Catalytic conversion of residual fine char recovered by aqueous scrubbing of syngas from urban biomass gasification

Biomass and Bioenergy 100 (2017) 98e107

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Research paper

Catalytic conversion of residual fine char recovered by aqueous scrubbing of syngas from urban biomass gasification Gnouyaro Palla Assima a, Stefano Dell’Orco a, Shahram Navaee-Ardeh b, Jean-Michel Lavoie a, * a
Universit
e de Sherbrooke, Chaire de Recherche Industrielle sur l'Ethanol Cellulosique et les Biocarburants (CRIEC-B), 4070, boul. de Portland, Sherbrooke, QC, J1L 2Y4, Canada b CRB Innovations, 4070, boul. de Portland, Sherbrooke, QC, J1L 2Y4, Canada

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 December 2016 Received in revised form 6 March 2017 Accepted 17 March 2017

Conversion of carbon contained in the solid residues (tars þ biochar) derived from urban biomass gasification named herein TC would allow enhancing the yield of carbon species (CO/CO2) in synthetic gas. For this purpose, three low cost materials have been tested as possible catalysts: iron species (reduced Fe), bone meal (BM), and ashes (ash) recovered from biochar complete oxidation. The parametric study used the following as variables: air GHSV, onset of reaction temperature, reaction time to optimize CO/CO2 molar ratio and tar content in the produced gas. Results showed an autocatalytic effect of biochar leading to the catalytic conversion of approximately 78% of tars by the native metals contained in TC. The catalytic effect was further enhanced by adding Fe, BM, and extra ash. Addition of Fe catalyst resulted in significant heat generation (temperature increase of ca. 500  C) and a twofold decrease in reaction time to consume all the carbon. Use of ash and BM as catalysts exhibit heat generation comparable to Fe, along with an improved reaction time, complete tars conversion and a CO/CO2 molar ratio to above 1.3 in the produced gas. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Oxidation Catalyst Urban biomass Tars reforming Biochar

1. Introduction Thermochemical conversion of renewable carbon-rich residues (such as biomass-containing residues) to gaseous fuel (biosyngas) is increasingly regarded as a promising, eco-efficient, and selfsustainable route for the production of energy, fuels, or various chemicals as it also offers a neutral carbon footprint on anthropogenic greenhouse gas emissions [1]. In Canada, the annual production of residual biomass (commercial harvest of forest and agricultural biomass) is estimated to ca. 1.43  108 metric tons carbon [2]. This residual biomass has a potential energy content of approximately 2.25  109 GJ which is tantamount to energy content of ca. 3.7  108 barrel of oil (on a LHV basis). As the daily production of Canadian Crude Oil is estimated to be around 3.9  106 barrels (National Energy Board, Government of Canada), the energy recoverable from the residual biomass annually is significant as it

Abbreviations: GHSV, Gas hourly space velocity; TC, Tar-char; BM, Bone meal; Ash, Ashes (inert) from TC complete oxidation. * Corresponding author. E-mail address: [email protected] (J.-M. Lavoie). http://dx.doi.org/10.1016/j.biombioe.2017.03.015 0961-9534/© 2017 Elsevier Ltd. All rights reserved.

might account for 9.3 days of oil production [3]. The thermal conversion of biomass into biosyngas or biosynthetic gas (known as gasification) is a process where the conversion is made to produce CO, H2, CO2, CH4, and minor quantities of other hydrocarbons, essentially low molecular weight alkanes/alkenes and aromatics. In some cases, steam is added to the feed to increase the level of H2 in the produced gas [4]. Typically, the resulting biosyngas has a low calorific value (3.8e5.6 MJ/m3 against 38 MJ/m3 for natural gas) [5]. Various types of gasification technology were developed over the years and applied for biomass based feedstocks conversion. They usually fall in three main classifications: fixed-bed or movingbed gasifiers (updraft or downdraft), entrained-flow gasifiers and fluidized-bed gasifiers [6,7]. The big challenge in the particular case of fluid beds is achieving total carbon conversion of the biomass into synthetic gas directly in the gasifier. This challenge comes from the very rapid formation of char when biomass is heated in an oxygen-limited environment. Generally, only 80e90% of the carbon in the biomass feed is converted into permanent gases at “normal” gasification temperatures, between 700 and 1000  C [8]. The unconverted carbon ends up in (a) entrained fine particles of char which contain some of the ash and tars or (b) in larger particles that

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are retrieved with the heat carrier through appropriate ducts. The larger particles are normally subjected to an oxidizing treatment to recover the heat from the carbon present, thus raising the temperature of the heat carrier that is then returned to the fluid bed. Regarding the fines particles, a possible approach to convert the entrained char and tars would be to raise the temperature along the gasifier (which implies adding O2-containing mixtures or simply air) by oxidizing part of the C in the char and tars and have steam and CO2 react with the remaining C. If the temperature is high enough, the C is consumed and permanent gases are produced in a time span that depends very much on the temperature at which the operation is carried out. Slag will be inevitably formed and its complexity will have to be appropriately handled. During the gas conditioning downstream the gasifier, a common practice in biomass gasification processes is to subject the primary gas, to one or several scrubbing treatments. The conditioning encompasses cleaning tar and other contaminants as well as downstream reforming processes. The cleaning step enables the removal of particulates, tars, alkali compounds, nitrogen, and sulfurcontaining compounds, while reforming processes are generally meant to achieve a desired composition of syngas for a specific usage. The scrubbing step(s) generate sludge which is often recovered as a tar-char cake typically containing 50 wt% moisture [9]. If desired, it can be further dried to lower moisture contents (20 wt% being relatively easy). The presence of tars, typically PAHs, makes it difficult to transport and use the recovered tar-char cake in burners (such as those in cement kilns) due to fouling issues. Also, careful control of emissions in the flue gas following combustion to recover the heat is required. To achieve full carbon conversion in such processes, the tar-char cake needs to be reprocessed whilst ensuring to convert all the carbon it contains (tars þ char). The char content is readily converted into gas at temperature 500  C while the tars conversion or destruction can only be achieved either under severe temperatures (temperature above 1000  C) [10] or catalytically (use of catalyst at temperature below 1000  C) [11]. Processing at temperature above 1000  C especially in autothermal process tantamount on the one hand sacrificing the tar-char indigenous carbon under enriched oxygen environment while producing elevated amounts of CO2 at the expense of the desired CO. Moreover, such severe temperatures might trigger the amalgamation of heavy metals enclosed in the tar-char matrix which typically start to melt, agglomerate and/or vaporize above 1000  C [12,13]. In this work, the <1000  C catalytic oxidation of the tar-char cake into synthetic gas has been explored to avoid complications related to slagging and minimize CO2 production. The catalytic conversion of tars and char is in fact widely covered in the open literature while the recent trend is to use cheap non-metallic compound for char and tars conversion. The common features with catalysts reported as effective for tars reforming such as dolomite (CaMg(CO3)2), calcium oxide (CaO), calcium carbonate (CaCO3), calcium hydroxide [Ca(OH)2], magnesium oxide (MgO) and potassium carbonate (K2CO3), olivine (Mg2SiO4), chrysotile (Mg3Si2O5(OH)4) were their contents in Ca, K, and Mg and Fe that were held liable for their catalytic behavior [14e22]. One of the main targets of this work is to gain additional knowledge of the catalytic effects on tars and char conversion using three additives: iron species (Fe), calcium phosphates (bone meal -BM) and ashes produced from the gasification of the feed material. Experimental investigations have been performed in a bench scale allothermal semi-batch reactor, controlling the partial pressure of O2 so that the produced gas would have a CO/CO2 molar ratio of ~1 (such as in the case of autothermal gasifier using O2 and steam). The gasifier was operated as a fixed bed reactor and the main parameters monitored were temperature in the reaction zone as well as in the free zone

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above the fixed bed, reaction time, residual PAHs, and carbon conversion into synthetic gas. Special consideration was also given to the heat generated during the oxidation reaction since achieving an autothermal mode of operation while minimizing the production of CO2 was targeted. 2. Material and methods 2.1. Materials characterization The tar-char cake (TC), iron species (Fe), bone meal (BM) which was rich in calcium and phosphorus, and ashes from combusted tar-char cake (ash) samples consisted of fine powder with D80  90 mm. TC was received from Enerkem Alberta Biofuels, a commercial gasification plant located in Alberta (Canada), processing urban biomass such as sorted Municipal Solid Waste (MSW), Institutional Commercial and Industrial Waste (ICI), Construction, Renovation & Demolition Waste (CRD) and treated wood (railway ties, spent power and telephone poles). Fe (grade  99%) was purchased from Sigma-Aldrich as fine powder and BM was purchased from ECOlogical FERTILIZERS, while ashes (ash) were produced from TC following calcination at 750  C overnight. The tars, char, and water content of TC were determined using soxhlet extractions, Karl Fischer titration and gravimetric measures. Water as well as some of the tars were extracted using isopropyl alcohol while the remaining tars were extracted using toluene. Water content was quantified using a Karl Fischer titration on the extracts. The residual solid was considered as composed of char (fixed carbon and ash). The ash content was thereafter determined following a complete combustion of the char. The organic content of BM was determined by weight difference between the initial mass and the one obtained after its complete combustion. TC and BM samples were analyzed using a Perkin-Elmer inductively coupled plasmaoptical emission spectrometry (ICPOES 43000DV) to quantify the following elements: Al, As, Ba, Be, B, Cd, Ca, Cr, Co, Cu, Fe, Pb, Mg, Hg, Ni, P, K, Se, Si, Ag, Tl, U, V, Zn, and Zr. CHNOS analysis was also carried out on TC and ash samples using a TruSpec Micro (LECO Corporation) to determine their respective contents in C, H, N, O and S. The sample's crystalline phases were identified using X-ray diffraction (XRD) spectra registered on a PANalytical X'Pert Pro MPD, powered by a Philips PW3040/60 X-ray generator and fitted with a PIXel 1D. X-rays were generated from a Cu anode supplied with 40 kV and a current of 50 mA. Diffraction data was acquired by exposing powder samples to Cu-Ka X-ray radiation at 1 /min (0.02 step size) over the 5e70 scattering angle range and at characteristic wavelengths of 1.5418 Å. Results were compared to those compiled in the Joint Committee on Powder Diffraction Standards library. The unconverted carbon and entrained metal levels were analyzed by Groupe EnvironeX using a GC-6890A MS-5973A (Agilent), a GC-7890A MS-5975C (Agilent) and a Q ICP-MS (Thermo Scientific) after concentrating the solution to identify and/or quantify the metals, PAHs and phenols, at each cleaning step of the produced gas. The metals were thereafter computed versus their initial amounts in the feed. Finally, an infrared gas monitor (Guardian Plus) was used to follow the production of CO and CO2 online. H2 was not monitored in this study as its production was negligible in the current steam-free process. 2.2. Oxidation setup and procedure TC conversion was performed in a vertical-electrically-heated Inconel alloy 600 reactor (ID ¼ 2.54 cm and H ¼ 93.98 cm) presented in Fig. 1. The experimental setup was mainly composed of (1) a gasifier where the reaction occurs, (2) a controlled air and N2 feed (3) a water scrubber to remove fine particles and phenols from

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Fig. 1. Diagram of the semi-continuous updraft centrifuge cake gasifier assembly [1]. Gasifier [2], Air/N2 flow controller [3], Water scrubber [4], Water trap [5], NMP bubbler [6], CO and CO2 infrared analyzer.

the produced gas, (4) a water trap to dry the washed gas, (5) an N
thyl-2-pyrrolidone (NMP) bubbler to trap the tars and (6) a Me Guardian Infrared gas analyzer for CO and CO2 online measurements. Before starting the reaction, the required air flow rate was set in order to achieve a define GHSV in the gasifier. The oxidant flow was preheated at 200  C and fed upwardly into the gasifier. The gasifier itself was externally heated by two separate heating elements which allowed preheating the gasifier along its length at two distinct temperatures. It was deliberately chosen to heat the free zone (upper one-third of the reactor) at higher temperatures than the reaction zone (bottommost two-thirds) to improve the chances of decomposition of any tar that would have escaped from the reaction zone. Temperatures in the two heated zones were monitored via two high temperature K-type thermocouples positioned respectively in the reaction zone (T1) and free zone (T2). Throughout this study, reaction zone preheating temperature of 500, 600 and 750  C and a free zone preheating temperatures of 750 and 900  C were tested. As the gasification was carried out in an updraft manner, the samples with and without catalysts were pelletized beforehand. In the case of the TC doped with catalysts, the mixture to be pelletized was manually done using a ceramic pestle and crucible while the catalyst was gradually added to the TC sample. Homogeneous mixtures of TC containing 5, 10, 20 wt%Fe; 10 wt%BM and 20 wt% Ash were made and tested. The average pellet size was ca. 21 mm3. The total amount of pellets fed at each experiment was targeted to maintain 10 g of dry TC in it. Once the desired temperatures T1 and T2 were reached, pellets were introduced from the top of the reactor via a lock hopper system. Insertion of pellets in the reactor coincided with the starting time of the reaction. As the reaction started, the hot gas produced in the gasifier was routed to the water scrubber where a shower of cold water washed the gaseous components, phenolics, and PAHs and trapped the entrained fine particles that might leave the gasifier. The scrubber water was initially set at pH 11 to boost its adsorption efficiency and was continuously chilled in order to reduce the temperature of the produced gas to around 19  C. The

produced gas leaving the water scrubber was thereafter dried in a water trap before bubbling in an NMP solution where the remaining PAHs were trapped. The clean gas leaving NMP bubblers was quantified using an online infrared CO and CO2 analyzers. The CO and CO2 signals were monitored using LabVIEW. The reaction time was deliberately restricted below 360 s in order to render viable a design for reactor sizing and/or feedstock treatment rate at industrial scale while insuring a residence time that allows complete oxidation of PAHs and fixed carbon. The lag between the gas production and its detection caused by an extended residence time of the produced gas in the system (reactor, scrubber and bubbler) made the direct determination of reaction time difficult. The latter was therefore approximated using the temperature profile which reflects the course of the oxidation reaction and refined using N2 injection as substitution for air to quench the reaction and check its completion through further combustion. A O2/C molar ratio of 1.82 was compounded beforehand as compulsory to convert the entire carbon content of TC to CO and CO2, considering the O2-carrying flow during the entire oxidation time. Lower O2/C ratios led either to uncompleted oxidation under higher GHSV (gas flow rate/sample volume under standard temperature and pressure) and/or longer reaction times under low GHSV. Within the time constraints imposed in this work, the impact of air flow rate and preheating temperature on catalystfree TC were comprehensively addressed while aiming at high carbon conversion and CO/CO2 molar ratio. The catalysts were thereafter used under the best conditions of GHSV and preheating temperature to grasp their effect and improve the process. It should be understood that during the interval between which high reaction rates are observed (as per CO and CO2 peaks), the O2 in contact with the pellet samples is well below that corresponding to stoichiometric combustion. 3. Results and discussion 3.1. Sample composition TC collected as centrifuge cake was received with 22 wt%

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moisture. The combined soxhlet extractions, thermal decomposition and gravimetric measures performed on TC reported tar, char (char þ ash) and ash (inert) contents of 6.02%, 93.97% and 21.51%, respectively (dry weight basis). The total carbon content of TC assessed by the carbon analysis was 52.56% (dry weight basis). A summary of the tars, char, and ash content as well as the CHNOS results are presented in Table 1. The ash collected after TC oxidation in the reactor had no tars and contained negligible amount of carbon (0-2.4 wt%). The carbon content (mineral and organic) in the BM was determined by oxidative thermal decomposition and amounted to 40.5 wt%, while the Fe sample was carbon free. XRD results of TC, BM and ash are presented in Fig. 2. Despite the organic and amorphous matrix mitigating any mineral phase diffraction, tiny peaks of quartz [SiO2] and magnetite [Fe3O4] were detected in the TC. BM was composed of calcite [CaCO3], quartz [SiO2], brushite [CaPO3(OH).2H2O], and hydroxylapatite [Ca10(PO4)6(OH)2]. The main crystalline phases composing the TC ashes were gehlenite [Ca2Al2SiO7] and perovskite [CaTiO3]. The other detected crystalline phases were minor and included calcium silicate [Ca2SiO4] and hatrunite [CaSiO6]. The results indicated that BM and ash were mainly composed of calcium, alumina and silicon bearing minerals. The metals analysis results corroborated that of the XRD. Ca, Al, Si, Fe and Mg were the major elements of TC ashes while BM predominantly contained Ca, P, Na, K, and Fe, respectively. Fe catalyst was composed of elemental iron at 99%. The elemental composition of TC, Fe, and BM are compiled in Table 2. 3.2. Oxidation of tar-char with no additive 3.2.1. Effect of air GHSV TC was oxidized under three air GHSVs of 36640, 45800, and 61067 h1 while the overall O2/C ratio was kept at 1.82. The reaction zone temperature (T1) was initially set at 750  C, while the free zone (T2) was set at 900  C. The impact of GHSV on the produced gas composition and the reaction zone and free zone temperature profiles are depicted in Fig. 3. In order to provide unbiased comparison of results obtained at different GHSVs the produced gases volume fractions were plotted as vol%*GHSV since TC feed was constant.

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As a key observation, CO is the first gas produced. It overlapped with CO2 (Fig. 3a) appearing later. Application of a Gaussian fitting to the produced CO and CO2 gas curves for each experiment always led to a two peak profile. Oxidation of the more reactive compounds happens first leading to a higher production of CO while combustion of the C residue comes after [23]. Since the TC is solely composed of tars and char (as carbon sources) an hypothesis would be that the first peak is related mainly to tars cracking/oxidation, while the second peak refers to fixed-carbon (char) oxidation. This assumption was confirmed by reacting TC in the same manner as described above but after its tars removal by thermal desorption under nitrogen at 500  C for 15min. The oxidation results of TC and desorbed TC obtained under GHSV of 36640, T1 and T2 of 750 and 900  C, respectively are presented in Fig. 4. In absence of tars, a much smaller CO peak followed by a broad CO2 peak was observed with regards to the sample containing tars. A CO/CO2 molar ratio of 0.58 was obtained in absence of tars versus 0.91 when they were non-pre-desorbed. The increase in CO2 production throughout the development of the reaction was attributed to the instantaneous O2/C molar ratio increase with the extent of the reaction owing to the constant air feed and the decarbonisation of the sample. The effect of air GHSV on the produced gas composition is presented in Fig. 3a. Increasing the GHSV from 36640 to 45800 and 61067 h1 respectively (GHSV1, GHSV2, GHSV3) resulted in shorter reaction time, low CO (thin peak), and high CO2 (large peak) production. Obviously, higher air flow rate appeared to enrich the reaction medium in O2, thus prompting CO2 formation as well as fast carbon depletion in the sample. The reaction times implemented under the various GHSVs are presented in Table 3. The best CO/CO2 ratio (0.91) was achieved under GHSV1 with a reaction time of 325s coinciding with complete carbon depletion in TC. Still, the comparison between the results obtained with full oxidation of TC and tar-devoid TC under the various GHSV suggested that only ~78% of the tars was cracked/oxidized during the reactions, leading to ca. 97.6% total carbon conversion. This rate corresponds to about 22% of the initial tars in the TC leaving the reactor through thermal desorption without being fully cracked/oxidized into CO or CO2. Thus the non-converted PAHs amounted roughly to 1 g/kg of TC. Those tars were assessed qualitatively by GC-MS analysis and up to five rings PAHs were recovered after the reaction. A summary of the

Table 1 Proximate and ultimate analysis results of tar-char (TC) and tar-char ashes (ash) achieved through combination of elemental, GC-MS, Karl fisher, and gravimetric analyzes. Humidity

CHONS and inert C H O N S Inert (ash) PAHs, phenols Type of PAHs in the as-received TC Naphthalene 2-ethynyl-naphthalene Acenaphthylene Acenaphthene Fluorene Phenanthrene Anthracene 4H-Cyclopenta [def] phenanthrene Fluoranthene 2.3-dihydrofluoranthene Cyclopenta [cd] pyrene

TC (char containing some tars)

Ash (from calcined char)

22

0

52.56 (±0.80) 1.28 (±0.10) 29.63 (±2) 0.77 (±0.02) 1 21.51 6.02

2.4 (±0.55) 0 (±0.07) e 0.13 (±0.02) e 98 0 Benzo [ghi] fluoranthene Chrysene Benzo [c] phenanthrene Benzo [a] pyrene Benzo [e] pyrene Benzo[ k] fluoranthene Perylene Benzo [ghi] perylene Indeno [1.2.3-cd] chrysene Coronene

wt % (wet)

wt wt wt wt wt wt wt

% % % % % % %

(dry) (dry) (dry) (dry) (dry) (dry) (dry)

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Fig. 2. XRD spectra in the 5e70 2q region (Cu Ka) of Tar-char (TC), bone meal (BM) and tar-char ash (ash).

Table 2 Results of metals analysis on tar-char (TC), iron species (Fe), bone meal (BM) and tarchar ash (ash) samples, achieved though ICP-OES. Metals

TC

Fe

BM

Concentration (mg/kg) with SD (0.21e8.38%) Aluminium (Al) Arsenic (As) Barium (Ba) Berylium (Be) Boron (B) Cadmium (Cd) Calcium (Ca) Chromium (Cr) Cobalt (Co) Copper (Cu) Iron (Fe) Lead (Pb) Manganese (Mn) Magnesium (Mg) Mercury (Hg) Nickel (Ni) Phosphorus (P) Potassium (K) Selenium (Se) Silicon (Si) Silver (Ag) Sodium (Na) Thallium (Tl) Uranium (U) Vanadium (V) Zinc (Zn) Zirconium (Zr)

29900 93 1005 0.3 34.3 8.75 123000 237.5 16.5 645 7250 210.5 266 6400 1.00 51.5 e 1750 1.0 26750 4.7 e 15.0 20.0 11 1415 10.6

e e e e e e e e e e 990000 e

800 e e e e e 343200 e e 100 1700 e

e e e e e e e e e e e e e e

5900 e e 188500 14300 e e e 15720 e e e 300 e

carbon conversion results as well as the type of tars desorbed and trapped in the NMP bubbler are presented in Table 3. Regarding the temperature profiles within the gasifier, a strong endothermic peak was perceived in the reaction zone (T1) in the

early moment of the reaction (first 30 s) under low GHSV, which tend to fade out when increasing the air flow rate from GHSV1 to GHSV3 (Fig. 3b). These endothermic peaks are related to water release from the sample while its behavioral change as the air's superficial velocity increases is attributed to a dilution factor and to an enhanced mass transfer. The heat generated by the carbon conversion to CO and CO2 once the water has been released led to an increase in the reaction zone temperature that peaked at around 1100  C, regardless of the GHSV. However, the autothermal domain (temperature higher than the heater set point) was wider with lower GHSV owing to the longer reaction hence the sustained heat production and the slow heat dissipation. Interestingly, a peculiarity was noticed at the free zone (T2 profile) where some exothermic behaviors were noticed for GHSV2 and GHSV3, but not for GHSV1. Such heat generation implied that oxidation reactions were also occurring at the top of the gasifier, pursuing those occurring in the reaction zone. Since the total amount of carbon converted remained constant under the various GHSVs, the exothermic peaks monitored at the free zone were not caused by the oxidation of the tars unconverted in the reaction zone. Otherwise, the total amount of carbon converted would be higher for GHSV2 and GHSV3. The two bumps discernible in the T2 profile clearly evoke the two different stages of oxidation previously identified as volatiles (tars) and fixed-carbon (char) oxidation. If the oxidation occurring at the free zone was that of tars, only one bump would be observed. Yet, two bumps were noticed at T2, referring to a second oxidation of products resulting from the reaction that took place in the reaction zone. These reactions are rather the conversion of produced CO into CO2 in the presence of the enriched O2 environment prompted by high GHSV. The absence of oxidation at the top of the reactor under GHSV1 indicate that the CO leaving the reaction zone is vented from the reactor prior to being oxidized to CO2 as proven by the high CO/CO2 ratio obtained under this air flow condition. It is noteworthy that the time axis of the produced gas

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Fig. 3. Impact of air GHSV on (a) syngas composition and (b) reaction zone (T1) and free zone (T2) temperatures.

Fig. 4. CO and CO2 production profiles from Tar-char (TC) oxidation with/without tars desorption under air GHSV of 36640 h1 and preheating temperature of 500  C.

Table 3 Results of tar-char (TC) oxidation under air GHSV of 36640e45800 and 61067 h1 and types of tar recovered in the NMP bubbler. Air GHSV (h1)

Total exposure (reaction) time (s)

36640 325 45800 260 61067 195 Type of PAHs recovered in the NMP bubblers and identified by GC-MS Naphtalene 1H-Indene, 1-ethylidene Biphenyl 1,4-Ethenonaphthalene, 1,4-dihydroAcenaphthylene Dibenzofuran 1H-Phenalene Phenanthrene Anthracene

CO/CO2 molar ratio

Carbon conversion (%)

T1/T2 ( C)

0.91 0.65 0.44

~97.6 ~97.6 ~97.6

750/900 750/900 750/900

monitoring needs to be adjusted to the time required for the gas to go from the reactor to the analyzer. The time axis of temperature, however, reflects the real reaction progress as the temperature was monitored without any time lag. At this point of the discussion, it is understood that the right oxygen supply is compulsory to avoid ongoing oxidation of large amounts of CO into CO2 downstream the

Mepivacaine Fluoranthene Pyrene Triphenylene Benz[e]acephenanthrylene Benzo[j]fluoranthene Benzo[k]fluoranthene Benzo[ghi]perylene

oxidation area where the temperature lies between 900 and 200  C. In the present study, GHSV1 was suitable for enhanced CO/CO2 molar ratio within the required reaction time span. Independently of the air flow rate, the white/gray solid residue (ash) resulting from TC oxidation maintained the initial size of the TC pellets introduced in the reactor. Ash CHN composition can be

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found in Table 1. The fact that TC ashes remained pellets at the end of the reaction suggested that the reaction occurs following a “shrinking core model”. This further confirmed the fact that under increased air flow rate the oxidant supply was faster than the product diffusion out of the pellets' porous structure, explaining the increased CO oxidation to CO2. The ash pellets were also found to maintain a good mechanical strength probably owing to the partial melting of metals, suggesting the feasibility of a downdraft or an updraft reactor. Such reactors operated at well-tuned GHSV could provide as producer gas with CO/CO2 ratio close to 1.

3.2.2. Effect of preheating temperature Combining preheating temperature of 750  C and free zone temperature of 900  C has led to high carbon (97.6%) and tars (78%) conversion into CO and CO2 in a reaction time that could be adapted to industrial conditions, either for a downdraft or updraft secondary reactor. The fact that no PAHs were oxidized at the top of the reactor at 900  C suggested that lower temperatures might also lead to comparable carbon conversion over similar time span. Since the air GHSV of 36640 h1 led to the highest CO/CO2 ratio, it was selected to investigate the effect of lower preheating temperatures on TC oxidation. Experiments were therefore conducted at preheating temperatures of 600 and 500  C combined with a free zone temperature of 750  C. The results are presented in Fig. 5. As showed in Fig. 5a the CO peak was found to increase with decreasing the preheating temperature, unlike the CO2 peak. Even though the CO2 production is delayed at lower preheating temperatures, the overall reaction duration for a complete C conversion was the same. Decreasing the reaction initiation temperature from 750 to 600 and 500  C, respectively led to CO/CO2 ratios of 0.91, 1.08, and 1.41. The increase of CO as compared to CO2 with the decrease of preheating temperature is mainly explained by the low carbon oxidation kinetics of char at low temperatures and an improved in-situ chemical-looping-combustion (carbon oxidation by indigenous metals within the TC which become oxidized over time) predominantly producing CO from the fixed carbon. Temperature variations throughout the reaction are presented in Fig. 5b. The reaction zone temperature (T1) exposed a higher heat production when the preheating temperature was elevated. This is consistent with Zhang and coworkers who demonstrated that the combustion heat release is positively correlated to the initial temperature [24]. Temperatures maxima of 1100, 780, and 773  C were reached under initial T1 of 750, 600, and 500  C respectively, and after reaction times of 168, 192, and 240 s. The time lag between the sample introduction and the heat generation

(Fig. 5b) can be correlated with the CO2 release (Fig. 5a). This implies that the heat generated in the early moments of the reaction by carbon conversion (into CO) served mainly to offset the heatdemanding water release (endothermic reaction). The fact that no heat production was perceived in the free zone (flat T2 profile) suggested that, even at low preheating temperatures (such as 500  C), the oxidation reactions were all occurring in the reaction zone and virtually no carbon-based structure reacted outside the fixed bed holding the samples. Keeping in consideration that the process is updraft, it is conceivable that the metals might leave the reactor through entrainment or evaporation. In order to verify this assumption, the scrubber water was analyzed to identify and quantify any entrained metals. Phenols and unconverted organic compounds that would have escaped from the reactor were quantified as well. The composition of the scrubber water is presented in Table 4. The results showed that the level of metals, phenols, and PAHs measured in the scrubber water are mere traces, being below the limit of quantification for the apparatus and method used. The low amount of PAHs detected in the scrubber water was explained by the fact that tars are hydrophobic and were rather collected in the NMP bubbler as previously showed in Table 3. Moreover, the low content of metals in the scrubbing water demonstrated that all the metals contained in the TC have remained virtually trapped in the resulting ash, offering a safe recovery and disposal option for these heavy and possibly hazardous metals.

3.3. Oxidation of tar-char with additives Fe, BM, and ash were tested as potential low cost catalysts to improve the conversion of tars contained in TC into synthetic gas. First, results of Fe-doped TC oxidation are presented in Fig. 6. In general, addition of iron species to TC led to higher CO2 production (Fig. 6a). Increasing the iron content from 0 to 20 wt% under GHSV1 at initial T1 of 500  C led the CO/CO2 molar ratio deterioration from 1.41 to 0.80, and 0.40 when 0, 5, and 10 wt% Fe were added, respectively. The said ratio increased back to 0.95 when 20 wt% Fe was added. Counterproductively, the presence of iron within the TC pellets catalysed the carbon conversion mainly into CO2 owing to the onset of the oxidation of elemental iron species. Yet, as the Fe content reached 20 wt%, the reaction pattern was reversed, yielding to an increased CO production. Still, the use of Fe at an elevated content such as 20 wt% did not provide any improvements in term of CO/CO2 ratio, as compared to Fe-free TC (0.95 versus 1.41). Nevertheless, the conversion time was found to decrease as a

Fig. 5. Impact of reaction preheating temperature on (a) syngas composition and (b) reaction zone (T1) and free zone (T2) temperatures.

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105

Table 4 Contents of metals, phenols and PAHs measured in the scrubber water at the outlet of the gasifier via Q ICP-MS (Thermo Scientific) and GC-6890A MS-5973A (Agilent), a GC7890A MS-5975C (Agilent), respectively. Metals

Content (mg/g) with SD (0e2.6%)

PAHs and phenols

Content (mg/g) with SD (0e2.6%)

Aluminium (Al) Silver (Ag) Arsenic (As) Cadmium (Cd) Chromium (Cr) Cobalt (Co) Copper (Cu) Tin (Sn) Mercury (Hg) Molybdenum (Mo) Nickel (Ni) Phosphorus (P) Lead (Pb) Selenium (Se) Zinc (Zn)

1.21 <0.06 <0.06 <0.03 <0.06 <0.06 0.24 <0.06 0.0003 <0.06 <0.06 0.19 0.04 <0.06 0.15

Acenaphthene Anthracene Benzo (a) anthracene Benzo (b) fluoranthene Benzo (j) fluoranthene Benzo (k) fluoranthene Benzo (a) pyrene Benzo (e) pyrene Chrysene Dibenzo (a, h) anthracene Dibenzo (a, i) pyrene Fluoranthene Fluorene Indeno (1,2,3-cd) pyrene Naphthalene Phenanthrene Pyrene Phenols

<0.06 0.17 <0.06 <0.06 <0.06 <0.06 <0.06 <0.06 <0.09 <0.06 <0.06 0.37 <0.90 <0.06 0.32 0.40 0.49 0.01

Fig. 6. Impact of Fe addition on (a) syngas composition and (b) reaction zone (T1) and free zone (T2) temperatures.

function of the Fe content and went from 325s to 316, 239 and 162s as 5, 10 and 20 wt% Fe were added, respectively. Moreover, the addition of Fe led to a quasi-complete conversion of TC (99%) thus virtually of tars as well. Regarding the heat generation, addition of Fe in TC pellets was found to increase the reaction temperature. As exposed in the previous section, oxidation of Fe-free TC from an initial temperature of 500  C induced an increase of temperature, owing to carbon oxidation, cumulating at 773  C. Unlike the Fe-free TC oxidation, the use of Fe-doped TC under the same reaction conditions yielded to elevated temperatures of 1008, 1221 and 1266  C when 5, 10, and 20 wt% Fe were used, respectively (Fig. 6b). Aside from the increase of amount of carbon converted into CO2, which contributed to generate more heat, iron oxidation in presence of air oxygen also greatly helped to the temperature upsurge [25]. Such high temperatures under oxic environment are known to enhance carbon conversion to CO2 to the expense of CO, but also to improve tars decomposition into CO and CO2. The simultaneous presence of Fe and C as two competitors for oxygen eased a double oxidation of carbon as air passed across the sample bed. Right from the time that the sample fell in the reactor, oxygen oxidized C to CO and/or CO2 and elemental iron into iron oxides. The produced CO thereafter swiftly reacted with the active sites of the oxidized iron available

within the pellets to generate CO2. The number of oxidizing sites appeared to be proportional to the amount of CO2 produced from the newly formed CO (5-10 wt% Fe). However, the use of a higher content of elemental iron such as 20 wt% as catalyst in the TC pellets gave rise to a partial oxidation of its iron content by the ascending oxygen in air. This implies an evolving bilayer configuration consisting of a bottommost oxidized iron and an uppermost elemental iron layer promoting the newly formed CO2 reduction into CO as it transits through the layer where elemental Fe is oxidized. Obviously, the reducing layer built when 20 wt% of Fe was used was not enough to increase the CO/CO2 ratio to that obtained with Fe-free TC. The residue collected after the reaction exhibited strong magnetic properties suggesting that the iron would have been oxidized mostly into magnetite. Oxidation of BM- and ash-doped TC exposed temperature profiles akin to that of Fe-doped TC. Maxima of 1229 and 1298  C were reached with addition of 20 wt% ash and 10 wt% BM, respectively. Such high heat productions contributed to a fast tars cracking (and oxidation of intermediates), while the metals constituting the BM and ash contributed to reduce the reaction time. A summary of catalyst-doped TC and the associated reaction time, carbon conversion rate, CO/CO2 ratio, preheating temperature, and temperature maxima reached are compiled in Table 5. From the results, it

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Table 5 Results of tar-char (TC) oxidation in presence of Fe, BM and Ash catalysts under GHSV of 36640h1, T1 ¼ 500  C and T2 ¼ 750  C. Sample

Reaction time (s)

Carbon conversion (%)

CO/CO2 molar ratio

Initial temperature T1 ( C)

Highest temperature reached ( C)

TC TCþ5 wt%Fe TCþ10 wt%Fe TCþ20 wt%Fe TCþ10 wt%BM TCþ20 wt%Ash

325 316 239 162 160 140

~97.6 ~99 ~99 ~99 ~99 ~99

1.41 0.80 0.40 0.95 1.28 1.33

500 500 500 500 500 500

773 1008 1221 1266 1298 1229

can be noted that akin to Fe, the use of BM and ash as catalysts considerably reduced the reaction time while providing both the catalytic metals and the elevated temperatures compulsory for a complete tars conversion. Furthermore, the CO/CO2 molar ratio achieved using 10 wt% BM and 20 wt% ash amounted to ca. 1.33 and 1.28, respectively and which proved to be convenient ratios for CO supply within industrial context. XRD spectra and elemental analysis of BM and ash presented in Fig. 2 and Table 2 showed that they were mainly composed of calcium-based minerals such as calcite [CaCO3], brushite [CaPO3(OH).2H2O], hydroxylapatite [Ca10(PO4)6(OH)2] and gehlenite [Ca2Al2SiO7], perovskite [CaTiO3], calcium silicate [Ca2SiO4], hatrunite [CaSiO6], respectively. Similar non-metallic compound are well documented in literature as having some activity in coal/ char e steam gasification and more specifically on tar cracking. Most work showed that tars content in syngas or producer gas were considerably reduced at the exit of the reactor or secondary reformer when alkali or alkaline earth metalsebearing minerals are used as catalyst [26]. The results obtained herein further demonstrates that the use of extra calcium mineral matrix such as gehlenite, perovskite, calcium silicate or calcite, brushite, and hydroxylapatite contained in ash and BM respectively, appeared to lower the apparent Arrhenius energy of activation while increasing the heat of reaction. Besides the heat generation, the calciumbearing minerals contained in BM and ash led to the complete tars conversion into gaseous carbon species (CO and CO2) and a substantial shortening of reaction. Finally, it was found that TC oxidation preferentially produced CO instead of CO2, unlike in the metallic-(Fe)-doped tar-char oxidation. This outcome is contrary to the acknowledged fact that CO2 production is positively correlated in presence of excess of oxygen to the oxidation temperature [24]. We therefore suggested that, in addition to the neutralization of the acid sites and the eutectic formation between calcium-bearing minerals that helped enhance the tar-char gasification, the arrangement of char and catalyst particles would also have an impact on the gas composition in CO and CO2. The mismatches between particle sizes of TC sample (very fine) and the ash or BM catalyst (containing coarser particles than TC and Fe) might prompt the release of the produced gas, thereby reducing the secondary CO oxidation into CO2 by the continuous air feed.

4. Conclusions Elemental iron (Fe), calcium phosphate-rich bone meal (BM), and ashes from biochar (ash) were used as low cost catalysts to improve the carbon conversion of a centrifuged cake (containing char and tars) derived from urban biomass gasification. Tar-char cake (TC) oxidation was performed in a semi-batch reactor operated in an updraft mode while insuring a suitable conversion of the fixed-carbon and the tars present in the sample within reaction times applicable to large scale processes using downdraft and updraft modes of operation. The following conclusions are drawn from this work:

(i) TC conversion into CO/CO2 with molar ratios above 1 is greatly affected by the addition of extra matrix Fe and Cabearing minerals. (ii) Tars desorption rate was lower than its oxidation rate in presence catalysts (Fe, BM, ash) leading to a quasi-total carbon conversion (above 99%). (iii) Elevated temperatures were achieved within the gasifier while sacrificing less carbon to CO2 thanks to the oxidation of metals in the catalysts. (iv) Addition of Fe led to a rapid TC conversion, high heat generation but poor CO/CO2 ratio when the amount of elemental iron added is below 20 wt%. (v) Addition of calcium-rich mineral such as BM and ash resulted into fast TC conversion, high heat generation and a high CO/ CO2 ratio with catalyst levels ranging from 10 to 20 wt%. Within the tested catalysts, simple ash derived from the feed material emerges as the best candidate for TC oxidation at large scale, since the latter is a product generated by the reaction and can be indefinitely collected and recycled without any associated cost related to its supply unlike the commercial metallic or the commercial non-metallic catalysts commonly used in pyrolysis and gasification processes. Furthermore, ash has displayed the behavior expected from a good catalyst such as high heat production, complete tars conversion, reduced reaction time as compared to noncatalytic reaction and CO-enriched mix of “CO þ CO2 syngas” production. AKNOWLDGEMENT The authors are grateful to funders of the Industrial Research
Chair on Cellulosic Ethanol and Biocommodities of the Universite de Sherbrooke and NSERC (CRDPJ 486964-2015) for their support, to Esteban Chornet for his guidance throughout this project, and to
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