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The O2-enriched air gasification of coal, plastics and wood in a fluidized bed reactor
Waste Management 32 (2012) 733–742
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The O2-enriched air gasification of coal, plastics and wood in a fluidized bed reactor Maria Laura Mastellone ⇑, Lucio Zaccariello, Donato Santoro, Umberto Arena Department of Environmental Sciences-Second University of Naples, Via Vivaldi, 43 81100 Caserta, Italy
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Article history: Received 24 May 2011 Accepted 1 September 2011 Available online 10 October 2011 Keywords: Co-gasification Fluidized bed Enriched-air Oxygen Plastics Wood Coal
a b s t r a c t The effect of oxygen-enriched air during fluidized bed co-gasification of a mixture of coal, plastics and wood has been investigated. The main components of the obtained syngas were measured by means of on-line analyzers and a gas chromatograph while those of the condensate phase were off-line analysed by means of a gas chromatography–mass spectrometer (GC–MS). The characterization of condensate phase as well as that of the water used as scrubbing medium completed the performed diagnostics. The experimental results were further elaborated in order to provide material and substances flow analyses inside the plant boundaries. These analyses allowed to obtain the main substance distribution between solid, gaseous and condensate phases and to estimate the conversion efficiency of carbon and hydrogen but also to easily visualise the waste streams produced by the process. The process performance was then evaluated on the basis of parameters related to the conversion efficiency of fuels into valuable products (i.e. by considering tar and particulate as process losses) as well as those related to the energy recovery. Ó 2011 Elsevier Ltd. All rights reserved.
1. Background and scope The growing interest to apply gasification to solid wastes coming from several sources (municipalities, waste recovery/recycling chain, industries) as a viable alternative to the conventional direct combustion mainly justified by the possibility to apply this process to a small-medium scale, i.e. units able to produce from 200 kWe to several MWe and to treat up to about 100 ktpa of municipal solid waste (Defra, 2007; Juniper, 2009; Arena, 2011). Moreover, the new gasification processes offer increased possibilities to obtain a wide range of valuable products from solid wastes, from clean fuel gases and electricity to bulk chemicals, like ammonia and methanol. In particular, they can be configured to utilize more efficient energy conversion systems, such as different kind of gas engines and turbines and therefore, they potentially have better electrical generation efficiencies (Wang et al., 2008; Yassin et al., 2009). The reducing reaction environment of gasification process determines a limited formation of nitrous and sulphur oxides (Knoef, 2005) but also that of dioxins, since the absence of an oxidizing atmosphere eliminates one of the steps of the dioxins synthesis mechanism (Stieglitz and Vogg, 1987; McKay, 2002; Vehlow, 2005). Other relevant potential advantages are the strong reduction of the process gas volume and the possibility to design and operate with different gasification agents (air, oxygen, steam, as alone or as a mixture). ⇑ Corresponding author. Tel.: +39 0823 274603; fax: +39 0823 274593, +39 0823 274605. E-mail address: [email protected] (M.L. Mastellone). 0956-053X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2011.09.005
Among all waste gasification technologies, that of bubbling fluidized beds is promising for a series of reasons: the operating flexibility of the technology makes possible to operate the process with different fluidizing (and gasification) agents, reactor temperatures and gas residence times, to add reagents along the reactor freeboard or riser and to operate with or without a specific catalyst (Arena and Mastellone, 2005; Basu, 2006). On the other hand, waste gasification is not yet a process sufficiently mature to be applied in a wide market, mainly because of conversion efficiency losses and syngas cleaning concerns, related to the production of carbonaceous particles and/or heavy hydrocarbons compounds. A limited or negligible formation of these byproducts is the true challenge of gasification since their presence leads to an increase of operating and maintenance plant costs, to a reduction of the fraction of energy contained in the fuel that is transferred to the syngas and, in several cases, to heavy difficulties to meet the rigorous specifications of some end-use devices, such as gas engines and turbines (Kauz and Hansen, 2007). The formation of carbonaceous materials (char or particle fines) and that of heavy compounds (usually indicated as tar) are strictly correlated to the fuel structure and composition (vanPaasen, 2004; Kiel et al., 2004). In particular, gasification of some fuels produces a large amount of particulate while that of others fuels generates a large amount of tars (Arena et al., 2009; Mastellone et al., 2010a). This suggested to investigate the possibility to utilize a co-gasification process (Mastellone et al., 2010a; Pohorely et al., 2006; Pinto et al., 2007, 2008), i.e. to feed in the gasifier a combination of different fuels since the possible synergy between their products and intermediates could lead to maximize the process performance, to
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reduce the carbon losses (in both particulate and tar fractions) and to increase the energy content of syngas. Few studies are available in the literature about the effect of different gasification agents on the gasification process and only a small part of these investigations are carried out with fluidized bed reactors. Attention is mainly oriented to the utilization of the staging of air or that of pure oxygen. On the other hand, the utilization of oxygen-enriched air as oxidizing agent stream is adopted by a number of companies as design and operation criteria for commercial scale gasifiers (Heermann et al., 2001). The experimental work presented in this study aims to investigate the effect of a gasifying stream composed by air enriched with pure oxygen on the performance of the co-gasification process of coal, plastics and wood fed as a mixture in the gasifier. The experiments were carried out by keeping almost fixed the fluidization velocity, bed temperature and equivalence ratio (defined as the ratio between the oxygen content of oxidant supply and that required for complete stoichiometric combustion) as much as it was possible by tuning the fluidizing gas pre-heating and by varying the molar oxygen fraction in the fluidizing and oxidizing stream from 21% (air) to 26% and 35%. The aim was to verify if an oxygen-enriched air stream could improve the material and energy conversion efficiency and enhance the environmental sustainability of the process by lowering the production of waste streams or changing their characteristics. 2. Experimental 2.1. Experimental apparatus The gasification apparatus is a bubbling fluidized bed (BFB) reactor made of AISI 316L, 102mmID and 2.5 m high. It is equipped
with a series of electrical shells able to bring the reactor temperature up to 850 °C in four sections of the reactor column (Fig. 1). Each couple of shells is controlled by a PID control device connected to a thermocouple located at the reactor internal wall so that the temperature can be independently set for each of reactor sections. The BFB reactor is equipped with a gas injection device that allows feeding into the reactor different oxidizing agents: air, oxygen, steam, carbon dioxide and their mixture. The gas distributor is formed by nozzles having a truncate pyramidal shape that was specifically designed in order to guarantee a uniform distribution of the fluidizing gas in the bed. The gasifier can be fed with different fuels by means of two mechanical feeders located at two levels: just above the top of the bed or at about 1 m above it. The reactor exit has a top end with a diameter reduction that allows the increasing of the mean velocity of the exit gas before its entrance into the cyclone, so improving particulate separation and collection. Just after the gas exit from the cyclone top, a hood divides the syngas stream in four parts: one is addressed to a hot filter connected to a heated line (where carbon particles are removed without any loss of hydrocarbons content) and then sent to a total hydrocarbon analyzer (THC); another part of the syngas stream is addressed to a conditioning line formed by two cold traps, three filters and a flow meter and then is sent to on-line gas analyzers (that measure CO, CO2 and O2 contents), to an on-line gas chromatograph and, finally, to the stack. These last two streams can be utilized to sample and analyse the content of acids (mainly HCl and H2S), ammonia and PAHs. Sampling of the condensable species was made at the reactor exit, for about 30 min, by means of four in-series cold traps, which are encased in an insulating container and chilled at a temperature of about 0 °C. The condensed samples are sent to offline analyses carried out as in the following steps: (a) dissolution
Fig. 1. Sketch of the gasifier and the connected sampling and measuring points.
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of organics in pure dichloromethane (DCM); (b) drying of the mixture of DCM and PAHs by means of a rotovapor to obtain the gravimetric tar for quantitative analysis; (c) weighing of the dried sample, dilution with DCM until to reach a known concentration (ppm); (d) injection in a gas chromatograph coupled with a mass spectrometer (GC–MS). The obtained results from the GC–MS represented the concentration (in ppm) of the detected compounds in the injected solution. The sum of these concentrations gave the total ‘‘detected heavy hydrocarbons’’. The remaining part of the sample represented the ‘‘undetected’’ fraction that the GC–MS was not able to detect because its molecular weight was mainly larger than 200 u.m.a. With this procedure the tars belonging to the classes from 2 to 5 of the tar classification system proposed by Kijel et al. (2004) have been recognized. Data obtained from on-line and off-line gas measurements and from chemical analyses of solid samples are processed to develop for each run complete mass balances on atomic species and the related energy balance. Carbon balance allows determining the content of carbon accumulated in the reactor as carbon loading. This latter value was experimentally verified at the end of each test by switching the operating conditions from reducing to oxidizing values at the end of each run and by recording and integrating the CO2 and CO contents produced by oxidation (Arena et al., 2009, 2010). 2.2. Materials and procedures Quartz sand with size distribution in the range 200–400 lm has been used during the experiments. It is the ‘‘typical’’ bed material of fluidized bed combustor and gasifier. The composition of the fuel mixture to be fed into the gasifier, as well as the best operating conditions in terms of equivalence ratio and gas velocity, have been chosen on the basis of a series of preliminary experiments carried out by using mixtures having different relative proportions between coal, plastics and wood (Mastellone et al., 2010a). A fuel mixture having 50% of German brown coal, 30% of plastic waste (polyethylene and polypropylene) and 20% of wood was chosen as reference fuel to simulate the co-gasification process. The ultimate analysis and the heating value of the tested fuel mixture are reported in Table 1. The fluidization velocity of fluidizing stream was fixed at 0.4 m/s while its composition was varied: streams of nitrogen and oxygen were mixed in the pre-heating section in order to reach the desired final value of oxygen that entered the fluidized bed gasifier. The feed rate of fuel was then adjusted in order to reach the selected value of the equivalence ratio. The composition of the gaseous, solid and liquid (condensate) products was analyzed and processed in order to evaluate the main performance parameters: syngas yield, defined as: SY = (Qsyngas/Qfuel) where Qsyngas is the mass flow rate of produced syngas and Qfuel is the fuel mass flow rate fed in the gasifier; specific energy, defined as: SE = (Qsyngas LHVsyngas)/ Qfuel, where LHVsyngas is the lower heating value of the syngas on Table 1 Ultimate analysis (as-it-is fuel based) and heating value of the tested mixture. Ultimate analysis of fuel mixture (%w) Carbon Hydrogen Nitrogen Sulphur Ashes Moisture Oxygen (by diff.) Lower heating value (kJ/kg)
50.5 7.4 0.5 0.2 4.6 14.8 22.0 21800
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volumetric basis evaluated on the basis of the LHVs of CO, H2, CH4 and CnHm with n < 6; cold gas efficiency, defined as: CGE = (Qsyngas LHVsyngas)/(Qfuel LHVfuel); carbon conversion efficiency, defined as: CCE = (Qsyngas Csyngas)/(Qfuel Cfuel), where Csyngasis the carbon fraction in the gaseous component of the syngas (i.e. dust and tar excluded) and Cfuel is the carbon fraction in the fuel.
3. Material and substance flow analyses All of the data obtained by analyzing gases, condensate and fine particles sampled during the tests were used to develop the material balances over the entire gasifier plant. The results were combined with an assessment tool, the Material Flow Analysis (MFA), which is named Substance Flow Analysis (SFA) when it is referred to a specific chemical. MFA/SFA is a systematic assessment of the flows and stocks of materials and elements within a system defined in space and time. It connects the sources, the pathways, and the intermediate and final sinks of each species in a specific process (Brunner and Rechberger, 2004). In this study MFA/SFA was applied to a system boundary that includes the BFB gasifier and the cleaning system for ash separation, i.e. cyclone, bubblers and filters (Fig. 1 and then Figs. 2–4). The software utilized to carry out the material and substance flow analyses is the STAN (short for subSTance flow ANalysis) by INKA software that is a freeware that helps to perform material flow analysis according to the Austrian standard ÖNorm S 2096 (Material flow analysis – Application in waste management). Each balance is performed with reference to a same block diagram on different layers, one of these is dedicated to the balance of ‘‘goods’’ (in this case, the total mass of reactants involved in the gasification reactions) and the others to different single substances (in the case of this paper carbon and hydrogen have been chosen). For each test, layers on total mass, carbon and hydrogen were elaborated. The layers ‘‘goods’’ allow calculating the overall yield of the fuel conversion into the syngas stream and those of the secondary streams, like fines and tar, whose management affects the plant operating costs and the potential environmental concerns. The carbon layers also allow to calculate the carbon conversion efficiency and to characterize the ‘‘waste’’ streams in term of carbon content. The accumulation of unconverted fuel in the gasifier is represented by the value reported inside the box ‘‘gasifier’’ and is due to the evidence that the kinetic is not infinite and then at steadystate it is necessary to have an amount of solid carbon (called ‘‘bed carbon loading’’) which reacts with the gasification medium to produce the final syngas. A part of this loading is made by fines that remain in the reactor under form of wall fouling. It is noteworthy that the yield of syngas, calculated by referencing to the fuel inlet, decreased with oxygen increasing, that the condensate did not change in an appreciable way and carbon fines escaped from the reactor, and collected at the cyclone, increased.
4. Experimental results The experimental tests were performed at given equivalence ratio, ER = 0.25, and superficial gas velocity, U = 0.4 m/s, while the volumetric oxygen content in the fluidizing gas stream injected at the gasifier bottom varied from 21%, to 26% and to 35%. In order to maintain fixed the ER value, the mass flow rate of fuel varied for each test accordingly with the oxygen flow rate. The results presented in the following describe the effect of oxygen concentration in the gasifying medium with reference to the energy balance (and then to the temperature profile along the reactor), mass balances (and then syngas composition and heating value) and yields of undesired by-products and waste.
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Fig. 2. Mass balance in the test carried out with air as fluidizing stream, with reference to total (A) carbon, (B) and hydrogen (C) mass flows.
4.1. Thermal effect The first effect of the utilization of an O2-enriched air is the decreasing of nitrogen fraction in the fluidizing gas. This implies that the thermal wheel action played by nitrogen gradually reduces, inducing a thermal accumulation in the reactor and then a temperature increase along it. In particular, for oxygen fraction in the fluidizing air larger than 26%, the rising of reactor temperature during the test was so high to make no safe the reactor operation, due to the reduced mechanical resistance of AISI 316L stainless steel, utilized as reactor building material. This imposed, for each experimental test, to arrange the set of the operating parameters in order to avoid a too high level of temperature. This is also in agreement with the consideration that it is not correct to compare results of tests carried out at a different temperature, since the latter strongly affects reaction rates and then syngas composition. In an auto-thermal gasifier, the reactor temperature can be seen as a process state variable, i.e. as the system answer
to a set of parameters, such as the equivalence ratio, residence time, waste chemical energy, quality of reactor insulation, composition and inlet temperature of the gasifying medium. It was then decided to obtain both a safe operation and a reliable comparison of results by tuning the pre-heating heat flux of the fluidizing gas (Fig. 5). In particular, the test with air (21% O2) was carried out by increasing the pre-heating temperature at about 790 °C in order to keep fixed the bed temperature at about 850 °C; during the test with 26% of O2 in the fluidizing gas, the temperature of this stream was lowered at 650 °C while for test with 35% of O2 it was decreased to 600 °C. The values reported in Fig. 5 suggest that the addition of pure oxygen to the air stream can be utilized to obtain a better control of the reactor temperature. It in fact gives the possibility to select (even though in a rather limited range) the best operating conditions from a kinetic or thermodynamic point of view as well as from that of an optimized pollutants control (e.g. low NOx, vitrified ashes, etc.). Fig. 5 also shows that the temperature in the dense bed
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Fig. 3. Mass balance in the test carried out with fluidizing stream having 26% of oxygen content, with reference to total (A) carbon, (B) and hydrogen (C) mass flows.
and in the splashing zone (i.e. the region just above the top of the bed, where there is the maximum turbulence induced by bubbles eruption) increased for all the tests up to 850 °C: this confirms that the exothermic reactions of partial oxidation mainly occur in these zones. Downstream of the splashing zone the temperature decreases at about 750 °C at the exit of the reactor, i.e. just before the cyclone entrance, as a consequence of the thermal losses and endothermic reactions, such as those involving radicals and polycyclic aromatic hydrocarbons (PAHs). 4.2. Syngas composition and yields Data reported in Fig. 6 refer to the syngas composition as continuously sampled after cleaning devices, in all the tests carried out at different values of oxygen in the fluidizing stream. As expected, the test carried out with enriched air with 35% of oxygen led to a less nitrogen-diluted syngas (40%, instead of about 60% obtained by using air). This implies higher concentrations of all syngas components: 13.8% for H2, 18.9% for CO, 6.0% for CH4, etc.
In order to remove most of the misleading effect due to the presence of different concentrations of the inert component in the syngas mixtures and to the (limited) difference of temperature, the mass fraction in the nitrogen-free syngas for each component were calculated. The comparison of these data is reported in Fig. 7: the only effect on mass fractions appears to be that on the carbon monoxide and carbon dioxide content (which increases and decreases, respectively, as a consequence of a larger oxygen enrichment of the gas stream) while there is a not substantial variations in the concentration of methane and just a slight increase in concentrations of total hydrocarbons and hydrogen. The relative contents of carbon monoxide and carbon dioxide are correlated to each other by means of the Boudouard reaction: CO2 + C 2CO. It could be argued that at a higher temperature, if there is an excess of carbon, the higher positive values of the Gibbs’ energy variation promotes the carbon dioxide breaking down to carbon monoxide (Basu, 2006). On the other hand, profiles of Fig. 5 indicate that the tests at 26% and 35% of oxygen fraction in the gasification medium were carried at an almost equal reactor temperature, as a result of the
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Fig. 4. Mass balance in the test carried out with fluidizing stream having 35% of oxygen content, with reference to total (A) carbon, (B) and hydrogen (C) mass flows.
adopted specific experimental procedure. Therefore, the reactor temperature did not determine the variation of CO and CO2 concentrations. A different amount of solid carbonaceous material inside the reactor is another possible explanation of the cited experimental evidence. The following discussion about the amount and characteristics of carbonaceous solids and their role during the gasification process aims to verify if this hypothesis is correct. On the basis on the material and flow analyses reported above, Table 2 reports the amount of bed carbon loading, i.e. the solid carbon that at steady-state condition is present inside the bed, and that of carbon that escapes from the reactor as elutriated fines, together with the specific volumetric and mass syngas yield. The latter appears to decrease as a consequence of a more oxygen content in the gasification medium: in particular, the specific mass of syngas obtained for air gasification is 1.55 times that obtained with O2-enriched air at 35% and the amount of elutriated carbon fines increased by about three times. As it can be deduced from the mass balances on atomic species reported in Figs. 2–4, there is a corresponding decrease in the bed carbon loading, from 134 gC/kgC-fuel
in the operation with air, to 114 gC/kgC-fuel and 90 gC/kgC-fuel for operation with 26 and 35% of O2-content in the fluidizing stream. On the other hand, data in Table 2 and Figs. 2–4 indicate that the total amount of solid carbon in the reactor under forms of bed carbon loading and elutriable carbon fines was, at steady-state conditions, substantially the same (about 170 gC/kgC-fuel). The distribution between these two phases was instead different, with progressively larger amounts of carbon fines in the splashing zone and freeboard in the O2-enriched air tests: this affected the equilibrium of Boudouard reaction and truly could explain the shift towards a larger CO production by confirming that the above cited hypothesis about the role of carbonaceous solids can be assumed as reasonable. Moreover, it might be noted that a so large increase of carbon fines generation is generally ascribed to a higher temperature increase or to the existence of catalytic activities of some metals (Mastellone and Arena, 2008). The absence of a plain difference of the mean temperatures along the rector wall leads to hypothesize that, at a mesoscale level, the exothermic reactions and the reduced thermal wheel action of nitrogen could generated
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Fig. 5. Temperature profiles along the fluidized bed reactor in the reported tests.
Fig. 6. Syngas composition on volumetric basis as obtained for the three tests carried out at different oxygen concentration of fluidizing stream.
local increases of temperature (hot spots) that cannot be recorded by thermocouples located at the wall and managed by a PID program that smooth out the fluctuations by averaging out data in the time. In a reaction environment with a very low presence of oxygen (Mastellone et al., 2010b), this could enhance the carbonization of hydrocarbons fragments directly to carbon solid phase (CnHm ? nC + m/2 H2) and limit the conversion into small and stable molecules (CO, CO2, CH4). This explanation appears to be in agreement with the measured parallel reduction of the condensate fraction that is constituted by molecules having a molecular weight larger than gaseous compounds but lower than carbonaceous fines. The different syngas composition led to a corresponding variation in the lower heating value (LHV) and specific energy of the
syngas. Table 3 reports the values obtained in the three tests and, in particular, shows the very large energy content of syngas obtained in the test with the highest oxygen concentration in the gasification medium: the LHV is about 9000 kJ/m3N and the specific energy is 4.3 kWh/kgfuel. This positive result is related to the reduced presence of nitrogen and then to higher volumetric fractions of hydrogen, carbon monoxide and methane, as it can be deduced from Figs. 6 and 7. As already recalled in the paragraph ‘‘materials and procedures’’, the gaseous products also contain heavy condensable compounds (i.e. having a molecular weight higher than benzene) that have been measured by means of condensation in a series of cold traps and quantified in yield and composition (Fig. 8 and Table 4). The socalled ‘‘condensate’’ amount represents the concentration of very
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Fig. 7. Syngas composition expressed as nitrogen-free mass fraction as obtained for the three tests carried out at different oxygen concentration of fluidizing stream.
Table 2 Distribution of different solid and condensate products in the experimental tests. ID test
Syngas specific yield
–
m3N/kgfuel–kg/kgfuel
Specific bed carbon loading gC-bed/kgC-fuel
Specific carbon fines production gC-fines/kgC-fuel
Specific carbon content in the condensate phase gC-tar/kgC-fuel
21% O2 26% O2 35% O2
2.5–3.1 2.2–2.6 1.7–2.0
134 114 90
37 59 82
6.9 6.7 5.8
Table 3 Low heating value of syngas and other performance parameters, as obtained in all the tests. ID –
LHV of syngas kJ/m3N
Specific energy of syngas kWh/kgfuel
CGE –
21% O2 26% O2 35% O2
5150 6850 8950
3.5 4.1 4.3
0.51 0.56 0.63
as of the carbon fines. A further support to these experimental data and to the proposed theoretical approach is given by the consideration that the presence of hot spots can enhance the metal extraction from the wall and from the fuel ashes and that these metals, under form of elements or clusters, can act as initiating nuclei for the formation of carbonaceous structures.
4.3. Effect of reactor height on syngas composition
heavy compounds to which must be added the BTX (benzene + toluene + xylene) quantity to obtain the tar amount, i.e. tar (mass/ time) = condensate (mass/time) + BTX (mass/time). The value of ‘‘tar’’ reported in Table 4 is constituted by benzene and other BTX for about 90%; this means that the condensate fraction that represents the true concern for syngas utilization is only 10% of the total ‘‘tar’’. Fig. 8 shows that the larger fraction of PAHs is always represented by naphthalene even though, for tests at higher oxygen concentrations, a larger amount of undetected species is present (last column of Table 4). These results suggest that the O2-enriched air gasification process has a limited influence on the content of light molecules in the syngas but the effect on the composition and yield of carbonaceous particulate and tar is instead remarkable. It is likely that the undetected fraction in the condensate fraction is composed by large fragments that are precursors of the formation of carbon nanoparticles and nanotubes (Arena et al., 2006) as well
A fluidized bed gasifier can be seen as a two-stage reactor having a first stage in the well-mixed dense bed and high turbulent splashing zone and the second one along the freeboard. Accordingly, two different sampling lines have been used to analyse the syngas composition with the aim to evaluate the relative importance of the freeboard with respect to dense bed + splashing zone. Figs. 6 and 9 report data measured at 210 cm (i.e. at the reactor exit) and 90 cm (i.e. just after the splashing zone), respectively. A comparative analysis of these data shows that there is a limited difference between the composition of syngas at 90 cm, i.e. just above the splashing zone, and that at the freeboard exit (at about 210 cm). It can be highlighted just an increase of aromatic compounds concentrations. This evidence confirms that in a bubbling fluidized gasifier most of gasification reactions completed inside the dense bed and the splashing zone (Knoef, 2005). The freeboard region is mainly the place where polycondensation reactions occur, in other words, the region where acetylene becomes benzene,
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Fig. 8. Composition of PAH detected and measured in the condensate phase for the three tests carried out with fluidizing streams having different oxygen content.
Table 4 Yield of total tar and PAHs detected in the heavy fraction of the syngas. ID
Tar (condensate + BTX) mg/m3N
Fraction of detected PAH in condensate (%)
21% O2 26% O2 35% O2
13500 21800 21700
76 33 28
benzene becomes naphthalene or phenantrene, multiple-ring compounds becomes clustered nanostructured material. The practical relevance to model the BFB gasifier as a sort of two-stage reactor lays on the real possibility to control the desired process in specific zone of the reactor where the process really occur. For instance, the controlling of gaseous products of syngas must be made in the bed + splashing zone; the controlling of PAHs and heavy compounds must be referred to the freeboard region. This means that a possible injection of reactants as secondary/
Fig. 9. Syngas composition on volumetric basis as measured just after the splashing zone (90 cm) in the three tests carried out at different oxygen concentration of fluidizing stream.
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tertiary streams, addition of catalysts, heating/cooling to enhance/ decrease the progressing of given reactions should be made with reference to the real reaction zone to be sure to obtain the expected result. 5. Conclusions The effect of the oxygen fraction in the fluidizing and gasification stream of a bubbling fluidized bed gasifier has been studied. A material and substance flow analyses were used to perform the mass balances and to highlight a series of data that can be utilized for a deeper understanding of the experimental results and then of the process mechanisms. It was experimentally confirmed that the main advantage of an O2-enriched air is the possibility to increase the bed temperature or, if a relatively low temperature of the reactor is desired (for instance, to prevent particles sintering), to reduce the size of the transversal cross section and the total cost of pre-heating. The tests were performed by keeping constant as much as it was possible the reactor temperature in order to allow a reliable comparison of the data. Under these conditions no crucial effect of O2-enriched air was detected. In particular, no remarkable effects were obtained in terms of tar reduction since its specific production slightly decreased when oxygen-enrichment was increased. On the contrary, tar composition changed by showing a decrease of recognized PAHs in the condensate and a corresponding increase in the carbon losses as elutriated fines. Moreover, the syngas specific yield decreased even though its specific energy remained almost unvaried. Acknowledgements The authors would like to thank the Italian Minister of University and Research for the financial support to this experimental activity, given in the framework of the PRIN-Research Project of National Interest 2008S22MJC_004. References Arena U., Mastellone M.L., 2005. Fluidized Pyrolysis and Gasification of Solid Wastes, in Proc. of Industrial Fluidization South Africa 2005, The South African Institute of Mining and Metallurgy (ISBN 1-919782-83-0), pp. 53–68. Arena, U., 2011. Gasification: an alternative solution for waste treatment with energy recovery. Waste Manag. 31, 405–406. Arena, U., Mastellone, M.L., Camino, G., Boccaleri, E., 2006. An innovative process for mass production of multi-wall carbon nanotubes by means of low-cost pyrolysis of polyolefins. Polym. Degrad. Stab. 91, 763–768.
Arena, U., Zaccariello, L., Mastellone, M.L., 2009. Tar removal during the fluidized bed gasification of a plastic waste. Waste Manag. 29, 783–791. Arena, U., Zaccariello, L., Mastellone, M.L., 2010. Fluidized bed gasification of wastederived fuels. Waste Manag. 30, 1212–1219. Basu P., 2006. Combustion and Gasification in Fluidized Beds. CRC Press. Brunner, P.H., Rechberger, V., 2004. Practical Handbook of Material Flow Analysis. CRC Press LLC, Boca Raton (FL). Defra, 2007. Advanced Thermal Treatment of Municipal Solid Waste, Enviros Consulting Limited on behalf of Department for Environment, Food & Rural Affairs (Defra), available on. Heermann, C., Schwager, F.J., Whiting, K.J., 2001. Pyrolysis & Gasification of Waste. A Worldwide Technology & Business Review, 2nd ed. Juniper Consultancy Serv. Ltd. Juniper Rating Gasification Report, 2009. . Kauz M. Hansen Ulf., 2007. The externally-fired gas-turbine (EFGT-Cycle) for decentralized use of biomass. Appl. Energy 84, 795–805. Kiel J.H.A., van Paasen S.V.B., Neeft J.P.A., Devi L., Ptasinski K.J., JanssenF.J.J.G., Meijer R., Berends R.H., Temmink H.M.G., Brem G., PadbanN., Bramer E.A., 2004. Primary measures to reduce tar formation in a fluidised-bed biomass gasifier. Report ECN-C–04–014, ECN. Petten, The Netherlands. Knoef H., 2005. Practical aspects of biomass gasification, chapter 3 in Handbook Biomass Gasification edited by H. Knoef. BTG-Biomass Technology Group (BTG), ISBN: 90-810068-1-9. Enschede, the Netherlands. Mastellone, M.L., Arena, U., 2008. Olivine as a tar removal catalyst during fluidized bed gasification of plastic waste. AIChE J. 54 (6), 1656–1667. MastelloneM, L., Zaccariello, L., Arena, U., 2010a. Co-gasification of coal, plastic waste and wood in a bubbling fluidized bed reactor. Fuel 89 (10), 2991–3000. Mastellone M.L., Santoro D., Zaccariello L., Arena U., 2010. The Effect of OxygenEnriched Air on the Fluidized Bed Co-Gasification of Coal, Plastics and Wood. 3rd Int. Symp. on Energy from Biomass and Waste, Venice, Italy 8–11 November 2010. CISA Publisher, Italy-ISBN 978-88-6265-008-3. McKay, 2002. Dioxin characterization, formation and minimization during municipal solid waste incineration: a review. Chem. Eng. J. 86, 343–368. Pinto, F., Lopes, H., André, R.N., Gulyurtlu, I., Cabrita, I., 2007. Effect of catalysts in the quality of syngas and by-products obtained by co-gasification of coal and wastes. 1. Tars and nitrogen compounds abatement. Fuel 86, 2052–2063. Pinto, F., Lopes, H., André, R.N., Gulyurtlu, I., Cabrita, I., 2008. Effect of catalysts in the quality of syngas and by-products obtained by co-gasification of coal and wastes. 2.Heavy metals, sulphur and halogen compounds abatement. Fuel 87, 1050–1062. Pohorely, M., Vosecky, M., Hejdova, P., Puncochar, M., Skoblja, S., Staf, M., Vosta, J., Koutsky, B., Svoboda, K., 2006. Gasification of coal and PET in fluidized bed reactor. Fuel 85, 2458–2468. Stieglitz, L., Vogg, H., 1987. On formation conditions of PCDD/PCDF in fly ash from municipal waste incinerators. Chemosphere 16, 1917–1922. vanPaasen S.V.B., Kiel JHA, 2004. Tar formation in a fluidized bed gasifier: impact of fuel properties and operating conditions. Report ECN-C-04-013, Petten, the Netherlands. Vehlow J., 2005. Dioxins in Waste Combustion – Conclusions from 20 Years of Research, presented at Bioenergy Australia 2005, Melbourne, December 12–13. Wang, L., Weller, C.L., Jones, D.D., Hanna, M.A., 2008. Contemporary issues in thermal gasification of biomass and its application to electricity and fuel production. Biomass Bioenergy 32, 573–581. Yassin L., Lettieri P., Simons S.J.R., Germanà A., 2009.Techno-economic performance of Energy-from-waste fluidized bed combustion and gasification processes in the UK context. Chem. Eng. J., 146, 315–327.
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Waste Management journal homepage: www.elsevier.com/locate/wasman
The O2-enriched air gasification of coal, plastics and wood in a fluidized bed reactor Maria Laura Mastellone ⇑, Lucio Zaccariello, Donato Santoro, Umberto Arena Department of Environmental Sciences-Second University of Naples, Via Vivaldi, 43 81100 Caserta, Italy
a r t i c l e
i n f o
Article history: Received 24 May 2011 Accepted 1 September 2011 Available online 10 October 2011 Keywords: Co-gasification Fluidized bed Enriched-air Oxygen Plastics Wood Coal
a b s t r a c t The effect of oxygen-enriched air during fluidized bed co-gasification of a mixture of coal, plastics and wood has been investigated. The main components of the obtained syngas were measured by means of on-line analyzers and a gas chromatograph while those of the condensate phase were off-line analysed by means of a gas chromatography–mass spectrometer (GC–MS). The characterization of condensate phase as well as that of the water used as scrubbing medium completed the performed diagnostics. The experimental results were further elaborated in order to provide material and substances flow analyses inside the plant boundaries. These analyses allowed to obtain the main substance distribution between solid, gaseous and condensate phases and to estimate the conversion efficiency of carbon and hydrogen but also to easily visualise the waste streams produced by the process. The process performance was then evaluated on the basis of parameters related to the conversion efficiency of fuels into valuable products (i.e. by considering tar and particulate as process losses) as well as those related to the energy recovery. Ó 2011 Elsevier Ltd. All rights reserved.
1. Background and scope The growing interest to apply gasification to solid wastes coming from several sources (municipalities, waste recovery/recycling chain, industries) as a viable alternative to the conventional direct combustion mainly justified by the possibility to apply this process to a small-medium scale, i.e. units able to produce from 200 kWe to several MWe and to treat up to about 100 ktpa of municipal solid waste (Defra, 2007; Juniper, 2009; Arena, 2011). Moreover, the new gasification processes offer increased possibilities to obtain a wide range of valuable products from solid wastes, from clean fuel gases and electricity to bulk chemicals, like ammonia and methanol. In particular, they can be configured to utilize more efficient energy conversion systems, such as different kind of gas engines and turbines and therefore, they potentially have better electrical generation efficiencies (Wang et al., 2008; Yassin et al., 2009). The reducing reaction environment of gasification process determines a limited formation of nitrous and sulphur oxides (Knoef, 2005) but also that of dioxins, since the absence of an oxidizing atmosphere eliminates one of the steps of the dioxins synthesis mechanism (Stieglitz and Vogg, 1987; McKay, 2002; Vehlow, 2005). Other relevant potential advantages are the strong reduction of the process gas volume and the possibility to design and operate with different gasification agents (air, oxygen, steam, as alone or as a mixture). ⇑ Corresponding author. Tel.: +39 0823 274603; fax: +39 0823 274593, +39 0823 274605. E-mail address: [email protected] (M.L. Mastellone). 0956-053X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2011.09.005
Among all waste gasification technologies, that of bubbling fluidized beds is promising for a series of reasons: the operating flexibility of the technology makes possible to operate the process with different fluidizing (and gasification) agents, reactor temperatures and gas residence times, to add reagents along the reactor freeboard or riser and to operate with or without a specific catalyst (Arena and Mastellone, 2005; Basu, 2006). On the other hand, waste gasification is not yet a process sufficiently mature to be applied in a wide market, mainly because of conversion efficiency losses and syngas cleaning concerns, related to the production of carbonaceous particles and/or heavy hydrocarbons compounds. A limited or negligible formation of these byproducts is the true challenge of gasification since their presence leads to an increase of operating and maintenance plant costs, to a reduction of the fraction of energy contained in the fuel that is transferred to the syngas and, in several cases, to heavy difficulties to meet the rigorous specifications of some end-use devices, such as gas engines and turbines (Kauz and Hansen, 2007). The formation of carbonaceous materials (char or particle fines) and that of heavy compounds (usually indicated as tar) are strictly correlated to the fuel structure and composition (vanPaasen, 2004; Kiel et al., 2004). In particular, gasification of some fuels produces a large amount of particulate while that of others fuels generates a large amount of tars (Arena et al., 2009; Mastellone et al., 2010a). This suggested to investigate the possibility to utilize a co-gasification process (Mastellone et al., 2010a; Pohorely et al., 2006; Pinto et al., 2007, 2008), i.e. to feed in the gasifier a combination of different fuels since the possible synergy between their products and intermediates could lead to maximize the process performance, to
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reduce the carbon losses (in both particulate and tar fractions) and to increase the energy content of syngas. Few studies are available in the literature about the effect of different gasification agents on the gasification process and only a small part of these investigations are carried out with fluidized bed reactors. Attention is mainly oriented to the utilization of the staging of air or that of pure oxygen. On the other hand, the utilization of oxygen-enriched air as oxidizing agent stream is adopted by a number of companies as design and operation criteria for commercial scale gasifiers (Heermann et al., 2001). The experimental work presented in this study aims to investigate the effect of a gasifying stream composed by air enriched with pure oxygen on the performance of the co-gasification process of coal, plastics and wood fed as a mixture in the gasifier. The experiments were carried out by keeping almost fixed the fluidization velocity, bed temperature and equivalence ratio (defined as the ratio between the oxygen content of oxidant supply and that required for complete stoichiometric combustion) as much as it was possible by tuning the fluidizing gas pre-heating and by varying the molar oxygen fraction in the fluidizing and oxidizing stream from 21% (air) to 26% and 35%. The aim was to verify if an oxygen-enriched air stream could improve the material and energy conversion efficiency and enhance the environmental sustainability of the process by lowering the production of waste streams or changing their characteristics. 2. Experimental 2.1. Experimental apparatus The gasification apparatus is a bubbling fluidized bed (BFB) reactor made of AISI 316L, 102mmID and 2.5 m high. It is equipped
with a series of electrical shells able to bring the reactor temperature up to 850 °C in four sections of the reactor column (Fig. 1). Each couple of shells is controlled by a PID control device connected to a thermocouple located at the reactor internal wall so that the temperature can be independently set for each of reactor sections. The BFB reactor is equipped with a gas injection device that allows feeding into the reactor different oxidizing agents: air, oxygen, steam, carbon dioxide and their mixture. The gas distributor is formed by nozzles having a truncate pyramidal shape that was specifically designed in order to guarantee a uniform distribution of the fluidizing gas in the bed. The gasifier can be fed with different fuels by means of two mechanical feeders located at two levels: just above the top of the bed or at about 1 m above it. The reactor exit has a top end with a diameter reduction that allows the increasing of the mean velocity of the exit gas before its entrance into the cyclone, so improving particulate separation and collection. Just after the gas exit from the cyclone top, a hood divides the syngas stream in four parts: one is addressed to a hot filter connected to a heated line (where carbon particles are removed without any loss of hydrocarbons content) and then sent to a total hydrocarbon analyzer (THC); another part of the syngas stream is addressed to a conditioning line formed by two cold traps, three filters and a flow meter and then is sent to on-line gas analyzers (that measure CO, CO2 and O2 contents), to an on-line gas chromatograph and, finally, to the stack. These last two streams can be utilized to sample and analyse the content of acids (mainly HCl and H2S), ammonia and PAHs. Sampling of the condensable species was made at the reactor exit, for about 30 min, by means of four in-series cold traps, which are encased in an insulating container and chilled at a temperature of about 0 °C. The condensed samples are sent to offline analyses carried out as in the following steps: (a) dissolution
Fig. 1. Sketch of the gasifier and the connected sampling and measuring points.
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of organics in pure dichloromethane (DCM); (b) drying of the mixture of DCM and PAHs by means of a rotovapor to obtain the gravimetric tar for quantitative analysis; (c) weighing of the dried sample, dilution with DCM until to reach a known concentration (ppm); (d) injection in a gas chromatograph coupled with a mass spectrometer (GC–MS). The obtained results from the GC–MS represented the concentration (in ppm) of the detected compounds in the injected solution. The sum of these concentrations gave the total ‘‘detected heavy hydrocarbons’’. The remaining part of the sample represented the ‘‘undetected’’ fraction that the GC–MS was not able to detect because its molecular weight was mainly larger than 200 u.m.a. With this procedure the tars belonging to the classes from 2 to 5 of the tar classification system proposed by Kijel et al. (2004) have been recognized. Data obtained from on-line and off-line gas measurements and from chemical analyses of solid samples are processed to develop for each run complete mass balances on atomic species and the related energy balance. Carbon balance allows determining the content of carbon accumulated in the reactor as carbon loading. This latter value was experimentally verified at the end of each test by switching the operating conditions from reducing to oxidizing values at the end of each run and by recording and integrating the CO2 and CO contents produced by oxidation (Arena et al., 2009, 2010). 2.2. Materials and procedures Quartz sand with size distribution in the range 200–400 lm has been used during the experiments. It is the ‘‘typical’’ bed material of fluidized bed combustor and gasifier. The composition of the fuel mixture to be fed into the gasifier, as well as the best operating conditions in terms of equivalence ratio and gas velocity, have been chosen on the basis of a series of preliminary experiments carried out by using mixtures having different relative proportions between coal, plastics and wood (Mastellone et al., 2010a). A fuel mixture having 50% of German brown coal, 30% of plastic waste (polyethylene and polypropylene) and 20% of wood was chosen as reference fuel to simulate the co-gasification process. The ultimate analysis and the heating value of the tested fuel mixture are reported in Table 1. The fluidization velocity of fluidizing stream was fixed at 0.4 m/s while its composition was varied: streams of nitrogen and oxygen were mixed in the pre-heating section in order to reach the desired final value of oxygen that entered the fluidized bed gasifier. The feed rate of fuel was then adjusted in order to reach the selected value of the equivalence ratio. The composition of the gaseous, solid and liquid (condensate) products was analyzed and processed in order to evaluate the main performance parameters: syngas yield, defined as: SY = (Qsyngas/Qfuel) where Qsyngas is the mass flow rate of produced syngas and Qfuel is the fuel mass flow rate fed in the gasifier; specific energy, defined as: SE = (Qsyngas LHVsyngas)/ Qfuel, where LHVsyngas is the lower heating value of the syngas on Table 1 Ultimate analysis (as-it-is fuel based) and heating value of the tested mixture. Ultimate analysis of fuel mixture (%w) Carbon Hydrogen Nitrogen Sulphur Ashes Moisture Oxygen (by diff.) Lower heating value (kJ/kg)
50.5 7.4 0.5 0.2 4.6 14.8 22.0 21800
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volumetric basis evaluated on the basis of the LHVs of CO, H2, CH4 and CnHm with n < 6; cold gas efficiency, defined as: CGE = (Qsyngas LHVsyngas)/(Qfuel LHVfuel); carbon conversion efficiency, defined as: CCE = (Qsyngas Csyngas)/(Qfuel Cfuel), where Csyngasis the carbon fraction in the gaseous component of the syngas (i.e. dust and tar excluded) and Cfuel is the carbon fraction in the fuel.
3. Material and substance flow analyses All of the data obtained by analyzing gases, condensate and fine particles sampled during the tests were used to develop the material balances over the entire gasifier plant. The results were combined with an assessment tool, the Material Flow Analysis (MFA), which is named Substance Flow Analysis (SFA) when it is referred to a specific chemical. MFA/SFA is a systematic assessment of the flows and stocks of materials and elements within a system defined in space and time. It connects the sources, the pathways, and the intermediate and final sinks of each species in a specific process (Brunner and Rechberger, 2004). In this study MFA/SFA was applied to a system boundary that includes the BFB gasifier and the cleaning system for ash separation, i.e. cyclone, bubblers and filters (Fig. 1 and then Figs. 2–4). The software utilized to carry out the material and substance flow analyses is the STAN (short for subSTance flow ANalysis) by INKA software that is a freeware that helps to perform material flow analysis according to the Austrian standard ÖNorm S 2096 (Material flow analysis – Application in waste management). Each balance is performed with reference to a same block diagram on different layers, one of these is dedicated to the balance of ‘‘goods’’ (in this case, the total mass of reactants involved in the gasification reactions) and the others to different single substances (in the case of this paper carbon and hydrogen have been chosen). For each test, layers on total mass, carbon and hydrogen were elaborated. The layers ‘‘goods’’ allow calculating the overall yield of the fuel conversion into the syngas stream and those of the secondary streams, like fines and tar, whose management affects the plant operating costs and the potential environmental concerns. The carbon layers also allow to calculate the carbon conversion efficiency and to characterize the ‘‘waste’’ streams in term of carbon content. The accumulation of unconverted fuel in the gasifier is represented by the value reported inside the box ‘‘gasifier’’ and is due to the evidence that the kinetic is not infinite and then at steadystate it is necessary to have an amount of solid carbon (called ‘‘bed carbon loading’’) which reacts with the gasification medium to produce the final syngas. A part of this loading is made by fines that remain in the reactor under form of wall fouling. It is noteworthy that the yield of syngas, calculated by referencing to the fuel inlet, decreased with oxygen increasing, that the condensate did not change in an appreciable way and carbon fines escaped from the reactor, and collected at the cyclone, increased.
4. Experimental results The experimental tests were performed at given equivalence ratio, ER = 0.25, and superficial gas velocity, U = 0.4 m/s, while the volumetric oxygen content in the fluidizing gas stream injected at the gasifier bottom varied from 21%, to 26% and to 35%. In order to maintain fixed the ER value, the mass flow rate of fuel varied for each test accordingly with the oxygen flow rate. The results presented in the following describe the effect of oxygen concentration in the gasifying medium with reference to the energy balance (and then to the temperature profile along the reactor), mass balances (and then syngas composition and heating value) and yields of undesired by-products and waste.
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Fig. 2. Mass balance in the test carried out with air as fluidizing stream, with reference to total (A) carbon, (B) and hydrogen (C) mass flows.
4.1. Thermal effect The first effect of the utilization of an O2-enriched air is the decreasing of nitrogen fraction in the fluidizing gas. This implies that the thermal wheel action played by nitrogen gradually reduces, inducing a thermal accumulation in the reactor and then a temperature increase along it. In particular, for oxygen fraction in the fluidizing air larger than 26%, the rising of reactor temperature during the test was so high to make no safe the reactor operation, due to the reduced mechanical resistance of AISI 316L stainless steel, utilized as reactor building material. This imposed, for each experimental test, to arrange the set of the operating parameters in order to avoid a too high level of temperature. This is also in agreement with the consideration that it is not correct to compare results of tests carried out at a different temperature, since the latter strongly affects reaction rates and then syngas composition. In an auto-thermal gasifier, the reactor temperature can be seen as a process state variable, i.e. as the system answer
to a set of parameters, such as the equivalence ratio, residence time, waste chemical energy, quality of reactor insulation, composition and inlet temperature of the gasifying medium. It was then decided to obtain both a safe operation and a reliable comparison of results by tuning the pre-heating heat flux of the fluidizing gas (Fig. 5). In particular, the test with air (21% O2) was carried out by increasing the pre-heating temperature at about 790 °C in order to keep fixed the bed temperature at about 850 °C; during the test with 26% of O2 in the fluidizing gas, the temperature of this stream was lowered at 650 °C while for test with 35% of O2 it was decreased to 600 °C. The values reported in Fig. 5 suggest that the addition of pure oxygen to the air stream can be utilized to obtain a better control of the reactor temperature. It in fact gives the possibility to select (even though in a rather limited range) the best operating conditions from a kinetic or thermodynamic point of view as well as from that of an optimized pollutants control (e.g. low NOx, vitrified ashes, etc.). Fig. 5 also shows that the temperature in the dense bed
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Fig. 3. Mass balance in the test carried out with fluidizing stream having 26% of oxygen content, with reference to total (A) carbon, (B) and hydrogen (C) mass flows.
and in the splashing zone (i.e. the region just above the top of the bed, where there is the maximum turbulence induced by bubbles eruption) increased for all the tests up to 850 °C: this confirms that the exothermic reactions of partial oxidation mainly occur in these zones. Downstream of the splashing zone the temperature decreases at about 750 °C at the exit of the reactor, i.e. just before the cyclone entrance, as a consequence of the thermal losses and endothermic reactions, such as those involving radicals and polycyclic aromatic hydrocarbons (PAHs). 4.2. Syngas composition and yields Data reported in Fig. 6 refer to the syngas composition as continuously sampled after cleaning devices, in all the tests carried out at different values of oxygen in the fluidizing stream. As expected, the test carried out with enriched air with 35% of oxygen led to a less nitrogen-diluted syngas (40%, instead of about 60% obtained by using air). This implies higher concentrations of all syngas components: 13.8% for H2, 18.9% for CO, 6.0% for CH4, etc.
In order to remove most of the misleading effect due to the presence of different concentrations of the inert component in the syngas mixtures and to the (limited) difference of temperature, the mass fraction in the nitrogen-free syngas for each component were calculated. The comparison of these data is reported in Fig. 7: the only effect on mass fractions appears to be that on the carbon monoxide and carbon dioxide content (which increases and decreases, respectively, as a consequence of a larger oxygen enrichment of the gas stream) while there is a not substantial variations in the concentration of methane and just a slight increase in concentrations of total hydrocarbons and hydrogen. The relative contents of carbon monoxide and carbon dioxide are correlated to each other by means of the Boudouard reaction: CO2 + C 2CO. It could be argued that at a higher temperature, if there is an excess of carbon, the higher positive values of the Gibbs’ energy variation promotes the carbon dioxide breaking down to carbon monoxide (Basu, 2006). On the other hand, profiles of Fig. 5 indicate that the tests at 26% and 35% of oxygen fraction in the gasification medium were carried at an almost equal reactor temperature, as a result of the
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Fig. 4. Mass balance in the test carried out with fluidizing stream having 35% of oxygen content, with reference to total (A) carbon, (B) and hydrogen (C) mass flows.
adopted specific experimental procedure. Therefore, the reactor temperature did not determine the variation of CO and CO2 concentrations. A different amount of solid carbonaceous material inside the reactor is another possible explanation of the cited experimental evidence. The following discussion about the amount and characteristics of carbonaceous solids and their role during the gasification process aims to verify if this hypothesis is correct. On the basis on the material and flow analyses reported above, Table 2 reports the amount of bed carbon loading, i.e. the solid carbon that at steady-state condition is present inside the bed, and that of carbon that escapes from the reactor as elutriated fines, together with the specific volumetric and mass syngas yield. The latter appears to decrease as a consequence of a more oxygen content in the gasification medium: in particular, the specific mass of syngas obtained for air gasification is 1.55 times that obtained with O2-enriched air at 35% and the amount of elutriated carbon fines increased by about three times. As it can be deduced from the mass balances on atomic species reported in Figs. 2–4, there is a corresponding decrease in the bed carbon loading, from 134 gC/kgC-fuel
in the operation with air, to 114 gC/kgC-fuel and 90 gC/kgC-fuel for operation with 26 and 35% of O2-content in the fluidizing stream. On the other hand, data in Table 2 and Figs. 2–4 indicate that the total amount of solid carbon in the reactor under forms of bed carbon loading and elutriable carbon fines was, at steady-state conditions, substantially the same (about 170 gC/kgC-fuel). The distribution between these two phases was instead different, with progressively larger amounts of carbon fines in the splashing zone and freeboard in the O2-enriched air tests: this affected the equilibrium of Boudouard reaction and truly could explain the shift towards a larger CO production by confirming that the above cited hypothesis about the role of carbonaceous solids can be assumed as reasonable. Moreover, it might be noted that a so large increase of carbon fines generation is generally ascribed to a higher temperature increase or to the existence of catalytic activities of some metals (Mastellone and Arena, 2008). The absence of a plain difference of the mean temperatures along the rector wall leads to hypothesize that, at a mesoscale level, the exothermic reactions and the reduced thermal wheel action of nitrogen could generated
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Fig. 5. Temperature profiles along the fluidized bed reactor in the reported tests.
Fig. 6. Syngas composition on volumetric basis as obtained for the three tests carried out at different oxygen concentration of fluidizing stream.
local increases of temperature (hot spots) that cannot be recorded by thermocouples located at the wall and managed by a PID program that smooth out the fluctuations by averaging out data in the time. In a reaction environment with a very low presence of oxygen (Mastellone et al., 2010b), this could enhance the carbonization of hydrocarbons fragments directly to carbon solid phase (CnHm ? nC + m/2 H2) and limit the conversion into small and stable molecules (CO, CO2, CH4). This explanation appears to be in agreement with the measured parallel reduction of the condensate fraction that is constituted by molecules having a molecular weight larger than gaseous compounds but lower than carbonaceous fines. The different syngas composition led to a corresponding variation in the lower heating value (LHV) and specific energy of the
syngas. Table 3 reports the values obtained in the three tests and, in particular, shows the very large energy content of syngas obtained in the test with the highest oxygen concentration in the gasification medium: the LHV is about 9000 kJ/m3N and the specific energy is 4.3 kWh/kgfuel. This positive result is related to the reduced presence of nitrogen and then to higher volumetric fractions of hydrogen, carbon monoxide and methane, as it can be deduced from Figs. 6 and 7. As already recalled in the paragraph ‘‘materials and procedures’’, the gaseous products also contain heavy condensable compounds (i.e. having a molecular weight higher than benzene) that have been measured by means of condensation in a series of cold traps and quantified in yield and composition (Fig. 8 and Table 4). The socalled ‘‘condensate’’ amount represents the concentration of very
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Fig. 7. Syngas composition expressed as nitrogen-free mass fraction as obtained for the three tests carried out at different oxygen concentration of fluidizing stream.
Table 2 Distribution of different solid and condensate products in the experimental tests. ID test
Syngas specific yield
–
m3N/kgfuel–kg/kgfuel
Specific bed carbon loading gC-bed/kgC-fuel
Specific carbon fines production gC-fines/kgC-fuel
Specific carbon content in the condensate phase gC-tar/kgC-fuel
21% O2 26% O2 35% O2
2.5–3.1 2.2–2.6 1.7–2.0
134 114 90
37 59 82
6.9 6.7 5.8
Table 3 Low heating value of syngas and other performance parameters, as obtained in all the tests. ID –
LHV of syngas kJ/m3N
Specific energy of syngas kWh/kgfuel
CGE –
21% O2 26% O2 35% O2
5150 6850 8950
3.5 4.1 4.3
0.51 0.56 0.63
as of the carbon fines. A further support to these experimental data and to the proposed theoretical approach is given by the consideration that the presence of hot spots can enhance the metal extraction from the wall and from the fuel ashes and that these metals, under form of elements or clusters, can act as initiating nuclei for the formation of carbonaceous structures.
4.3. Effect of reactor height on syngas composition
heavy compounds to which must be added the BTX (benzene + toluene + xylene) quantity to obtain the tar amount, i.e. tar (mass/ time) = condensate (mass/time) + BTX (mass/time). The value of ‘‘tar’’ reported in Table 4 is constituted by benzene and other BTX for about 90%; this means that the condensate fraction that represents the true concern for syngas utilization is only 10% of the total ‘‘tar’’. Fig. 8 shows that the larger fraction of PAHs is always represented by naphthalene even though, for tests at higher oxygen concentrations, a larger amount of undetected species is present (last column of Table 4). These results suggest that the O2-enriched air gasification process has a limited influence on the content of light molecules in the syngas but the effect on the composition and yield of carbonaceous particulate and tar is instead remarkable. It is likely that the undetected fraction in the condensate fraction is composed by large fragments that are precursors of the formation of carbon nanoparticles and nanotubes (Arena et al., 2006) as well
A fluidized bed gasifier can be seen as a two-stage reactor having a first stage in the well-mixed dense bed and high turbulent splashing zone and the second one along the freeboard. Accordingly, two different sampling lines have been used to analyse the syngas composition with the aim to evaluate the relative importance of the freeboard with respect to dense bed + splashing zone. Figs. 6 and 9 report data measured at 210 cm (i.e. at the reactor exit) and 90 cm (i.e. just after the splashing zone), respectively. A comparative analysis of these data shows that there is a limited difference between the composition of syngas at 90 cm, i.e. just above the splashing zone, and that at the freeboard exit (at about 210 cm). It can be highlighted just an increase of aromatic compounds concentrations. This evidence confirms that in a bubbling fluidized gasifier most of gasification reactions completed inside the dense bed and the splashing zone (Knoef, 2005). The freeboard region is mainly the place where polycondensation reactions occur, in other words, the region where acetylene becomes benzene,
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Fig. 8. Composition of PAH detected and measured in the condensate phase for the three tests carried out with fluidizing streams having different oxygen content.
Table 4 Yield of total tar and PAHs detected in the heavy fraction of the syngas. ID
Tar (condensate + BTX) mg/m3N
Fraction of detected PAH in condensate (%)
21% O2 26% O2 35% O2
13500 21800 21700
76 33 28
benzene becomes naphthalene or phenantrene, multiple-ring compounds becomes clustered nanostructured material. The practical relevance to model the BFB gasifier as a sort of two-stage reactor lays on the real possibility to control the desired process in specific zone of the reactor where the process really occur. For instance, the controlling of gaseous products of syngas must be made in the bed + splashing zone; the controlling of PAHs and heavy compounds must be referred to the freeboard region. This means that a possible injection of reactants as secondary/
Fig. 9. Syngas composition on volumetric basis as measured just after the splashing zone (90 cm) in the three tests carried out at different oxygen concentration of fluidizing stream.
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tertiary streams, addition of catalysts, heating/cooling to enhance/ decrease the progressing of given reactions should be made with reference to the real reaction zone to be sure to obtain the expected result. 5. Conclusions The effect of the oxygen fraction in the fluidizing and gasification stream of a bubbling fluidized bed gasifier has been studied. A material and substance flow analyses were used to perform the mass balances and to highlight a series of data that can be utilized for a deeper understanding of the experimental results and then of the process mechanisms. It was experimentally confirmed that the main advantage of an O2-enriched air is the possibility to increase the bed temperature or, if a relatively low temperature of the reactor is desired (for instance, to prevent particles sintering), to reduce the size of the transversal cross section and the total cost of pre-heating. The tests were performed by keeping constant as much as it was possible the reactor temperature in order to allow a reliable comparison of the data. Under these conditions no crucial effect of O2-enriched air was detected. In particular, no remarkable effects were obtained in terms of tar reduction since its specific production slightly decreased when oxygen-enrichment was increased. On the contrary, tar composition changed by showing a decrease of recognized PAHs in the condensate and a corresponding increase in the carbon losses as elutriated fines. Moreover, the syngas specific yield decreased even though its specific energy remained almost unvaried. Acknowledgements The authors would like to thank the Italian Minister of University and Research for the financial support to this experimental activity, given in the framework of the PRIN-Research Project of National Interest 2008S22MJC_004. References Arena U., Mastellone M.L., 2005. Fluidized Pyrolysis and Gasification of Solid Wastes, in Proc. of Industrial Fluidization South Africa 2005, The South African Institute of Mining and Metallurgy (ISBN 1-919782-83-0), pp. 53–68. Arena, U., 2011. Gasification: an alternative solution for waste treatment with energy recovery. Waste Manag. 31, 405–406. Arena, U., Mastellone, M.L., Camino, G., Boccaleri, E., 2006. An innovative process for mass production of multi-wall carbon nanotubes by means of low-cost pyrolysis of polyolefins. Polym. Degrad. Stab. 91, 763–768.
Arena, U., Zaccariello, L., Mastellone, M.L., 2009. Tar removal during the fluidized bed gasification of a plastic waste. Waste Manag. 29, 783–791. Arena, U., Zaccariello, L., Mastellone, M.L., 2010. Fluidized bed gasification of wastederived fuels. Waste Manag. 30, 1212–1219. Basu P., 2006. Combustion and Gasification in Fluidized Beds. CRC Press. Brunner, P.H., Rechberger, V., 2004. Practical Handbook of Material Flow Analysis. CRC Press LLC, Boca Raton (FL). Defra, 2007. Advanced Thermal Treatment of Municipal Solid Waste, Enviros Consulting Limited on behalf of Department for Environment, Food & Rural Affairs (Defra), available on