5 Experimental volumetric gas composition.
At this stage, the numerical model is validated by performing simulations able to reproduce experimental conditions. Table 6 gives the obtained results.
Fig. 5. Experimental and numerical volumetric fraction of H2.
Fig. 6. Experimental and numerical volumetric fraction of CO.
No. test
FR
H2
CO
CO2
CH4
Total
1 2 3 4 5
0.33 0.5 0.67 1 1.3
52.4 55.8 54.1 56.3 57.0
13.5 16.0 15.1 14.9 17.7
4.2 5.9 7.3 8.5 12.6
29.9 22.3 23.5 20.3 12.7
100 100 100 100 100
Fig. 7. Experimental and numerical volumetric fraction of CO2.
Fig. 4. Volumetric fraction trends in the experiments.
Table 6 Numerical gas composition at equilibrium. No. test
FR
H2
CO
CO2
CH4
Total
1 2 3 4 5
0.33 0.5 0.67 1 1.3
52.7 52.7 52.7 52.7 52.7
18.1 18.1 18.1 18.1 18.1
7.0 7.0 7.0 7.0 7.0
22.2 22.2 22.2 22.2 22.2
100 100 100 100 100
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Fig. 8. Experimental and numerical volumetric fraction of CH4.
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Table 7 Numerical gas composition and solid and gas yields at equilibrium. Mass flow-rate of tyres Gtyres (kg h1)
Mass flow-rate of steam Gsteam (kg h1)
FR
H2
CO
CO2
CH4
Total
Solid yield (w/w)
Gas yield (w/w)
3 3 3 3 3 3 3 3 3 3 3 3 3 3 3
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5
0.03 0.07 0.10 0.13 0.17 0.20 0.23 0.27 0.30 0.33 0.37 0.40 0.43 0.47 0.50
42.6 44.4 46.0 47.4 48.7 49.8 50.8 51.8 52.6 52.7 52.7 52.7 52.7 52.7 52.7
5.0 7.3 9.4 11.2 12.8 14.3 15.6 16.9 18.0 18.1 18.1 18.1 18.1 18.1 18.1
10.7 10.0 9.4 8.9 8.4 8.0 7.7 7.3 7 7.0 7.0 7.0 7.0 7.0 7.0
41.7 38.3 35.2 32.5 30.1 27.9 25.9 24.0 22.4 22.2 22.2 22.2 22.2 22.2 22.2
100 100 100 100 100 100 100 100 100 100 100 100 100 100 100
13.52 12.04 10.56 9.07 7.59 6.11 4.63 3.15 1.66 1.5 1.5 1.5 1.5 1.5 1.5
89.81 94.63 99.44 104.26 109.07 113.89 118.7 123.52 128.33 128.87 128.87 128.87 128.87 128.87 128.87
Looking at the numerical results, it can be observed that the gas composition does not vary. It means that, starting from the steam lower quantity sent to the reactor (FR = 0.33), any steam addition does not react with tyres and gas produced at the reference temperature. This is because the value 0.33 represents the reaction stoichiometric limit at the considered process conditions (FR and residence time). In experimental test, the excess steam could react with methane (steam gasification) or carbon monoxide only in some fixed conditions: the steam reforming reaction of methane is subordinated to the presence of catalytic species (generally nickel) and the residence time into the reactor, sufficient to guarantee an appreciable methane conversion, while for carbon monoxide conversion, the thermodynamic as well as the kinetics aspects play an important role. Moreover, the excess steam does not react with solid residue, because the rate of reaction of steam with the carbon is such that would require the use of a longer time than this of contact between the two phases in the reactor. The exothermic CO shift reaction, in fact, is basically not favored in the reference range of temperature and the conversion is not almost appreciable. This implies some differences when comparing experimental and numerical gas composition. Indeed, Figs. 5–8 show obtained experimental and numerical results: as it can be noticed, there is a good correspondence for H2 and CO, better than that for CO2 and CH4. The theoretical trend, in fact, takes into account the values at thermodynamic equilibrium so that, once defined the fluid dynamic parameters, such as temperature, pressure, and stoichiometric ratio, the model allows to get the compositions at equilibrium in case of all tyres are degraded into gas mixture components and char. At the output section of the
Fig. 9. Gas composition at equilibrium at different FR.
rotating drum, char is still present and may react with gas mixture components as well as remaining char, leading to a series of secondary reactions not yet considered in modeling reaction. This re-
Fig. 10. Solid and gas yields at different FR.
Table 8 Required energy for gasification and gas energy content in numerical simulations. FR
LHV (MJ kg1gas)
Gas yield (w/w)
Required energy in gasification per kg of feeding (MJ kg1tyres)
Energy production per kg of feeding (MJ kg1tyres)
0.03 0.07 0.10 0.13 0.17 0.20 0.23 0.27 0.30 0.33 0.37 0.40 0.43 0.47 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 1.40
33.4 32.8 32.2 31.7 31.3 30.8 30.3 29.9 29.6 29.5 29.5 29.5 29.5 29.5 29.5 29.5 29.5 29.5 29.5 29.5 29.5 29.5 29.5 29.5
89.8 94.6 99.4 104.3 109.1 113.9 118.7 123.5 128.3 128.9 128.9 128.9 128.9 128.9 128.8 128.9 128.9 128.9 128.9 128.8 128.9 128.8 128.9 128.8
7.79 8.61 9.42 10.24 11.03 11.86 12.67 13.50 14.30 14.39 14.38 14.38 14.37 14.37 14.37 14.36 14.35 14.34 14.33 14.33 14.32 14.31 14.30 14.30
33.77 34.85 35.94 37.02 38.09 39.18 40.25 41.34 42.41 42.53 42.53 42.53 42.53 42.53 42.53 42.53 42.53 42.53 42.53 42.53 42.53 42.53 42.53 42.53
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Fig. 11. Gas energy content in simulation tests and required energy in experimental test for different FR ratios.
sults in large differences between theoretical and experimental methane and carbon dioxide compositions. Experimental data show that the volumetric composition of methane decreases when the FR increases, confirming that, in case of high excess steam, the steam reforming reaction is favorite. It is also confirmed by hydrogen and carbon monoxide composition increase. The raise of the carbon dioxide volumetric composition may depend from the combined action of the Boudouard reaction, which tends to favor the carbon monoxide formation, and the CO shift reaction, which consumes it in the presence of large excess of steam. Their combined effect globally involves the increase of the carbon dioxide volumetric fraction. Moreover, it can be noticed that H2 content in experimental test is much than one obtained numerically: this could be due also to presence of trace components e.g. Ni in residual ash that are catalytically active and help to achieve this. Starting from the obtained experimental data, numerical simulations have been carried out by varying FR in the range 0–0.5 with an increasing step of 0.033. Table 7 gives the volumetric gas composition at equilibrium. Fig. 9 reports each species for different FR values. The stoichiometric limit of FR at the considered process conditions (FR*) is in the range 0.3–0.33: higher values do not contribute to vary the volumetric gas composition (N2 and H2O free) while implying a greater overall process energy consumption due to the unnecessary use of steam. Numerical results also allow to determine the solid residue and the gas produced during the steam gasification process by Eqs. (7) and (8). Table 7 reports the obtained numerical results. Fig. 10 outlines the corresponding trends.
solid yield ¼ char flow-rate=waste tyres flow-rate
ð7Þ
gas yield ¼ dry gas flow-rate=waste tyres flow-rate
ð8Þ
Obtained data give back information on the gas energy content. Table 8 and Fig. 11 show results obtained by varying FR: here are reported the energy required for carry out the gasification of 1 kg tyres at different values of FR ratio (both experimental and numerical simulation) and the energy obtained from combustion of gas produced in the gasification carried out with the relative FR ratio. It is noticed that the energy obtained from combustion of gas produced increases as FR increases as well, reaching the maximum vaTable 9 Gas energy content in experimental tests. No. test
FR
LHV ðMJ kggas Þ
1 2 3 4 5
0.33 0.5 0.67 1 1.3
36.55 32 30.8 29.1 22.56
1
lue in correspondence of FR* and, in any case, it is always more than the one required by process. The LHV decreases when FR increases, as found in the experiments (Table 9). Hence, it can be said that waste tyres steam gasification at FR* represents the optimum condition in terms of resulting energy. Higher FR value only contribute to increase the process input energy, useful for heating steam from atmospheric to process temperature. 5. Conclusions The aim of this work has been to analyze key aspects on waste tyres steam gasification. Both the process performance and the gas features have been experimentally and numerically evaluated by varying FR. First, experiments have been performed. Then, obtained results have been used to validate a numerical model developed in the ChemCADÒ framework. Once assessed the consistency of the model, the effect of varying FR on gas composition and energy content has been studied. There are some differences when comparing experimental and numerical gas composition. This is because the theoretical trend takes into account the values at thermodynamic equilibrium so that, once defined the fluid dynamic parameters, such as temperature, pressure, and stoichiometric ratio, the model allows to get the compositions at equilibrium in case of all tyres are degraded into gas mixture components and char. Instead at the output section of the rotating kiln char is still present and may react with gas mixture components as well as remaining char, leading to a series of secondary reactions not yet considered in modeling reaction. Currently the model cannot replace the experimentation, due to approximations still present, but it can be used as support to the experimental tests because it supplies results near those real one; Both experimental and numerical have given back the following outcomes: (i) hydrogen and carbon monoxide in the gas increase as FR increases as well; methane correspondingly decreases. This is basically due to the high steam quantity which makes possible the steam reforming reaction of methane (5), giving back hydrogen, carbon monoxide and carbon dioxide; (ii) the FR stoichiometric limit at the considered process conditions (FR*) is in the range 0.3–0.33; higher values do not contribute to vary the volumetric gas composition (N2 and H2O free); (iii) the energy production obtained by combustion of gas produced increases as FR increases as well, reaching the maximum value in correspondence of FR*. Moreover, for every FR ratio, the energy obtained by combustion of gas produced is more than the energy required by process. The LHV
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decreases when FR increases, as found in the experiments (Table 9). Hence, it can be said that waste tyres steam gasification at FR* represents the optimum condition in terms of resulting energy. Higher FR value only contribute to increase the process input energy, useful for heating steam from atmospheric to process temperature.
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[9] Galvagno S, Casu S, Casabianca T, Calabrese A, Cornacchia G. Pyrolysis process for the treatment scrap tyres; preliminary and experimental results. Waste Manage 2002;22:917–23. [10] Galvagno S, Casciaro G, Casu S, Martino M, Mingazzini C, Russo A, et al. Steam gasification of refuse-derived fuel (RDF); influence of process temperature on yield and product composition. Waste Manage 2009;29:678–89. [11] Gonzalez JF, Encinar JM, Canito JL, Rodriguez JJ. Pyrolysis of automobile tyre waste. Influence of operating variables and kinetics study. J Anal Appl Pyrol 2001;58–59:667–83. [12] Higman G, Van Der Burgt M. Gasification. USA: Elsevier; 2003. [13] Jeong-Hyun S, Kwang-Suk K, Jeong-Han L, Wan-Suk Y. Regeneration of the road profile to compensate tyre nonlinearity for virtual testing. Int J Vehicle Des 2004;35:210–23. [14] Klein A, Whiting K, Archer E, Schwager J. Gasification and pyrolysis: what is the current situation for waste management? Waste Manage World 2004:71–5. [15] Leung DYC, Wang CL. Fluidized-bed gasification of waste tire powders. Fuel Process Technol 2003;84:175–96. [16] Mastral AM, Alvares R, Callen MS, Clemente C, Murillo R. Characterization of chars from coaltire copyrolysis. Ind Eng Chem Res 1999;38:2856–60. [17] Mitta NR, Ferrer-Nadal S, Lazovic AM, Parales JF, Velo E, Puigjaner L. Modelling and simulation of a tyre gasification plant for synthesis gas production. Comput Aided Chem Eng 2006;21:1771–6. [18] Raman KP, Walawender WP, Fan LT. Gasification of waste tires in a fluidized reactor. Conserv Recycl 1981;4:79–88. [19] Tang Y, Curtis CW. Thermal and catalytic co-processing of waste tire with coal. Fuel Process Technol 1996;46:195–215. [20] Xiao G, Ni MJ, Chi Y, Cen KF. Low-temperature gasification of waste tire in a fluidized bed. Energy Convers Manage 2008;49:2078–82. [21] Xiao YL, Eckehard S. Mean residence time and hold-up of solids in rotary kilns. Chem Eng Sci 2006;61:5176–81.
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High energy syngas production by waste tyres steam gasification in a rotary kiln pilot plant. Experimental and numerical investigations A. Donatelli a,*, P. Iovane a, A. Molino b a ENEA – Italian National Agency for New Technologies, Energy and Environment – Environment, Global Change and Sustainable Development Department (ACS) – STP Division – SS 106 Jonica km 419.500, 75026 Rotondella, MT, Italy b ENEA – Italian National Agency for New Technologies, Energy and Environment – Energy Technologies, Renewable Energy Sources and Energy Saving Department (TER) – SS 106 Jonica km 419.500, 75026 Rotondella, MT, Italy
a r t i c l e
i n f o
Article history: Received 3 December 2009 Received in revised form 10 March 2010 Accepted 23 March 2010 Available online 1 April 2010 Keywords: Waste tyres Steam gasification Rotary kiln Syngas
a b s t r a c t This paper presents experimental and numerical results on steam gasification of waste tyres in a rotary kiln pilot plant. Both the process performance and the gas features have been evaluated varying the feeding ratio (FR), defined as the steam/tyres mass ratio. First, several experimental tests have been performed. Then, the obtained experimental results have been used to verify the consistency of a numerical model developed with the aid of the commercial code ChemCADÒ. Once done, the effect of increasing the FR on the gas energy content has been evaluated. Numerical results showed that the gas energy content increases as the FR increases as well, achieving a maximum value for FR = 0.33 that produced a gas which composition N2 free is (H2 = 52.7%vol, CO = 18.1%vol, CO2 = 7.0%vol, CH4 = 22.2%vol) in correspondence of which the lower heating value (LHV) is equal to 29.5 MJ kggas1. Higher FR values do not produce a further increase of the gas energy content, rather require a greater amount of input energy for heating the steam from the atmospheric to the process temperature. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The rapid increase in number of road vehicles has been accompanied worldwide by a major generation of waste tyres. Almost 5 million tonnes per year of waste tyres are actually generated, 2% of the total annual solid wastes. In 2003 the United States (US) produced around 290 million of waste tyres, the European Union (EU) around 260 million (http://www.environment-agency.gov.uk/). In the past, about 40% of waste tyres produced in Europe were sent to landfill. After the implementation of the EU Directive on Waste Landfill in 2006, this practice could no longer be done. In spite of this, the 1999/31/EC Landfill Directive, the 2000/53/EC End-of-Life Vehicle Directive, and the 2000/76/EC Waste Incineration Directive guaranteed that the percentage of waste tyres sent to landfill was recycled by using alternative technologies (http:// www.environment-agency.gov.uk/). With the aim of recovering energy from waste tyres, many researchers studied different utilization techniques, such as pyrolysis [4,5,9,11], coal co-pyrolysis [16,19], incineration/combustion
* Corresponding author. Tel.: +39 0835974423; fax: +39 0835974284. E-mail address: [email protected] (A. Donatelli). 0016-2361/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.03.040
[2,8], gasification [10,17,15,18,20], and, more recently, tyre refreshment and rubber regeneration [7,13]. Gasification, in particular, is a thermal process that can convert carbonaceous materials, such as organic waste or biomass, into carbon monoxide and hydrogen with a controlled amount of oxygen or steam. The resulting gas mixture, also called synthesis gas or syngas, is a fuel able to power gas turbines or fuel cells [12]. Typically, a gasification plant consists of a gasifier unit, a gas cleaning system and an energy recovery system. Gasification reactors can be basically classified as fixed beds, fluidized beds, or entrained beds. This process has several advantages over traditional combustion of solid waste: a limited formation of dioxins and of nitrous and sulphur oxides; a strong reduction in the process gas volume; production of an energy carrier that can be integrated with combined cycle turbines or reciprocating engines; a reduced amount of secondary wastes; and the possibility to apply the process at a smaller scale [14,1]. On the other hand, during gasification, tars, heavy metals, halogens and alkaline compounds are released within the gas and can cause environmental and operational problems so it is important an improved syngas cleaning, able to meet defined specifications [6,3]. Only a few studies were conducted to investigate the gasification of waste tyres, either in laboratory-scale or pilot-scale. In a laboratory-scale fluidized-bed gasifier [15], granulated tyres were
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Nomenclature FR LHVi HHVi t h L B S
steam-tyres ratio (kgsteam kg1tyres) lower heating value (MJ kg1species) higher heating value (MJ kg1species) solid residence time (min) response angle of matrix (°) length of the kiln (m) correction factor slope of the kiln
D n DHr T Gi E
gasified with air in the temperatures range of 350–900 °C, giving a gaseous product having a heating value of about 6 MJ Nm3 and a yield of solid residue (char) of 24–37% and oil of 0–37% with an energy recovery of 15–38%. Xiao et al. [20] tested the low-temperature gasification of waste tyres in a lab-scale fluidized bed at 400–800 °C. The air equivalence ratio (ER), written as the air/waste tyres mass ratio, was in the range of 0.2–0.6. The syngas LHV (about 4–9 MJ Nm3) increased with increasing temperature or decreasing ER and its yield (about 1.8–3.7 Nm3 kg1) was linearly proportional to ER. Surface area and yield of carbon black were respectively in range of 20– 30 m2 g1 and 550–650 g kg1. Waste tyres steam gasification in a laboratory-scale rotary kiln was already studied [10]. Results showed that the obtained syngas had a higher hydrogen content (almost 45% v/v), a higher yield of methane, ethylene, and ethane, and finally a lower yield of oxygenates than that produced by gasification of poplar and refuse derived fuel (RDF) at the same process conditions. Waste tyres steam gasification in a pilot-scale fluidized-bed reactor [18] highlighted a gas yield decrease from 0.76 Nm3 kg1tyres DAF to 0.21 Nm3 kg1tyres DAF, where DAF is dry ash free, and a heating value increase from 22.2 MJ Nm3 to 39.6 MJ Nm3, when decreasing gasification temperature from 787 °C to 627 °C. Finally, another interesting study on a waste tyres air/steam gasification in a fluidized-bed reactor [17] provided a numerical
kiln diameter (m) angular velocity (min1) reaction enthalpy (kJ kg1species) temperature (°C) mass flow-rate of i-species (kgspecies h1) required energy for gasification process or energy production by combustion of gas (MJ)
scheme for predicting gas composition under different working conditions, including flow-rate, composition and temperature of feed materials as well as operating pressure and temperature. Within a range of small deviations, the model provided results similar to those of experiments. This work is in this area. Unlike previous works, it deals with experimental and numerical tests on waste tyres steam gasification in a rotary kiln pilot plant. Its aim is to evaluate both the process performance and the gas features when varying the feeding ratio (FR), written as the steam/tyres mass ratio. First, experiments have been carried out. Then, a numerical model, whose consistency has been tested on the basis of experimental results, has been considered to assess the effect of increasing the FR on the gas energy content. The numerical scheme has been designed in the framework of the commercial code ChemCAD 5.2Ò, a pretty intuitive suite of chemical process simulation software (http://www.chemstations.com/Why_ChemCAD).
2. Experimental work 2.1. Feedstock matrix The raw materials chosen for experiments have been rubber grain derived from waste tyres: the reinforced fibers and the steel belts have been extracted and removed then samples have been
Fig. 1. Thermogram of waste tyres.
A. Donatelli et al. / Fuel 89 (2010) 2721–2728
2723
Fig. 2. Scheme of pilot plant.
shredded to 1–2 cm diameter particle size by a cold mechanical grinding. The materials have been stored under dry conditions before feeding the reactor via a screw feeder system. They have been characterized by an elementary analyzer EA 1110 CHNS-O (ThermoQuest), a thermo-balance 2950 (TA Instruments), and a calorimetric bomb IKA C5000 in agreement with the UNI TS norms. These norms are defined by UNI, an Italian private association founded in 1921 and appointed by the Italian Government and the European Union to develop, approve and publish technical standards in all economic sectors. The role of UNI is recognized by the 83/189/EEC European Directive, implemented in Italy by the national Law 317/86.
rotary kiln reactor. The 50 dm3 storage tank has an appropriate system to prevent bridge formation. The reactor is fed by the top.
2.2. Pilot plant and gasification process
T ¼ ð1; 77
The experiments have been performed in a continuous pilot plant, having a feeding capacity of 5 kg/h. The plant is basically a rotary kiln gasifier with an indirect electric heating system and its layout is sketched in Fig. 2; each component will be described in detail in the following sections.
The geometric features of the rotary kiln reactor are reported in Table 3.
2.3. Fuel system The feeding system consists of a closed tank for matrix storage and a screw conveyor which transports the matrix directly into the Table 1 Elementary analysis of waste tyres.
a
2.4. Rotary kiln reactor The reactor is an indirectly heated rotary kiln with an internal diameter of 400 mm and a length of 1000 mm and then has an overall volume of about 126 dm3. The speed rotation can be varied in the range of 0.5–3 (min1) and the temperature in the range 400–1100 °C. The residence time of the solid matrix can be varied changing appropriately the slope and the speed of the kiln. It can be analytically computed by Eq. (1) which reads [21]:
pffi
h B LÞ=ðS n DÞ
ð1Þ
2.5. Gas treatment system The produced gas treatment system consists of a gravity settler to collect the large char particles eventually present into syngas, a scrubber (quencher) to collect fine particles and tars and cool down the gas. On the scrubber water recirculation line, a water-heat exchanger cools down the water. 2.6. Procedure
Material
N
C
H
S
Ash
Oa
Tyres (w/w)
0.4
85.2
7.3
2.3
4.4
0.4
Determined by difference.
The raw materials (waste tyres) have been loaded into the feed hopper. The tank of the quencher has been loaded with a measured quantity of water. At the process temperature (reached by electrically heating the reactor), the raw materials and the gasifying agent (steam) have been fed to the rotary kiln. The transport of the raw materials from the hopper to the reactor has been realized
Table 2 Proximate analysis of waste tyres. Volatile (% w/wdry) Fixed carbon (% w/wdry) Ash (% w/wdry) Moisture (% w/w) HHV (MJ kg1)
61.3 33.5 4.4 0.8 37.1
Table 3 Geometric characteristics of the rotary kiln. N (min1) S (°) D (m)
0.7–0.5 3.75° 0.4
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Table 4 Process parameters for experimental tests. No. test
T (°C)
Gasifying agent
Tyres flow-rate (kg h1)
FR ðwH2 O =wtyres Þ
N2 flow-rate (Nm3 h1)
Solid residence time (min)
Gas residence time (s)
1 2 3 4 5
850 850 850 850 850
Steam Steam Steam Steam Steam
3 3 3 3 1.5
0.33 0.5 0.67 1 1.3
1 1 1 1 0.7
15 15 15 15 15
13 10.4 10.1 9 11
by a screw conveyor whose rotation has been controlled by an inverter in order to obtain the desired flow-rate. The steam has been first produced into a heat exchanger by the combustion of LPG and syngas and then it has sent to the reactor. The steam mass flow-rate has been controlled by a valve and a volumetric counter, directly posed on the water adduction circuit, for monitoring the set FR value. In order to analyze the product features, different tests have been carried out at different FR. The process parameters used in the experiments are shown in Table 4. For FR = 1.3 the tyres mass flow-rate and the N2 mass flow-rate have been reduced to 1.5 kg/h and 0.7 Nm3/h, respectively. This has been basically due to the need of avoiding high gas mass flow-rates and, consequently, process line occlusion for dragging of fine particles. The tyres as well as the char have been fed and discharged from the reactor in a continuous way. The char has been collected into a tank and then unloaded after the test. The produced gas crossed the quencher for cooling and condensing the excess steam, and the washing section for removing acid components eventually present in the gas. Then it crossed a hydraulic guard with 50 mm water column for assuring a small overpressure and avoiding undesired air entrance. Finally, it has been analyzed by using a gas chromatograph and quantified by using a volumetric counter. Once done, the gas has been sent to a torch for realizing the complete oxidation. A hydraulic emergency guard, placed on the line between the quencher and the reactor, assured the drainage of gas in the torch in case of an undesired overpressure. During the experiments, a fixed nitrogen mass flow-rate has been introduced in the plant in order to: create an inert atmosphere during the reactor heating starting phase (washing); remove produced gases from the reactor (carrier).
Fig. 3 illustrates it: the pyrolysis gas (circle 1) has been mixed with steam (circle 2) and then the mix has been heated till 850 °C (circle 3) and sent to the gasification section (circle 4). Reactions have been evaluated in both reactors (i.e. pyrolysis and steam gasification), according with the simplification assumptions at equilibrium. For that it concerns the pyrolysis section, only five components have been considered: H2, CO, CO2, CH4 and C so that five linearly independent equations have been introduced. Three of them describe the balance on the atomic species (C, H, and O) at equilibrium, considering the atomic species as invariant during the process. The remaining two describe the reactions involving the species at equilibrium: 1
Ca Hb Oc $ ða b=4 c=2ÞC þ b=4CH4 þ c=2CO2
DHr ¼ 218:9 kJ mol
ð2Þ 1
C þ CO2 $ 2CO DHr ¼ 172:5 kJ mol
ðBoudouard reactionÞ
For that it concerns the steam gasification section, the components considered at equilibrium have been six: H2, CH4, CO, CO2, C and H2O. Similarly, the linearly independent equations have been the atomic balance of the species at equilibrium (C, H, and O) and three reactions of equilibrium as follows:
C þ H2 O $ CO þ H2
DHr ¼ 131:3 kJ mol
CH4 þ H2 O $ CO þ 3H2
1
ð4Þ
DHr ¼ 205:8 kJ mol
1
ðmethane steam reformingÞ CO þ H2 O $ CO2 þ H2
ð5Þ 1
DHr ¼ 50:8 kJ mol
ðCO shift reactionÞ
ð6Þ
The experiments showed that the steam gasification of a matrix with organic components produces a gas containing at least two additional components: ethane (C2H6) and ethylene (C2H4). Moreover, their fractions are smaller than those of the other components.
Moreover, a secondary nitrogen mass flow-rate has been sent to the mechanical seals for preventing gas leakage from the reactor to the external. At the end of the experiments, a total mass balances have been done, unloaded and weighed residual tyres in the hopper.
3. Numerical model The influence of key parameters (e.g. temperature, feeding ratio) on waste tyres steam gasification process have been numerically evaluated with the aid of a simulation tool created by ChemCAD 5.2Ò. The numerical model is able to determine the gas composition at equilibrium by following the two subsequent operating steps: waste tyres pyrolysis at atmospheric pressure and 400 °C, under inert atmosphere conditions; pyrolysis gas steam gasification at atmospheric pressure and 850 °C.
ð3Þ
Fig. 3. Numerical model detailed scheme (ChemCad 5.2Ò).
A. Donatelli et al. / Fuel 89 (2010) 2721–2728
4. Results The waste tyres elementary analysis (ASTM D5373-08) and proximate analysis (ASTM D3175-77 and ASTM D2866-83) are shown in Tables 1 and 2, respectively. The corresponding higher heating value (ASTM D5865) is reported in Table 2. Fig. 1 shows the waste tyres thermo-gravimetric analysis (TGDTG) in a nitrogen atmosphere until 900 °C with a constant heating rate of 20 °C min1 followed by air combustion. The waste tyres de-volatilization starts at about 220 °C and is practically complete at 600 °C. At 900 °C the remaining carbon black is oxidized with the corresponding mass loss. The decomposition occurs through a series of peaks which represent the simultaneous degradation of the main components of the tyres, mainly natural rubber (NR), styrene–butadiene rubber (SBR), and butadiene rubber (BR). The maximum degradation rates are in the temperature range of 375 °C for NR, 445 °C for SBR, and 465 °C for BR. The last peak at 900 °C accounts for the mass loss due to air combustion [10]. Table 5 shows the volumetric gas nitrogen free composition in different experiments. For each species, the trend is reported in Fig. 4. Hydrogen, carbon monoxide, and carbon dioxide contents increase as the FR increases as well. Correspondingly, methane content decreases. This is basically due to the high steam quantity which makes possible the steam reforming reaction of methane (5), giving back hydrogen, carbon monoxide and carbon dioxide. The gas reported in graphs is nitrogen free, to make able a direct comparison with the composition of gas obtained from numerical simulation, when nitrogen is not present.
Table 5 Experimental volumetric gas composition.
At this stage, the numerical model is validated by performing simulations able to reproduce experimental conditions. Table 6 gives the obtained results.
Fig. 5. Experimental and numerical volumetric fraction of H2.
Fig. 6. Experimental and numerical volumetric fraction of CO.
No. test
FR
H2
CO
CO2
CH4
Total
1 2 3 4 5
0.33 0.5 0.67 1 1.3
52.4 55.8 54.1 56.3 57.0
13.5 16.0 15.1 14.9 17.7
4.2 5.9 7.3 8.5 12.6
29.9 22.3 23.5 20.3 12.7
100 100 100 100 100
Fig. 7. Experimental and numerical volumetric fraction of CO2.
Fig. 4. Volumetric fraction trends in the experiments.
Table 6 Numerical gas composition at equilibrium. No. test
FR
H2
CO
CO2
CH4
Total
1 2 3 4 5
0.33 0.5 0.67 1 1.3
52.7 52.7 52.7 52.7 52.7
18.1 18.1 18.1 18.1 18.1
7.0 7.0 7.0 7.0 7.0
22.2 22.2 22.2 22.2 22.2
100 100 100 100 100
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Fig. 8. Experimental and numerical volumetric fraction of CH4.
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Table 7 Numerical gas composition and solid and gas yields at equilibrium. Mass flow-rate of tyres Gtyres (kg h1)
Mass flow-rate of steam Gsteam (kg h1)
FR
H2
CO
CO2
CH4
Total
Solid yield (w/w)
Gas yield (w/w)
3 3 3 3 3 3 3 3 3 3 3 3 3 3 3
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5
0.03 0.07 0.10 0.13 0.17 0.20 0.23 0.27 0.30 0.33 0.37 0.40 0.43 0.47 0.50
42.6 44.4 46.0 47.4 48.7 49.8 50.8 51.8 52.6 52.7 52.7 52.7 52.7 52.7 52.7
5.0 7.3 9.4 11.2 12.8 14.3 15.6 16.9 18.0 18.1 18.1 18.1 18.1 18.1 18.1
10.7 10.0 9.4 8.9 8.4 8.0 7.7 7.3 7 7.0 7.0 7.0 7.0 7.0 7.0
41.7 38.3 35.2 32.5 30.1 27.9 25.9 24.0 22.4 22.2 22.2 22.2 22.2 22.2 22.2
100 100 100 100 100 100 100 100 100 100 100 100 100 100 100
13.52 12.04 10.56 9.07 7.59 6.11 4.63 3.15 1.66 1.5 1.5 1.5 1.5 1.5 1.5
89.81 94.63 99.44 104.26 109.07 113.89 118.7 123.52 128.33 128.87 128.87 128.87 128.87 128.87 128.87
Looking at the numerical results, it can be observed that the gas composition does not vary. It means that, starting from the steam lower quantity sent to the reactor (FR = 0.33), any steam addition does not react with tyres and gas produced at the reference temperature. This is because the value 0.33 represents the reaction stoichiometric limit at the considered process conditions (FR and residence time). In experimental test, the excess steam could react with methane (steam gasification) or carbon monoxide only in some fixed conditions: the steam reforming reaction of methane is subordinated to the presence of catalytic species (generally nickel) and the residence time into the reactor, sufficient to guarantee an appreciable methane conversion, while for carbon monoxide conversion, the thermodynamic as well as the kinetics aspects play an important role. Moreover, the excess steam does not react with solid residue, because the rate of reaction of steam with the carbon is such that would require the use of a longer time than this of contact between the two phases in the reactor. The exothermic CO shift reaction, in fact, is basically not favored in the reference range of temperature and the conversion is not almost appreciable. This implies some differences when comparing experimental and numerical gas composition. Indeed, Figs. 5–8 show obtained experimental and numerical results: as it can be noticed, there is a good correspondence for H2 and CO, better than that for CO2 and CH4. The theoretical trend, in fact, takes into account the values at thermodynamic equilibrium so that, once defined the fluid dynamic parameters, such as temperature, pressure, and stoichiometric ratio, the model allows to get the compositions at equilibrium in case of all tyres are degraded into gas mixture components and char. At the output section of the
Fig. 9. Gas composition at equilibrium at different FR.
rotating drum, char is still present and may react with gas mixture components as well as remaining char, leading to a series of secondary reactions not yet considered in modeling reaction. This re-
Fig. 10. Solid and gas yields at different FR.
Table 8 Required energy for gasification and gas energy content in numerical simulations. FR
LHV (MJ kg1gas)
Gas yield (w/w)
Required energy in gasification per kg of feeding (MJ kg1tyres)
Energy production per kg of feeding (MJ kg1tyres)
0.03 0.07 0.10 0.13 0.17 0.20 0.23 0.27 0.30 0.33 0.37 0.40 0.43 0.47 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 1.40
33.4 32.8 32.2 31.7 31.3 30.8 30.3 29.9 29.6 29.5 29.5 29.5 29.5 29.5 29.5 29.5 29.5 29.5 29.5 29.5 29.5 29.5 29.5 29.5
89.8 94.6 99.4 104.3 109.1 113.9 118.7 123.5 128.3 128.9 128.9 128.9 128.9 128.9 128.8 128.9 128.9 128.9 128.9 128.8 128.9 128.8 128.9 128.8
7.79 8.61 9.42 10.24 11.03 11.86 12.67 13.50 14.30 14.39 14.38 14.38 14.37 14.37 14.37 14.36 14.35 14.34 14.33 14.33 14.32 14.31 14.30 14.30
33.77 34.85 35.94 37.02 38.09 39.18 40.25 41.34 42.41 42.53 42.53 42.53 42.53 42.53 42.53 42.53 42.53 42.53 42.53 42.53 42.53 42.53 42.53 42.53
A. Donatelli et al. / Fuel 89 (2010) 2721–2728
2727
Fig. 11. Gas energy content in simulation tests and required energy in experimental test for different FR ratios.
sults in large differences between theoretical and experimental methane and carbon dioxide compositions. Experimental data show that the volumetric composition of methane decreases when the FR increases, confirming that, in case of high excess steam, the steam reforming reaction is favorite. It is also confirmed by hydrogen and carbon monoxide composition increase. The raise of the carbon dioxide volumetric composition may depend from the combined action of the Boudouard reaction, which tends to favor the carbon monoxide formation, and the CO shift reaction, which consumes it in the presence of large excess of steam. Their combined effect globally involves the increase of the carbon dioxide volumetric fraction. Moreover, it can be noticed that H2 content in experimental test is much than one obtained numerically: this could be due also to presence of trace components e.g. Ni in residual ash that are catalytically active and help to achieve this. Starting from the obtained experimental data, numerical simulations have been carried out by varying FR in the range 0–0.5 with an increasing step of 0.033. Table 7 gives the volumetric gas composition at equilibrium. Fig. 9 reports each species for different FR values. The stoichiometric limit of FR at the considered process conditions (FR*) is in the range 0.3–0.33: higher values do not contribute to vary the volumetric gas composition (N2 and H2O free) while implying a greater overall process energy consumption due to the unnecessary use of steam. Numerical results also allow to determine the solid residue and the gas produced during the steam gasification process by Eqs. (7) and (8). Table 7 reports the obtained numerical results. Fig. 10 outlines the corresponding trends.
solid yield ¼ char flow-rate=waste tyres flow-rate
ð7Þ
gas yield ¼ dry gas flow-rate=waste tyres flow-rate
ð8Þ
Obtained data give back information on the gas energy content. Table 8 and Fig. 11 show results obtained by varying FR: here are reported the energy required for carry out the gasification of 1 kg tyres at different values of FR ratio (both experimental and numerical simulation) and the energy obtained from combustion of gas produced in the gasification carried out with the relative FR ratio. It is noticed that the energy obtained from combustion of gas produced increases as FR increases as well, reaching the maximum vaTable 9 Gas energy content in experimental tests. No. test
FR
LHV ðMJ kggas Þ
1 2 3 4 5
0.33 0.5 0.67 1 1.3
36.55 32 30.8 29.1 22.56
1
lue in correspondence of FR* and, in any case, it is always more than the one required by process. The LHV decreases when FR increases, as found in the experiments (Table 9). Hence, it can be said that waste tyres steam gasification at FR* represents the optimum condition in terms of resulting energy. Higher FR value only contribute to increase the process input energy, useful for heating steam from atmospheric to process temperature. 5. Conclusions The aim of this work has been to analyze key aspects on waste tyres steam gasification. Both the process performance and the gas features have been experimentally and numerically evaluated by varying FR. First, experiments have been performed. Then, obtained results have been used to validate a numerical model developed in the ChemCADÒ framework. Once assessed the consistency of the model, the effect of varying FR on gas composition and energy content has been studied. There are some differences when comparing experimental and numerical gas composition. This is because the theoretical trend takes into account the values at thermodynamic equilibrium so that, once defined the fluid dynamic parameters, such as temperature, pressure, and stoichiometric ratio, the model allows to get the compositions at equilibrium in case of all tyres are degraded into gas mixture components and char. Instead at the output section of the rotating kiln char is still present and may react with gas mixture components as well as remaining char, leading to a series of secondary reactions not yet considered in modeling reaction. Currently the model cannot replace the experimentation, due to approximations still present, but it can be used as support to the experimental tests because it supplies results near those real one; Both experimental and numerical have given back the following outcomes: (i) hydrogen and carbon monoxide in the gas increase as FR increases as well; methane correspondingly decreases. This is basically due to the high steam quantity which makes possible the steam reforming reaction of methane (5), giving back hydrogen, carbon monoxide and carbon dioxide; (ii) the FR stoichiometric limit at the considered process conditions (FR*) is in the range 0.3–0.33; higher values do not contribute to vary the volumetric gas composition (N2 and H2O free); (iii) the energy production obtained by combustion of gas produced increases as FR increases as well, reaching the maximum value in correspondence of FR*. Moreover, for every FR ratio, the energy obtained by combustion of gas produced is more than the energy required by process. The LHV
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decreases when FR increases, as found in the experiments (Table 9). Hence, it can be said that waste tyres steam gasification at FR* represents the optimum condition in terms of resulting energy. Higher FR value only contribute to increase the process input energy, useful for heating steam from atmospheric to process temperature.
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