| This study was aimed at producing light oil（L-O）from SS over transition metal catalysts（TMCs）,Ca-based catalysts（CBCs）and char established at 700 ℃ and its metal-loaded forms(C700s)in a quartz tube reactor.The study also considered the synthesis of char catalyst established at 700 ℃(C700)and it modified derivatives by loading of Ni and Fe metals to obtain Ni-C700 and Fe-C700 respectively.The study also compared the products of C700s with the commercially synthesized ones（i.e.TMCs and CBCs）at the selected temperatures（i.e.600 and 650 ℃）to draw inference vis-à-vis their feasible application and environment friendliness.That is considering the recovery of value-added products or the L-Os application as a conventional fuel vis-à-vis pollution and the emission of GHGs.Also,the nitrogen conversion mechanism during catalytic pyrolysis（CP）of SS over the TMCs,CBCs and the C700s was studied in brief to draw inference in order to know its impact on the environment and possible application opportunities as well.The catalytic pyrolysis made use of three groups of catalysts including three C700s consisting of C700,5%Ni-C700,and 5%Fe-C700,three TMCs consisting of Ni/Al203 from Japan,Indonesian limonite and Co Mo/Al2O3,and finally two CBCs including calcined dolomite and Ca O.The results discussion was captured under four main headings which included product yield,gas yield,GC/MS analysis of L-Os and nitrogen distribution taking into account the non-catalyst and catalyst tests so as to draw inference.The product yield discussed the carbon distribution of the experiments to understand the conversion of organic carbon during the pyrolysis process.The product distributions changed notably by raising the pyrolysis temperature from 500 to 700 ℃.For the non-catalyst test raising the temperature from 500 to 700 ℃ resulted in the decrease in char-C（carbon in char）due to the progressive pyrolysis conversion.At 500℃ 37.9%of carbon was retained in char and decreased to 33.5%at 700 ℃,with insignificant change in the char-C above 600 ℃,implying that,the char-C decreased from 35.1%at 600 to 33.5%at 700 ℃.This can be ascribed to the elevated secondary cracking of carbonaceous and char particles gathered atop the frit encounter with pyrolysis vapors.The yield of gas-C appreciated markedly from 6.6 to 17.0%while the oil-C slightly decreased from 55.5 to 49.5%by raising the pyrolysis temperature from500 to 700 ℃ respectively.The oil-C yields at 500,600,650 and 700 ℃ were 55.5,54.8,53.2 and 49.5%respectively,indicating that,the oil-C yield decreases by raising pyrolysis temperature and with maximum yield at 500 ℃.The pattern of decrease in oil-C suggests that,the SS volatiles experienced complex cracking reaction and were converted to NCG like CO,CO2and H2 at the elevated temperatures,which subsequently caused increase in the gas-C yields.Inferring from catalytic test and with reference to the experiment run without catalyst depicts that,the introduction of catalyst decreased the oil-C and increased the gas-C progressively at 600 and 650 ℃ respectively.The decrease in oil-C and increase in gas-C can be due to the cracking of pyrolysis vapors（also from the char-C）over the catalysts to produce light hydrocarbons,CO and CO2.The effect of the catalyst conversion of SS carbon decreased in the order:TMCs>C700s>CBCs.The order might have been influenced by the active metals such as Ni in Ni/Al2O3and 5%Ni-C700,Co and Mo in Co Mo/Al2O3,the Fe in limonite and 5%Fe-C700.The coke-C（carbon in coke）decreased when the reaction temperature was raised from 600 to 650 ℃ over all the catalysts.The non-catalyst test for the gas composition showed that,the overall gas yield of NCG appreciated sharply and significantly from 10.3 to 23.1%（daf）,when the pyrolysis temperature was raised from 500 to 700 ℃ respectively indicating that,the pyrolysis temperature influenced the gas composition.At 500 ℃,CO2 was identified as the predominant gaseous product（GP）.The gas yield increased significantly when the pyrolysis temperature was raised from 500 to 700 ℃ notably for H2 and CO by increasing from 0.3 to 4.7 mmol/g and 0.5 to 2.3 mmol/g respectively.This fashion can be attributed to the secondary decomposition of SS primary volatiles.At 500 ℃ a negligible amount（0.1 mmol/g）of olefins was produced as against the noticeable amount of 0.7 mmol/g at 700 ℃.C1-C4 alkanes were produced across the temperature range of 500 to 700 ℃ with the highest yield of 1.0 mmol/g at 700 ℃.The changes realized in the yields of olefins and C1-C4 alkanes can be attributed to the thermal cracking of gas-phase hydrocarbons at high temperatures.For the catalytic test,the total yields of NCG were significantly enhanced by the introduction of catalysts.The addition of catalysts(TMCs,CBCs and C700s)developed a pattern with respect to the pyrolysis temperatures selected.The gas yields decreased in the order:TMCs>C700s>CBCs.Amongst the TMCs Ni/Al2O3 was the most effective thereby producing the highest gas yield both at 600 and 650 ℃.Similarly,Ni/Al2O3 produced the highest CO yields of 7.5 mmol/g and 14.0 mmol/g at 600 and 650 ℃ respectively,showing an excellent syngas conversion energy compared to all the catalysts used and by also effectively promoting the production of high H2compared with the non-catalyst test.Firstly,this can be ascribed to the high activity of Ni/Al2O3 with respect to its nickel particle size（NPS）and high SBET.Another interpretation for the increase in H2 and CO yield over Ni/Al2O3 could be the ability for Ni-based catalyst to promote depolymerization and cracking reactions of biomass matrix and tar.Conversely,CO2yield over Ni/Al2O3decreased by 1.0 mmol/g when the temperature was raised from600 to 650 ℃.An analogous outcome was enumerated by Wang et al.The CO2 yield over the CBCs decreased significantly compared with the non-catalyst test.The reasonable explanation may be the presence of Mg O-Ca O phase in the calcined dolomite and the pure Ca O,which increased the catalysts physiochemical properties for tar cracking and CO2absorption.Owing to the effective CO2capture in the 600-650℃ range,the initial gas produced was composed of about 76%H2,and lower concentrations of CO（5%）,Olefins（4%）and C1-C4 alkanes（13%）which is in agreement with the attribute that,Ca-based catalyst acts as deoxygenation catalyst due to their relatively high capacity in removing oxygenated compound in the form of CO2and CO under decarboxylation and decarbonylation reaction respectively.Also,the alkanes and olefins increased over the CBCs as compared to the non-catalyst test,which may be due to the decarboxylation and decarbonylation reaction respectively.Adding C700s ensured increase in the entire gas yield from 600 to 650 ℃,particularly the H2yield was about two（at 600 ℃）and three（at 650 ℃）times higher than that of the non-catalyst test.The CO2 yield was about two times that of the non-catalyst test especially over C700and the activity of the C700s decreased in the order:5%Fe-C700>5%Ni-C700>C700.This outcome could chiefly be due to the porous microstructure and to some extent the specific surface area（SSA）of the char support,which aided in the adsorption of organic volatiles on its surface.Additionally,the SS used as the char support for the synthesis of the C700s has a characteristic high ash content（59.1 wt%）and may contain quantities of alkali or alkali earth metal elements such as Ca and Mg,which may be active for tar cracking hence increasing the gas yields.With the catalyst test all the C700s showed appreciable amounts of CO increase compared with the non-catalyst test,especially over 5%Fe-C700and 5%Ni-C700implying that,they have good syngas conversion energy better than the CBCs but closer to that of Co Mo/Al2O3and limonite.The plastic,pesticides and rubber manufacturing outlets use arenes as solvent and starting materials.The GC/MS results depict benzene,toluene,indene,xylene,styrene,naphthalene,and phenanthrene as the main aromatic rings detected in the L-Os.With the non-catalytic test,the relative content of arenes realized from 500 to 700 ℃ increased significantly from 21.8%to 35.6%respectively,with the highest relative content obtained at 650 and 700 ℃.There was no decline in relative contents between500 and 600 ℃ and that between 650 and 700 ℃ respectively.Among the arenes benzene and alkylbenzenes were found to be the predominant species for the non-catalyst test.At high temperatures the L-Os produced registered especially bicyclics including,benzene and alkylbenzenes and methyindene,and polycyclics such as naphthalene,alkylnaphthalene,styrene,acenaphthene,methylphenanthrene,methylanthracene and benzo（c）phenanthrene.The most abundant species produced at the high temperatures were the benzene and alkylbenzenes.The generation of arenes at high temperatures can be attributed to three main mechanisms namely;the Diels-Alder reaction reported by Williams and Cunliffe the alkane dehydrogenation to alkenes and dienes coupled with cyclization and aromatization explained by Cypres and Fairburn and the hydrogen abstraction acetylene addition account by Frenklach.Aliphatics are essential compounds when considering application of bio-oils to be used as transport fuel.Ideally the aliphatics in bio-oils should include long-chain n-alkanes and n-alkenes from C12-C28,since they have low viscosity and high heating value.The results from the GC/MS indicate that,the L-Os contains straight-chain n-alkanes and n-alkenes from C3-C19and C27（Cholestane）,a branched hydrocarbon often found in the analysis of organic compounds in petroleum.With the non-catalyst test the relative content of alkanes and alkenes in the L-Os decreased notably from 4.4 to 1.7%and 11.6 to 10.1%when the reaction temperature was raised from 500 to 700 ℃ respectively.The same trend was observed for the total relative content of aliphatics when the reaction temperature was raised from 500 to 700 ℃ with the highest relative content at 600 ℃.A rational justification could be assigned to the cracking of the long-chain aliphatics to shorter chains or the formation of other kinds of compounds such as arenes at elevated temperatures.The explanation is well consistent with the results obtained in the non-catalyst test in that,the relative content of arenes increased from 21.8 to 35.6%when the reaction temperature was raised from 500 to 700 ℃.The relative content of OOSs in the L-Os for the non-catalyst test were 25.2,23.2,22.7 and 15.9%at 500,600,650 and 700 ℃ respectively.The relative contents decreased significantly from 25.2%to 15.9%from 500 to 700 ℃ respectively,due to the secondary thermal cracking of the volatiles.Bio-oils contain almost all kinds of oxy compounds;the main OOSs detected in the L-Os are alcohols,organic acids,esters,ketones,phenols aldehydes and other heterocyclics.The abundant alcohols and phenols groups in the L-Os suggest the wide presence of oxygen with relative contents of 10.4%（600 ℃）and 7.5%（500 ℃）respectively.Additionally,corresponding high relative content of OOSs were obtained in relation to alcohol and phenol with totals of 23.2%at 600 ℃ and 25.2%at 500 ℃ respectively.The significant decrease in the overall relative content of OOSs seen when the reaction temperature was raised from 500 to700 ℃ is in agreement with the corresponding increase of CO and CO2 as depicted in the non-catalytic gas yield and consistent with the report by Huang et al.ONSs are formed due to the devolatilization of the proteic fraction（nucleic acids and proteins）in the SS originating from dead organic matter.The composition of the L-Os compared to other sludge derived bio-oils reveals that,large amount of ONSs including;nitriles,amines,amides,N-heterocycles,and lactams were discovered in the L-Os from the pyrolysis of SS and as analyzed by the GC/MS.The N-heterocyclics were the most predominant ONSs detected in the L-Os with the largest relative conent of 28.1%and corresponding highest total relative content of ONSs of 37.0%at 500 ℃.The abundant N-heterocyclics in the L-Os included;pyridines,pyrroles,pyrazins,and indoles with carbozole and quinoline detected only when the catalyst was introduced.The high relative contents of N-heterocyclics could be due to the dehydrogenation of amino acid groups present in proteins and nucleic acids.Similarly,it may be due to the concentration of amine groups into pyrrolic and pyridinic species to form N-heterocyclics.The nitriles including aromatics and aliphatics were the second predominant ONSs detected in the L-Os.Most of the aromatic nitriles were formed due to the linkage between benzene rings and nitriles.Both alkanenitriles and alkene nitriles from C3to C18 were detected in the L-Os which could be formed during pyrolysis by the reaction of fatty acids with NH3.The relative content of aliphatic nitriles increased while aromatic nitriles derease when the reaction temperature was raised from 500 to700 ℃,but with a slight decrease at 700 ℃ compared to 600 and 650 ℃.The trend may be due to the thermal cracking of amines and amides with subsequent polymerization to form aliphatic nitriles at high temperatures.The non-catalyst tests recorded no relative content of lactams.The GC/MS results showed that,the highest relative content of arenes was detected over limonite both at 600 and 650 ℃ with reference to the test without catalyst and those over catalysts.The relative content of arenes were higher at 650 ℃ over all the catalysts,indicating that,all the catalysts efficiently promoted the OOSs cracking and finally left the aromatic ring which was difficult to crack.When catalysts were applied the relative content of aliphatics in the L-Os decreased significantly.On a moderate note the L-O obtained over Ca O(CaL-O)contained substantial relative content（9.3%）of aliphatics at 600 ℃ even though lesser than the least relative content（10.3%）detected in the non-catalyst test at 650 ℃.The results from the GC/MS suggest that,limonite promoted the cracking of aliphatics and generated a large number of aromatics at 600 and 650 ℃ in the L-Os over limonite(LiL-O).The relative content of OOSs over all the catalyst decreased notably at both 600 and650 ℃ but more significantly at 650 ℃,indicating that,high temperatures influence the reduction of OOSs even with the addition of catalyst.Furthermore,the removal of functional groups such as aldehydes and ketones almost completely over all the catalysts and leaving negligible relative contents in L-O over C700(C7L-O),L-O over5%Ni-C700(N7L-O)and L-O over 5%Fe-C700(F7L-O)at 600 ℃ and F7L-O and CaL-O at650 ℃ respectively,show the efficiency of the catalysts’ability to upgrade the L-Os produced.The secondary cracking reactions decreased the relative contents of ketones sharply in all the L-Os by raising the temperature to 650 ℃,even though calcined dolomite is noted for ketonization of bio-oil oxygenate.This pattern was obvious for all the other oxygenated groups,since high temperatures leads to the formation of gaseous products.The removal of aldehydes and ketones hindered the aldol condensation reaction so that no large molecules are formed during storage which should have increased the viscosity of the L-Os.The observable relative content of ketones in the range of 0.8-6.1%detected in the L-Os from the non-catalyst test can increase the production cost by increasing the viscosity which results in high-pressure drop-in pipelines,thus needs to be minimized.Hence limonite and the CBCs showed very high potential for the ketone’s complete elimination.The trend of elimination and minimization is well in agreement with the non-catalyst test establishing that,ketones minimization is influenced by high temperatures.Phenols are noted for their ability to lower heating values,thus have to be minimized as well.The acids produced with the non-catalyst test were completely removed over all the catalyst while the phenols were completely converted over limonite,but partially converted at 600 ℃ over the CBCs and with the least conversion by the C700s.The potential for phenols removal decreased in the order;TMCs>CBCs>C700s.The LiL-O contained no phenols at both 600 and 650 ℃,the L-O over dolomite(DoL-O),CaL-O contained scanty relative contents at only 600 ℃,while C7L-O,N7L-O and F7L-O contained phenols at both 600 and 650 ℃ but higher at 600 ℃ in the range of 3.6-6.8%.The relative content of ONSs decreased over all the catalysts by raising the temperature from 600 to 650 ℃,with about 47%decrement over limonite and the least over the C700s.Among the C700s,there was about 16%decrement in the ONSs detected in F7L-O at 650 ℃.This implies that,the Fe-based catalysts(limonite and 5%Fe-C700)have a unique ONSs removal ability but the rate might depend on the amount of Fe present in the catalyst.The CBCs showed a better ONSs removal compared to the C700s.The relative contents of N-heterocyclics decreased when the temperature was raised from 600 to 650 ℃.Limonite showed a corresponding trend of decrease（43%）in the N-heterocyclics as well.The relative content of N-heterocyclics detected in DoL-O and CaL-O at 600 ℃ was higher than that detected in the non-catalyst tests.The amines,amides and lactams detected over the catalyst were insignificant.The aliphatic nitriles were the main nitriles detected over the catalysts,and decreased when the reaction temperature was raised from 600 to 650 ℃.The low sulfur content bio-oils derived from SS makes it advantageous over many fossil fuels.The highest relative content of OSSs detected in the L-Os was 1.6%at 600℃ without catalyst,while no OSSs were detected in the L-Os at 500 and 650 ℃.At700 ℃ a relative content of 1.2%of sulfur species was recorded.The outcome is in agreement with the almost negligible content of sulfur in the SS specimen used.The introduction of catalysts significantly decresed the relative contents of OSSs in all the L-Os.The TMCs and CBCs showed effective reduction of OSSs in their corresponding L-Os with NiL-O,LiL-O and DoL-O presenting with no OSSs although L-O over CoL-O showed neglegible content of 0.1%.For the C700s even though they showed some ability for OSSs mimization with some tracable amount,which could be due to the char support used for the catalyst synthesis.For the catalyst test,the higher temperature（650 ℃）caused an insignificant increase in the relative contents of sulfur species in all the L-Os from the GC/MS results.Similarly,LiL-O did not contain any sulfur species at 600 ℃ but showed a negligible content（0.1%）at 650 ℃,indicating that Fe-based catalysts have effect on OSSs minimization.The OSSs detected in the L-Os in this study include both mono heteroatoms（e.g.thiophene and thiophene-2-methyl,benzo（c）thiophene）andbinaryheterotoms（e.g.benzene,[（methylsulfinyl）methyl]-;1,4-dithiepan-2-one,3-phenyl-;and 3,5-dithiahexanol 5,5-dioxide）.From the non-catalyst experiment it can be inferred that,the nitrogen distribution of char-N（nitrogen in char）,coke-N（nitrogen in coke）,NH3 and HCN is chiefly influenced by pyrolysis temperature.Char-N and coke-N decreased when the temperature was raised from 400-700 ℃.At 400 ℃ 46.2%of SS-N（nitrogen in SS）remained in char.At 700 ℃ the char-N was consistently decreased to 21.6%owing to the progressive conversion of char-N to volatile nitrogen.The same trend was observed for coke-N reducing from 37.6 to 19.4%accounting for 100%reduction both for char-N and coke-N.The discharge amount of SS-N illustrates a complete and contrasting trend as seen in the SS-C,such that insignificant amount was liberated above 600 ℃.HCN and NH3were determined to be the main nitrogen containing gaseous species（GSs）with yields increasing when the reaction temperature was raised.NH3 was identified as the main nitrogen containing GSs.At 400 ℃ an observable amount（12.2%）of NH3 was formed but was sharply increased to 30.7%when the reaction temperature was raised to 500 ℃,and with a further increase of about 14.2%from 600 to 700 ℃.The trend observed in this work is contrary to the research findings on coal pyrolysis where the release of NH3 started above 600 ℃.Referring from the FTIR and TG analysis indicates that,the SS encompasses pronounced quantity of proteins and lignin which is decomposed around the 400-550 ℃ and as such explains the observable amount of NH3 formation below 500 ℃.The findings in this case is similar to the report which explained that,under 400-500 ℃ amino moieties in sludge are partly decomposed during pyrolysis to form NH3.The HCN formation trend in this work is distinct from that of NH3.The HCN yield under 500 ℃ was very low（2.9%）,indicating that,insignificant yield of HCN is produced from protein pyrolysis with temperatures under 500 ℃.The HCN yield appreciated markedly to 9.3%when the reaction temperature was raised to 700 ℃.A finding analogous to this was reported in the pyrolysis of wastes.The distribution of HCN,NH3,and coke-N efficiently relied on catalyst and temperature.The yields of NH3 and HCN increased while coke-N decrease with increase in temperature over almost all the catalysts except for limonite.In the 500-700℃ temperature range NH3 was the principal product and increased by raising the temperature in the range aforementioned.Comparing the HCN yield over all the catalysts with that of the test without catalyst was very low at the selected temperatures.In the same vein the NH3 yield over Ni/Al2O3compared to the test without catalyst at both 600 and 650 ℃ was lower.The lowest amount of HCN and NH3 yield were determined over Ni/Al2O3.The high activity of Ni/Al2O3for nitrogen volatiles（NV）conversion to be fixed in char can be attributed to the high content of nickel in Ni/Al2O3.An experiment conducted on Ni/Al2O3catalysts with varying Ni loadings established that,the activity of Ni/Al2O3for NH3 conversion efficiently relied on the amount of nickel present in the catalyst,and concluded that,the highest nickel content achieved the highest rate of NH3conversion.Nickel based catalysts（NBCs）are active for the decomposition of NH3 even in the 400-600 ℃ range.There was almost no HCN detected at 650 ℃ over Ni/Al2O3.At 600 ℃ HCN was converted to NH3 over Co Mo/Al2O3and limonite.At 650 ℃ NH3was effectively decomposed by Co Mo/Al2O3and limonite.Compared with TMCs,CBCs behave differently and not that active for NV conversion.The NH3 yield detected over the CBCs was the highest amongst all the catalysts employed in this work.Calcined dolomite seems to be slightly active than the pure Ca O for HCN conversion compared to NH3.The conversion of HCN to NH3effectively improved at 650 ℃ over calcined dolomite.As shown in formula（1）,TMC,especially Ni/Al2O3,promotes the conversion of HCN to NH3 by hydrogenation.In addition,N2 may be produced by the subsequent decomposition of NH3（formula（2））.HCN+3H2→NH3+CH4（1）2NH3→N2+3H2（2）The relative content of aromatics in tar increased,at 650 ℃ over all the catalysts,indicating that,all the catalysts effectively promoted the OOSs cracking,and finally left the aromatic ring which was difficult to crack.The L-Os contain straight-chain n-alkanes and n-alkenes from C3-C19and C27.Substantial relative content（9.3%）of aliphatics was detected in CaL-O at 600 ℃ even though lesser than the least content（10.3%）detected in the non-catalyst test at 650 ℃.Indicating that,the potential of the L-Os obtained do have very scanty conventional fuel properties.At 650 ℃ the secondary cracking reactions decreased the relative contents of ketones sharply in all the L-Os,even though calcined dolomite is noted for ketonization of bio-oil oxygenates.Limonite and the CBCs showed very high potential for ketones complete elimination,with the strongest potential for phenols removal over limonite.On the other hand,some allowable amount of ketones which when oxidized forms esters can be considered as a desirable bio-fuel compound while the extraction of phenol from SS can serve as an economical renewable resin that can substitute the exorbitant petroleum-based phenol used by most industries,hence the L-Os obtained over the C700s are a potential option for such resins.Reports on SS derived bio-oils establishes that,OOSs confers numerous undesirable properties such as high viscosity,low calorific value,non-volatility,corrosiveness,thermal lability,liable to polymerization in storage and transportation,and inferior miscibility with fossil fuels on the bio-oil.The effective reduction of OOSs at 650 o C can remedy these setbacks mentioned above.The N-heterocyclics were the most predominant ONSs detected in the L-Os for the non-catalyst test with the largest relative amount of 28.1%and corresponding highest total ONSs of 37.0%at 500 ℃.The removal of ONSs over limonite at 650 ℃ was the most significant.Since ONSs accounts for about 30%of the composition of the L-Os,the appropriate application of the L-Os would be their recovery as value-added compounds such as medicines and drugs rather than removing them would be the most suitable option in order to prevent the emission of the GHGs.HCN and NH3 were determined to be the main nitrogen containing GSs with yields increasing when the reaction temperature was raised.NH3 was identified as the main nitrogen containing GSs.The lowest NH3 and HCN yield were detected over Ni/Al2O3.At 600 ℃ HCN was converted to NH3 over Co Mo/Al2O3and limonite.At 650 ℃ NH3was effectively decomposed by Co Mo/Al2O3and limonite.The C700s produced the highest yields of HCN amongst the catalyst used both at 600 and 650 ℃ but lower than that detected in the non-catalyst test.The CBCs and C700s can be considered for the production of fertilizer due to their ability to convert SS-N（nitrogen in SS）to NH3. |
