2284
Energy & Fuels 2006, 20, 2284-2288
Steam Gasification of Refuse-Derived Fuel (RDF): Influence of Process Temperature on Yield and Product Composition Sergio Galvagno,* Stefania Casu, Giovanni Casciaro, Maria Martino, Antonio Russo, and Sabrina Portofino Thermal Process DeVelopment Laboratory, EnVironmental DiVision, C. R. ENEA Trisaia SS Jonica 106, km 419.5 s 75026 Rotondella (MT), Italy ReceiVed May 26, 2006. ReVised Manuscript ReceiVed July 24, 2006
The opportunity of using refuse-derived fuel (RDF) to produce fuel gas seems to be promising, and particular attention has been focused on alternative process technologies such as pyrolysis and gasification. Within this frame, present work relates to experimental tests and obtained results of a series of experimental surveys on RDF gasification with steam, performed by means of a bench-scale rotary kiln plant at different process temperatures, using thermogravimetry (TG) and infrared spectrometry (Fourier transform infrared, FTIR) to characterize the incoming material and online gas chromatography to qualify the gaseous stream. Experimental data show that the gas yield increases with temperature and, with respect to the gas composition, the hydrogen content increases, mainly at the expense of the other gaseous compounds, which highlights the major extension of secondary cracking reactions into the gaseous fraction at higher temperature. Syngas obtained at processing temperatures of 950 °C or higher seems to be suitable for producing hydrogen for ammonia synthesis or for fuel cell applications, whereas, at lower processing temperatures, it seems usable for Fischer-Tropsch synthesis. The low organic content of solid residue does not suggest any other exploitation of the char apart from the landfilling.
Introduction The Italian government has acknowledged European strategies on waste management (minimization, reuse, recycling, and recovery), adopted at the community level, with a series of specific regulations. In this context, to face the declining availability of landfills and to promote the energetic valorization of waste, up-to-date rules and technical features for the utilization of nonconventional waste-derived fuels (RDF) have been set up. Nevertheless, until now, the exploitation of RDF has not met expectations, on one hand, because its production is strongly regulated by prescriptive limits (related to typology, origin, and characteristics of waste), on the other hand, because the heterogeneous composition of RDF restricts the class of possible secondary users (such as cement works, steelworks, and so on) that would prefer more-selective incoming fluxes. Because the opportunity to use RDF to produce fuel gas seems to be promising, particular attention has been focused on process technologies such as pyrolysis and gasification.1-3 Within this framework, the present study reports results of RDF gasification with steam, according to a series of trials performed using a bench-scale rotary kiln reactor, with the final goal of determining the influence of processing temperature on yields and product composition. Experimental Section Materials. Samples used for the experimental work were commercial products that were supplied by an Italian producer. * Author to whom correspondence should be addressed. Fax: +390835974284. E-mail:
[email protected]. (1) Kiran, N.; Ekinci, E.; Snape, C. E. Resour. ConserV. Recycl. 2000, 29, 273-283. (2) Morris, M.; Waldheim, L. Waste Manage. 1998, 18, 557-564. (3) Bridgewater, A. V.; Meier, D.; Radlein, D. Org. Geochem. 1999, 30, 1479-1493.
Because of the high moisture content of RDF (∼25%-30%), the samples were first dried and then milled into particles up to 0.2 mm in diameter. After being ground, samples were maintained under ambient conditions. Approximately 250 g of material was used for each test. Apparatus and Procedures. A schematic diagram of the experimental device is reported in Figure 1. RDF gasification tests were conducted on a bench-scale rotary kiln reactor, produced by Lenton (PTF 16/75/610 model) (see Figure 1). Rotary kilns have not been widely used for gasification treatment; meanwhile, they offer such flexible adjustments configurations to be able to achieve an easy management of the process, even with a highly heterogeneous material, such as RDF. The material was loaded into a feeder hopper with a maximum capacity of 5 L, which was fitted with an air-tight closure system and a mechanical stirrer; during the experiments, the material was continuously fed into the beginning of the alumina reactor by means of a screw-driver device, whose rotation was ruled by an inverter. Inside the reactor, the material rolled down the length of the kiln, thanks to a series of shields, mutually tilted at 30°, that assisted the transport and limited heat dispersions outside the refractory shell. Feeding began after the reactor reached the temperature that was selected for the experiment. The rotation speed of the kiln is adjusted by an inverter, and the reactor slope can be varied up to 10°; the furnace is externally heated, and three different thermocouples are provided to measure the temperature axially along the reactor. The solid residue was continuously discharged in a tank at the outlet of the reactor while the process gas was headed for the cleaning system: the stream passed through an ice-jacketed condenser trap that cooled the stream further to room temperature and removed any oil particles from the gaseous products; subsequently, it was first bubbled into a 1 M NaOH solution, which served as a basic scrubber for acid removal, and then it entered into a water-based bubbling system, which prevented any further charcoal transport. The extent of product condensation throughout the cleaning system was detected by weight difference. After
10.1021/ef060239m CCC: $33.50 © 2006 American Chemical Society Published on Web 09/02/2006
Steam Gasification of RDF
Energy & Fuels, Vol. 20, No. 5, 2006 2285
Figure 1. Schematic diagram of the experimental apparatus. Table 1. Bench-Scale Plant Characteristics parameter
value
oven heating maximum power maximum temperature maximum working temperature heating zone length material reactor reactor length inside diameter external diameter reactor volume heating zone volume
electric, three independent zones 9.2 kW 1600 °C 1550 °C 610 mm recrystallized alumina 1550 mm 80 mm 94 mm 7.79 dm3 3.06 dm3
Table 2. Proximate and Ultimate Analysesa Ultimate Analysis 48.78% (by weight) 7.76% (by weight) 0.74% (by weight) 0.00% (by weight) 0.00% (by weight) 29.22% (by difference)
C H N S Cl O volatile matter fixed carbon ash gross heating value apparent density a
Proximate Analysis 79.66% (by weight) 6.84% (by weight) 13.50% (by weight) 19.88 MJ/kg 0.27 g/mL
All weights given on a dry basis. Table 3. Operating Parameters parameter
value
rotational speed of reactor reactor slope gas flux (N2) feeding rate
2 rpm 7° 1.5 L/min 1.32 g/min
treatment, gas flow was measured by a gas meter, before being analyzed and then discharged into the final abatement system. All the characteristics of the experimental device are reported in Table 1. Gasification tests have been conducted with steam that was produced at a temperature of 80-120 °C in a heated flask, through which the nitrogen transfer line was forced to pass: by bubbling the gas stream through the boiling water, the inert gas becomes enriched with saturated steam before entering the furnace. Such a system guarantees the production of a diluted syngas stream and the creation of safe working conditions. A nitrogen stream, after it has been charged with steam, flows through a thermally insulated transfer line and is able to avoid water condensation before entering the reactor. Before starting the gasification experiments, the initial nitrogen flow rate was maintained at 1.5 L/min to purge the system completely, until the prescribed process temperature was reached. The gases produced were monitored on-line via process gas chromatography and off-line via Fourier transform infrared (FTIR)
Figure 2. Refuse-derived fuel (RDF) thermogram, showing a ramp at 10 °C/min in N2 atmosphere to 900 °C, followed by air combustion.
analysis performed on gas samples that had been discontinuously collected; the gas heating value (GHV) was calculated from the composition data. A TA Instruments model TGA 2950 system, coupled with a Thermo Optek FTIR spectrometer, was used to set up the thermal process and characterize the RDF samples, with respect to proximate analysis4 (that is, moisture, volatile matter, fixed carbon, and ash content of the material); the ash content was further evaluated by incineration at 1000 °C. Thermogravimetry (TG) curves were recorded at different heating rates, using pure nitrogen as a inert purge gas, at a constant flow rate of 100 mL/min. Ultimate analysis was obtained with a Thermo Quest model EA 1110 analyzer. Such analysis gives the weight percent of carbon, hydrogen, nitrogen, and sulfur in the samples simultaneously; afterward, determination of the oxygen content can be obtained by difference. The heating value of the material was estimated using a bomb calorimeter (model IKA C5000) in adiabatic modality. Flue gas from the combustion chamber was allowed to pass through a sampling bottle filled with a NaHCO3/Na2CO3 buffer solution, to be further analyzed for chlorine determination. On-line gas analysis was conducted with a gas chromatograph (Agilent model 3000A) that was able to provide precise analysis of the principal gas components (H2, O2, N2, CO, CO2, CH4, C2H4, C2H6) within no more than 2-3 min. The instrument was equipped with two different columns that were working in parallel (Molsieve 5A e Poraplot Q) and used a thermal conductivity detector (TCD); the carrier gas was argon in all analyses.
Results and Discussions The decomposition profile of a RDF sample from thermogravimetry-differential thermogravimetry (TG-DTG) in a (4) Mayoral, M. C.; Izquierdo, M. T.; Andre`s, J. M.; Rubio, B. Thermochim. Acta 2001, 370, 91-97.
2286 Energy & Fuels, Vol. 20, No. 5, 2006
GalVagno et al. Table 4. Tests Parameters
test
process gas
thermal process
temp (°C)
solid residence time (min)
gas residence time (s)
pressure (atm)
RDFa RDFb RDFc RDFd
N2 N2 + vapor N2 + vapor N2 + vapor
pyrolysis gasification gasification gasification
850 850 950 1050
>15 >15 >15 >15
5 4 3 2
1 1 1 1
nitrogen atmosphere, at a constant heating rate of 10 °C/min, is reported in Figure 2. The curves show that the thermal decomposition is practically complete at ∼800 °C;5,6 such decomposition occurs through a series of complex peaks that are related to the simultaneous degradation of the various fractions (paper, plastics, wood, fabrics, etc.) contained in RDF. Table 2 reports proximate and ultimate analyses of the RDF; the data indicate a very high volatile content (up to 80%) and an ash quantity of almost 14%. As shown by the ultimate analysis, the material presents a good carbon content, while there is no evidence of the presence of either sulfur or chlorine. The steam gasification of RDF was performed in a series of trials at different processing temperatures, in the range of 8501050 °C, holding all other operational parameters constant (rotational speed of reactor, reactor slope, feeding rate, carrier gas flux), as reported in Table 3; such parameters, in turn, allow us to fix the solid and gas residence times, according to the equations described in the following discussions. Kiln residence time (ts) is expressed by the formula7
ts )
1.77θ1/2BL SND
where θ is the dynamic angle of repose (dependent from the material), L the kiln length, B the correction factor (equal to 1 for undammed kilns, >1 for dammed kilns), S the kiln slope, N the rotational speed of the kiln, and D the inside diameter. Such a formula is a simplified empirical expression of the solids residence time, correlating various operational variables, kiln geometry parameters, and material properties; nevertheless, theoretical results have been controlled by measuring the kiln crossing time of RDF through the cold reactor under the same operational conditions: the agreement between calculated and experimental data was very good. The gas residence time (tg) is calculated assuming an ideal behavior, according to
tg )
V FT
where V is the heating zone volume and FT is the flux at the processing temperature. All these parameters are shown in Table 4, which summarizes the entire experimental scheme. According to such a scheme, to give the boundary conditions due consideration, the experimental trials included a blank gasification test (with no material in the reactor) and a RDF pyrolysis trial: the first one allowed us to evaluate the contribution of the process gas to any liquid transfer within the cleaning system, which must be taken into account into the mass balance of the process; the pyrolysis experiment instead highlights the (5) Casu, S.; Galvagno, S.; Calabrese, A.; Casciaro, G.; Martino, M.; Russo, A.; Portofino, S. J. Therm. Anal. Calorim. 2005, 80, 477-482. (6) Stenseng, M.; Jensen, A.; Dam-Johansen, K. J. Anal. Appl. Pyrolysis 2001, 58-59, 765-780. (7) Sullivan, J. D.; Maier, C. J.; Ralson, O. C. U.S. Bur. Mines Tech. Pap. 1927, 384.
Table 5. Fraction Yields test
char yield (wt %)
oil yield (wt %)
gas yield (wt %)
RDFa RDFb RDFc RDFd
33.9 36.0 32.9 16.6
5.7
60.4 81.3 84.7 89.0
contribution of the thermal degradation reactions, with no steam, to the overall gas production. The influence of temperature on the process variables has been studied through comparison of the different trials, with respect to the yields, to the chemical composition and to the mass balances of the main components of the process. Influence of Temperature on the Gas Fraction. Table 5 reports the yields of the various process fractions (char, oil, and gas) at different temperatures: the data show that, except for the pyrolysis test (RDFa), the liquid fraction is almost negligible and the mass balance exceeds 100%, because of the introduction of steam; it is important to remark that the oil fraction was determined by weight difference of the cold trap, and no evidence of condensed matter was observed elsewhere in the cleaning system. On the other hand, literature data8 for biomass gasification, which was performed under quite similar experimental conditions, report an average yield of ∼7-10 g tar /kg daf biomass, which accounts for 65%, while the other gaseous components gradually decrease (other than carbon monoxide, the level of which remains fairly constant) over the range of the temperatures. This situation is probably dependent on the more favorable thermal cracking and steam reforming reactions that were caused by higher temperatures, which result in the major extensions of secondary cracking reactions into the gas fraction.8,9 Thermodynamical data show that the water-gas shift reaction,
CO + H2O f CO2 + H2
(∆H298K ) -41.2 kJ/mol)
is less important at higher temperature; thus, the main gasification reactions (water-gas and Boudouard reactions) (8) Turn, S.; Kinoshita, C.; Zhang, Z.; Ishimura, D.; Zhou, J. Int. J. Hydrogen Energy 1998, 23 (8), 641-648. (9) Lin, S.; Harada, M.; Suzuki, Y.; Datano, H. Fuel 2004, 83, 869874.
Steam Gasification of RDF
Energy & Fuels, Vol. 20, No. 5, 2006 2287 Table 6. Gas Composition versus Process Temperature
test
O2 (% vol)
H2 (% vol)
CH4 (% vol)
CO (% vol)
CO2 (% vol)
C2H4 (% vol)
C2H6 (% vol)
propane (% vol)
H2/CO ratio
RDFa RDFb RDFc RDFd
1.70 0.00 0.00 2.01
27.28 42.69 55.64 66.02
22.83 15.83 11.11 9.98
29.13 17.86 18.16 17.37
10.06 17.63 14.05 4.59
8.00 5.60 0.95 0.01
0.52 0.39 0.09 0.00
0.47 0.00 0.00 0.00
0.94 2.39 3.06 3.80
Table 7. Gas Characterization Value parameter
RDFa
RDFb
RDFc
RDFd
C (wt %) H (wt %) O (wt %) gas heating value GHV (MJ/m3) GHV (MJ/kg of feed) gas density at n.c. (kg/m3)
48.45 9.27 42.28
42.60 9.73 47.67
37.70 11.08 51.22
36.71 16.46 46.83
22.11 15.26 0.88
17.80 18.33 0.79
14.48 19.11 0.64
14.60 28.91 0.45
have a more prevalent role:
C + H2O f CO + H2
(
CnHm + 2nH2O f 2n +
(∆H298K ) 131.3 kJ/mol) m H + nCO2 2 2
C + CO2 f 2CO
)
(∆H298K > 0)
(∆H298K ) 172.5 kJ/mol)
Such reactions, together with the secondary cracking tar reactions, are the main factors responsible for the increase in hydrogen. The methane formation reaction,
CO + 3H2 f CH4 + H2O
(∆H298K ) -206.1 kJ/mol)
does not really seem to be favored at higher temperature as well,10 which accounts for the slow decrease of methane content with increases in temperature. Because of the different gas compositions, the H2/CO ratio in the exit gas increases from 2.39 to 3.80 by changing the temperature from 850 °C to 1050 °C. It is well-known that synthesis gas that has different levels of H2/CO ratios are suitable for different applications: for example, synthesis gas that has a H2/CO ratio in the higher range is advisable for producing hydrogen for ammonia synthesis or for fuel cell applications, whereas synthesis gas that has a H2/CO molar ratio in the range of 1-2 is highly desirable as feedstock for Fischer-Tropsch synthesis for the production of transportation fuels. As far as our results are concerned, Table 6 shows that such a composition
Figure 3. Example of Fourier transform infrared (FTIR) spectrum of synthesis gas.
that has H2 and CO in the latter range could be obtained by conducting the experiment at the lowest investigated temperature, which is 850 °C, at a steam flow rate of ∼4 g/min; beyond 850 °C, the synthesis gas, at the same steam flow rate, seems to be suitable for hydrogen production.11 Syngas usage for those applications (fuel cells and Fischer-Tropsch reactions) is strictly dependent on trace contaminants, such as chlorine or sulfur, which are responsible for the poisoning effects on such sensible catalytic processes12 and, therefore, it requires an adequate cleaning before specific application; also note that neither RDF characterization or FTIR analysis on the outlet process gas gave evidence of noticeable quantities of chlorine or sulfur. Besides the main gas components, as reported in Table 6, FTIR off-line analyses of the gaseous stream (see Figure 3) have revealed the presence of some other species, such as ethane and ethyne, benzene, toluene, and xylene, whose amount is, indeed, almost negligible; for this reason, it has not been taken into consideration for the evaluation of the entire gas composition;13 nevertheless, such species can be used as an indication of the extent of cracking reactions during secondary tar conversion.14,15 Table 7 reports the elemental composition (carbon, hydrogen, and oxygen) of the synthesis gas, which shows that, as the temperature increases, the carbon content continuously decreases, while the hydrogen content increases; the hydrogen, being the main component of the gas fraction and having a small atomic weight, is responsible for the progressive reduction of the gas density at higher temperature. Furthermore, the heating value decreases from 17.8 MJ/m3 to 14.6 MJ/m3, with an increase of temperature from 850 °C to 1050 °C; however, on the other hand, the energy content of the total gas shows a remarkable increase (from 18.3 MJ/kg of charge to 28.9 MJ/kg of charge). As reported in Table 8, the higher temperatures, together with the presence of the steam, strongly influence the total volume of the gas produced, and with an increase in temperature from 850 °C to 1050 °C, the gas volume per kilogram of feeding almost doubles. This aspect becomes more significant if each contribution of the components of the gas mixture is taken into account; as it would be expected, hydrogen mainly affects the gas production at higher temperature, reaching a value of 1.31 m3/kg at 1050 °C, over a total production of 1.98 m3/kg. The progressive increase of hydrogen and carbon monoxide content is a clear indication of the major extension of secondary cracking reactions.15-17 A careful comparison between bibliographic and experimental data is difficult to accomplish, because of the scarce experiences on RDF gasification in rotary kilns configu(10) Franco, C.; Pinto, F.; Gulyurtlu, I.; Cabrata, I. Fuel 2003, 82, 835842. (11) Chaudhari, S. T.; Bej, S. K.; Bakhshi, N. N.; Dalai, A. K. Energy Fuels 2001, 15, 736-742. (12) Hamelinck, C. N.; Faaij, A. P. C.; den Uil, H.; Boerrigter, H. Energy 2004, 29, 1743-1771. (13) Lu, R.; Purushothama, S.; Yang, X.; Hyatt, J.; Pan, W.-P.; Riley, J. T.; Lloyd, W. G. Fuel Process. Technol. 1999, 59, 35-50. (14) Jess, A. Fuel 1996, 75 (12), 1441-1448. (15) Morf, P.; Hasler, P.; Nussbaumer, T. Fuel 2002, 81, 843-853. (16) Boroson, M. L.; Howard, J. B.; Longwell, J. P.; Peters, W. A. Energy Fuels 1989, 3, 735-740. (17) Encinar, J. M.; Gonzalez, J. F.; Gonzalez, J. Fuel Process. Technol. 2002, 75, 27-43.
2288 Energy & Fuels, Vol. 20, No. 5, 2006
GalVagno et al.
Table 8. Gas Composition at Different Temperatures test
O2 (m3/kg)
H2 (m3/kg)
CH4 (m3/kg)
CO (m3/kg)
CO2 (m3/kg)
C2H4 (m3/kg)
C2H6 (m3/kg)
C3 (m3/kg)
total (m3/kg)
RDFa RDFb RDFc RDFd
0.01 0.00 0.00 0.04
0.19 0.44 0.73 1.31
0.16 0.16 0.15 0.20
0.20 0.18 0.24 0.34
0.07 0.18 0.19 0.09
0.06 0.06 0.01 0.00
0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00
0.69 1.03 1.32 1.98
Table 9. Char Characterization at Different Temperatures test
organic (wt %)
ash (wt %)
conversion
RDFa RDFb RDFc RDFd
47.3 33.2 16.3 12.3
52.7 66.8 83.7 87.7
0.91 0.86 0.94 0.98
Table 10. Char Elemental Analysis test
N (wt %)
C (wt %)
H (wt %)
O (by difference) (wt %)
RDFa RDFb RDFc RDFd
0.71 0.21 0.13 0.12
41.20 18.52 16.33 4.20
0.65 0.69 0.56 0.56
4.77 13.75 0.00 7.45
rations;18,19 nevertheless, good agreement has been achieved with reported data on biomass gasification under the same operation conditions.8,17 Influence of Temperature on the Solids Fraction. Table 9 reports the proximate analysis of the char fraction at different processing temperatures; the data clearly show that the effect of temperature is such to increase the ash amount onto the solids residue; furthermore, the low organic content at higher temperature strongly limits any possibility of char recycling,20,21 and, more likel, makes this product suitable for landfilling applications. With respect to elemental composition (Table 10), the data show a gradual decrease in carbon content, relative to temperature, while the hydrogen content remains constant. In addition, the same table shows the conversion factor of the process, which, holding all the other operational parameters constant, increases with temperature and, at 1050 °C, is equal to ∼1, highlighting the completeness of the reactions; conversion is calculated, for gasification tests RDFb-RDFd, from the following formula:
conversion )
reacted organic matter total organic matter
For pyrolysis test RDFa, the conversion is calculated as follows:
conversion )
volatile matter (oil + gas) theoretical volatile matter
Table 11 shows the distribution of carbon and hydrogen on the different process fractions and the corresponding mass balance, with respect to their initial amount on the feeding. As (18) Braekman-Danheux, C.; D’haeyere, A.; Fontana, A.; Laurent, P. Fuel 1998, 77 (1/2), 55-59. (19) Malkow, T. Waste Manage. 2004, 24, 53-79. (20) Nakagawa, K.; Tamon, H.; Suzuki, T.; Nagano, S. J. Porous Mater. 2002, 9, 25-33. (21) Mui, E. L. K.; Ko, D. C. K.; McKay, G. Carbon 2004, 42, 27892805.
Table 11. Distribution of Carbon and Hydrogen into the Various Fractions Distribution (wt %) test
char
gas
difference
Carbon RDFb RDFc RDFd
6.67 5.29 0.69
34.65 31.93 32.66
7.47 11.56 15.44
RDFb RDFc RDFd
0.25 0.18 0.09
Hydrogen 7.91 9.38 14.63
-0.50 -1.90 -7.06
we see, as far as hydrogen is concerned, the minus sign expresses an over-difference that is due to the introduction of steam, which is responsible for the increase in hydrogen. With respect to carbon, the mass balance is slightly defective, and this loss should be partly ascribed to CO2 scrubbing into the gas cleaning system (as proven from the weight difference registered for the NaOH trap), and partly to soot formation: the latter amount was very difficult to estimate, because of the minimum load and the limited experimental configuration. Conclusions In the present work, the steam gasification of refuse-derived fuel (RDF) was performed in a series of trials, by varying the process temperature within the range of 850-1050 °C, to investigate the effect of the processing temperature on the properties of the products. The data show that a higher temperature results in a higher conversion of the total organic content, which accounts for greater syngas production. Furthermore, when all the operational parameters are held constant, the temperature definitely affects gas composition, in favor of a higher hydrogen production at higher temperature; with respect to the calorific value, the syngas seems to be comparable to natural gas. The gas composition, and particularly the H2/CO molar ratio, allow the syngas to be suitable for the production of hydrogen for ammonia synthesis or for fuel cell applications, for process temperature of 950 °C or higher, whereas, at lower processing temperatures, it seems usable for Fischer-Tropsch synthesis. Mass balances of some elements on the solid and gaseous fractions prove that the gas becomes richer in hydrogen, which is the main factor responsible for the increase in gas production, at the expense of solids residue, at higher temperatures; moreover, the hydrogen content onto the gas fraction is always in excess, because of the water contribution, while the carbon content shows an almost constant loss. The solids residue shows a relevant ash content; nevertheless, the extremely low organic content does not suggest any other exploitation of the char, other than landfilling applications. EF060239M
