Direct gasification of dewatered sewage sludge in supercritical water. Part 1: Effects of alkali salts

In the present study, the catalytic effects of alkali salts [NaOH, KOH, K₂CO₃, Na₂CO₃ and Ca(OH)₂] on the direct gasification of dewatered sludge in supercritical water were investigated by using a high-pressure autoclave at a constant temperature of 450 °C and a residence time of 30 min. The hydrogen yield increased in the presence of the alkali salts, except for Ca(OH)₂. Specifically, the hydrogen yield increased from 0.68 to 3.45 mol/(kg OM) as the K₂CO₃ concentration increased from 0 to 8 wt%. Although Ca(OH)₂ did not significantly impact the catalytic effect on the hydrogen yield, it did impact the CO₂ yield. Generally, the addition of alkali salts did not affect the organic matter or total phenol concentrations in the liquid residue. Moreover, char formation was considerably suppressed by the alkaline catalyzed hydrolysis of the dewatered sludge [except in the case of Ca(OH)₂].     

Catalytic steam reforming of bio-oil model compounds for hydrogen production over coal ash supported Ni catalyst

The development of a high performance and low cost catalyst is an important contribution to clean hydrogen production via the catalytic steam reforming of renewable bio-oil. Solid waste coal ash, which contains SiO₂, Al₂O₃, Fe₂O₃ and many alkali and alkaline earth metal oxides, was selected as a superior support for a Ni-based catalyst. The chemical composition and textural structures of the ash and the Ni/Ash catalysts were systematically characterized. Acetic acid and phenol were selected as two typical bio-oil model compounds to test the catalyst activity and stability. The conversion of acetic acid and phenol reached as much as 98.4% and 83.5%, respectively, at 700 °C. It is shown that the performance of the Ni/Ash catalyst was comparable with other commercial Ni-based steam reforming catalysts.     

Influence of alkali metal modification and reaction conditions on the catalytic activity and stability of Ni containing smectite-type material for steam reforming of acetic acid

A Ni incorporated mesoporous smectite-like material, SM(Ni), was modified by various alkali metals, such as, Li, Na, K, Rb or Cs, and tested for the steam reforming of acetic acid as a model compound of aqueous phase of bio-oil derived from biomass pyrolysis. Initial conversion of acetic acid and concentration of H₂ produced are drastically enhanced by the modification with these alkali metals. 1.0 wt% K-modified SM(Ni) catalyst exhibits the highest activity among the modified SM(Ni) materials tested. Addition of K promotes the reduction of Ni species incorporated in the smectite, yielding more metallic Ni species than in the original SM(Ni) sample. Therefore, the K-modified SM(Ni) catalyst gives higher initial activity compared with the original smectite catalyst. However, these modified materials lose their activities due to carbon deposition on their surface during the course of reaction, similar to the original SM(Ni). The influence of reaction conditions, such as O₂ or H₂ addition, steam to carbon ratio (S/C) and reaction temperature, was also investigated. Higher and more stable activity was obtained with unmodified SM(Ni) catalyst at a high reaction temperature of 973 K and at an S/C ratio of 3.3.     

The effects of Fe2O3 catalyst on the conversion of organic matter and bio-fuel production during pyrolysis of sewage sludge

In this paper, the pyrolysis treatment of sewage sludge is studied in a fixed bed reactor at temperatures range of 400–600 °C. Meanwhile, the catalytic effect of Fe₂O₃ on the characteristics of the resulting gases, bio-oil and bio-char are also investigated. The experimental results indicate that the yields of gases and bio-oil respectively increase from 8.69 wt% and 32.54 wt% to 11.62 wt% and 38.74 wt%, and the char yield decreases from 58.77 wt% to 49.64 wt% during Fe-embedded sewage sludge pyrolysis when Fe₂O₃ is added equal to 5% in the dried sewage sludge. Meanwhile, Fe₂O₃ promotes the CO and H₂ formation and inhibits the CH₄ formation, while it exhibits no significantly effect on the composition of the bio-oil. Moreover, the bio-oil should be direct combustion for power generation due it contains higher oxygenated hydrocarbons. In addition, the bio-char exhibits good desulfurization activity.     

Catalytic hydropyrolysis of crop straws with different biochemical composition

In this study, four kinds of straws with different biochemical compositions, including soybean straw (SS), peanut straw (PS), rice straw (RS), and corn straw (CS), were subjected to catalytic hydropyrolysis (HyPy) to explore the influence of biochemical composition on the products distribution and properties of the pyrolysis oil. The HyPy reactions were performed at 400 °C for 2 h with added 10 wt% Pd/C and 4 MPa H₂. During the HyPy, hydrogen and catalyst broke the coating structure and hydrogen bond between cellulose (CL) and hemicellulose (HCL), and thus significantly weakened the biochemical composition effect on the yield and elemental composition of the bio-oil. The bio-oil yield varied between 11.75 wt% and 13.05 wt%, and the C, H, N, O, and S content fell into the following ranges of 82.06–85.15 wt%, 9.24–9.61 wt%, 1.18–1.43 wt%, 4.62–7.86 wt%, 45–130 ppm, respectively. Biochemical composition of straw, especially the mass ratio of CL to HCL (m_CL/m_HCL), markedly influenced the molecular composition of the bio-oil. Hydrocarbons (20.15–46.66%) and phenolic compounds (17.01–47.98%) accounted for the vast majority of the identified compounds. SS and PS with higher m_CL/m_HCL (1.92 and 1.80, respectively) tended to produce bio-oils with more aromatics (22.63% and 20.70%, respectively) and fewer phenolic compounds (17.01% and 22.56%, respectively).     

Thermodynamic analysis of aqueous phase reforming of three model compounds in bio-oil for hydrogen production

Thermodynamic analysis with Gibbs free energy minimization was performed for aqueous phase reforming of methanol, acetic acid, and ethylene glycol as model compounds for hydrogen production from bio-oil. The effects of the temperature (340-660 K) and pressure ratio Psys/PH₂O (0.1-2.0) on the selectivity of H₂ and CH₄, formation of solid carbon, and conversion of model compounds were analyzed. The influences of CaO and O₂ addition on the formation of H₂, CH₄, and CO₂ in the gas phase and solid phase carbon, CaCO₃, and Ca(OH)₂ were also investigated. With methanation and carbon formation, the conversion of the model compounds was >99.99% with no carbon formation, and methanation was thermodynamically favored over hydrogen production. H₂ selectivity was greatly improved when methanation was suppressed, but most of the inlet model compounds formed solid carbon. After suppressing both methanation and carbon formation, aqueous phase reforming of methanol, acetic acid and ethylene glycol at 500 K and with Psys/PH₂O = 1.1 gave H₂ selectivity of 74.98%, 66.64% and 71.38%, respectively. These were similar to the maximum stoichiometric hydrogen selectivity of 75.00% (methanol), 66.67% (acetic acid), and 71.43% (ethylene glycol). At 500 K and 2.90 MPa, as the molar ratio of CaO/BMCs increased, the normalized variation in H₂ increased and that for CH₄ decreased. Formation of solid carbon was effectively suppressed by addition of O₂, but this was at the expense of H₂ formation. With the O₂/BMCs molar ratio regulated at 1.0, oxidation and CO₂ capture increased the normalized variation in H₂ to 33.33% (methanol), 50.00% (acetic acid), and 60.00% (ethylene glycol), and the formation of solid carbon decreased to zero.     

Effects of solvents and catalysts in liquefaction of pinewood sawdust for the production of bio-oils

Liquefaction of biomass with proper solvents and catalysts is a promising process to produce liquid biofuels and valuable chemicals. In this study, pinewood sawdust was liquefied in the presence of various supercritical solvents (carbon dioxide, water, acetone, and ethanol) and catalysts (alkali salts and acidic zeolites). The liquid, gas and solid products were analyzed using GC–MS, FT-IR, elemental analyzer, ¹H NMR, ¹³C NMR. The experimental results showed that both solvent and catalyst can significantly improve the liquefaction process by increasing the yield of liquid oil and suppressing the formation of solid residue. K₂CO₃ showed the best performance by doubling the yield of bio oil. Meanwhile, the maximum bio-oil yield (30.8 wt%) and the minimum solid residue yield (28.9 wt%) were obtained when ethanol was employed as the solvent. Solvents can also strongly affect the distribution of liquid products. 2,4,5,7-tetramethyl-phenanthrene and bis(2-ethylhexyl) phthalate were the premier compounds in liquid product as supercritical carbon dioxide is used as solvent while 2-methyl-naphthalene became the main composition when water is used as solvent.     

Upgrading of liquid fuel from the vacuum pyrolysis of biomass over the Mo–Ni/γ-Al2O3 catalysts

High amounts of acid compounds in bio-oil not only lead to the deleterious properties such as corrosiveness and high acidity, but also set up many obstacles to its wide applications. By hydrotreating the bio-oil under mild conditions, some carboxylic acid compounds could be converted to alcohols which would esterify with the unconverted acids in the bio-oil to produce esters. The properties of the bio-oil could be improved by this method. In the paper, the raw bio-oil was produced by vacuum pyrolysis of pine sawdust. The optimal production conditions were investigated. A series of nickel-based catalysts were prepared. Their catalytic activities were evaluated by upgrading of model compound (glacial acetic acid). Results showed that the reduced Mo–10Ni/γ-Al₂O₃ catalyst had the highest activity with the acetic acid conversion of 33.2%. Upgrading of the raw bio-oil was investigated over reduced Mo–10Ni/γ-Al₂O₃ catalyst. After the upgrading process, the pH value of the bio-oil increased from 2.16 to 2.84. The water content increased from 46.2 wt.% to 58.99 wt.%. The H element content in the bio-oil increased from 6.61 wt.% to 6.93 wt.%. The dynamic viscosity decreased a little. The results of GC–MS spectrometry analysis showed that the ester compounds in the upgraded bio-oil increased by 3 times. It is possible to improve the properties of bio-oil by hydrotreating and esterifying carboxyl group compounds in the bio-oil.     

Comparative studies of thermochemical liquefaction characteristics of microalgae using different organic solvents

The effect of different organic solvents, such as methanol, ethanol and 1,4-dioxane, on thermochemical liquefaction characteristics of Spirulina (a kind of high-protein microalgae) was systematically studied. The liquefaction experiments were conducted in a 1000 mL autoclave at different temperatures from 573 to 653 K with a fixed solid/liquid ratio. Liquefaction of Spirulina processed in methanol and ethanol favored the conversion rate and bio-oil yield compared with that in 1,4-dioxane solvent. The bio-oil generated in methanol contained higher C and H concentrations but a lower O content, resulting in a higher caloric value (39.83 MJ/kg). The results of FT-IR (Fourier Transform Infrared Spectroscopy) and GC-MS (Gas Chromatography-Mass Spectroscopy) analyses indicated that the compositions of bio-oil products were greatly affected by the type of solvent used for the liquefaction process. The major component of bio-oil produced with methanol was hexadecanoic acid methyl ester (C₁₇H₃₄O₂, 35.53%). However, ethanol favored the formation of hexadecanoic acid ethyl ester (C₁₈H₃₆O₂, 26.27%). When Spirulina were operated with 1,4-dioxane, the bio-oil was dominated by hexadecanenitrile (C₁₆H₃₁N, 22.7%). The presence of methanol and ethanol might promote the formation of esters. Low-boiling-points compounds with phenol ring structure or heterocyclics can be generated when 1,4-dioxane was employed as solvent.     

Hydrogen production by low-temperature reforming of organic compounds in bio-oil over a CNT-promoting Ni catalyst

Present study reports on high catalytic activity of CNTs-supported Ni catalyst (x% Ni-CNTs) synthesized by the homogeneous deposition–precipitation method, which was successfully applied for low-temperature reforming of organic compounds in bio-oil. The optimal Ni-loading content was about 15 wt%. The H₂ yield over the 15 wt% Ni-CNTs catalyst reached about 92.5% at 550 °C. The influences of the reforming temperature (T), the molar ratio of steam to carbon fed (S/C) and the current (I) passing through the catalyst, on the reforming process of the bio-oil over the Ni-CNTs' catalysts were investigated using the stream as the carrier gas in the reforming reactor. The features of the Ni-CNTs' catalysts with different loading contents of Ni were investigated via XRD, XPS, TEM, ICP/AES, H₂-TPD and the N₂ adsorption–desorption isotherms. From these analyses, it was found that the uniform and narrow distribution with smaller Ni particle size as well as higher Ni dispersion was realized for the CNTs-supported Ni catalyst, leading to excellent low-temperature reforming of oxygenated organic compounds in bio-oil.     

Bio-oil catalytic reforming without steam addition: Application to hydrogen production and studies on its mechanism

A novel process for hydrogen production via bio-oil catalytic reforming without steam addition was proposed. The liquid feedstock was a distillation fraction from crude bio-oil molecular distillation. The fraction obtained was enriched with the low-molecular-weight organics (acids, aldehydes, and ketones), and contained nearly all of the water from crude bio-oil. The highest catalytic performance, with a carbon conversion of 95% and a H₂ yield of 135 mg g⁻¹ organics, was obtained by processing the distillate over Ni/Al₂O₃ catalyst at 700 °C. The steam involved in the reforming reaction was derived entirely from the water in the crude bio-oil. The fresh and spent catalysts were characterized by N₂-physisorption, thermogravimetric analysis, and high-resolution transmission electron microscopy. To further understand the reaction mechanisms, symmetric density functional theory calculations for decomposition were performed on four model compounds in bio-oil (acetic acid, hydroxyacetone, furfural, and phenol) over the Ni(111) surface. In addition, the decomposition of H₂O∗ to OH∗ and O∗ and their subsequent steam reforming reactions with carbon precursors (CH∗ and CH₃C∗) were also examined.     

Carbon deposition on Ni/ZrO2–CeO2 catalyst during steam reforming of acetic acid

Steam reforming of acetic acid, one model compounds of bio-oil, was studied on the Ni/ZrO₂–CeO₂ catalysts which were prepared by the impregnation method. The results showed that high acetic acid conversion and hydrogen yield were obtained in the temperature range of 650–750 °C when H₂O/HAC ratio was 3. Nevertheless, the catalyst deactivation was caused by carbon deposition eventually with time-on-stream. In order to discuss the behavior of the carbon deposition on the Ni/ZrO₂–CeO₂ catalyst during steam reforming of bio-oil, the structure and morphology of carbon deposition were investigated by BET, XRD, TG/DTA, TPR, SEM and EDX techniques. All the experimental results showed acetone and CO were the important carbon precursors of acetic acid reforming and the graphitic-like carbon was the main type of carbon deposition on the surface of the deactivated 12%Ni/CeO₂–ZrO₂ catalyst.     

Hydrogen production from bio-oil: A thermodynamic analysis of sorption-enhanced chemical looping steam reforming

The steam reforming of pyrolysis bio-oil is one proposed route to low carbon hydrogen production, which may be enhanced by combination with advanced steam reforming techniques. The advanced reforming of bio-oil is investigated via a thermodynamic analysis based on the minimisation of Gibbs Energy. Conventional steam reforming (C-SR) is assessed alongside sorption-enhanced steam reforming (SE-SR), chemical looping steam reforming (CLSR) and sorption-enhanced chemical looping steam reforming (SE-CLSR). The selected CO₂ sorbent is CaO(s) and oxygen transfer material (OTM) is Ni/NiO. PEFB bio-oil is modelled as a surrogate mixture and two common model compounds, acetic acid and furfural, are also considered. A process comparison highlights the advantages of sorption-enhancement and chemical looping, including improved purity and yield, and reductions in carbon deposition and process net energy balance.The operating regime of SE-CLSR is evaluated in order to assess the impact of S/C ratio, NiO/C ratio, CaO/C ratio and temperature. Autothermal operation can be achieved for S/C ratios between 1 and 3. In autothermal operation at 30 bar, S/C ratio of 2 gives a yield of 11.8 wt%, and hydrogen purity of 96.9 mol%. Alternatively, if autothermal operation is not a priority, the yield can be improved by reducing the quantity of OTM. The thermodynamic analysis highlights the role of advanced reforming techniques in enhancing the potential of bio-oil as a source of hydrogen.     

Thermodynamic analysis of hydrogen production via steam reforming of selected components of aqueous bio-oil fraction

This work presents thermodynamics analysis of hydrogen production via steam reforming of bio-oil components. The model compounds, acetic acid, ethylene glycol and acetone, representatives of the major classes of components present in the aqueous fraction of bio-oil were used for the study. The equilibrium product compositions were investigated in a broad range of conditions like temperature (400–1300 K), steam to fuel ratio (1–9) and pressure (1–20 atm). Any of the three model compounds can be fully reformed even at low temperatures producing hydrogen with maximum yield ranging from 80% to 90% at 900 K. Steam to fuel ratio positively affect the hydrogen content over the entire range of temperature studied. Conversely, higher pressure decreases the hydrogen yield. The formation of solid carbon (graphite) does not constitute a problem provided that reforming temperatures higher than 600 K and steam to fuel ratios higher than 4 for acetic acid and ethylene glycol and 6 for acetone are to be used. Thermal decomposition of the bio-oil components is thermodynamically feasible, forming a mixture containing _C(s)${\mathrm{C}}_{(\mathrm{s})}$, CH₄, H₂, CO, CO₂, and H₂O at various proportions depending on the specific nature of the compound and the temperature. Material and energy balances of complete reforming system demonstrated that the production of 1 kmol/s hydrogen from bio-oil steam reforming requires almost the same amount of energy as with natural gas reforming.     

Catalyst modification strategies to enhance the catalyst activity and stability during steam reforming of acetic acid for hydrogen production

A high energy content (∼122 MJ/kg H₂) and presence of hydrogen-bearing compounds abundance in nature make hydrogen forth runner candidate to fulfill future energy requirements. Biomass being abundant and carbon neutral is one of the promising source of hydrogen production. In addition, it also addresses agricultural waste disposal problems and will bring down our dependency on fossil fuel for energy requirements. Biomass-derived bio-oil can be an efficient way for hydrogen production. Acetic acid is the major component of bio-oil and has been extensively studied by the researchers round the globe as a test component of bio-oil for hydrogen generation. Hydrogen can be generated from acetic acid via catalytic steam reforming process which is thermodynamically feasible. A number of nickel-based catalysts have been reported. However, the coke deposition during reforming remains a major challenge. In this review, we have investigated all possible reactions during acetic acid steam reforming (AASR), which can cause coke deposition over the catalyst surface. Different operating parameters such as temperature and steam to carbon feed ratio affect not only the product distribution but also the carbon formation during the reaction. Present review elaborates effects of preparation methods, active metal catalyst including bimetallic catalysts, type of support and microstructure of catalysts on coke resistance behavior and catalyst stability during reforming reactions. The present study also focuses on the effects of a combination of a variety of alkali and alkaline earth metals (AAEM) promoters on coke deposition. Effect of specially designed reactors and the addition of oxygen on carbon deposition during AASR have also been analyzed. This review based on the available literature focuses mainly on the catalyst deactivation because of coke deposition, and possible strategies to minimize catalyst deactivation during AASR.     

Production of hydrogen rich bio-oil derived syngas from co-gasification of bio-oil and waste engine oil as feedstock for lower alcohols synthesis in two-stage bed reactor

High efficient production of lower alcohols (C₁–C₅ mixed alcohols) from hydrogen rich bio-oil derived syngas was achieved in this work. A non-catalytic partial oxidation (NPOX) gasification technology was successfully applied in the production and conditioning of bio-oil derived syngas using bio-oil (BO) and emulsifying waste engine oil (EWEO) as feedstock. The effects of water addition and feedstock composition on the gasification performances were investigated. When the BO₂₀ and EWEO₃₀ was mixed with mass ratio of 1: 0.33, the maximum hydrogen yield of 93.7% with carbon conversion of 96.7% was obtained, and the hydrogen rich bio-oil derived syngas was effectively produced. Furthermore, a two-stage bed reactor was applied in the downstream process of lower alcohols synthesis from hydrogen rich bio-oil derived syngas (H₂/CO/CO₂/CH₄/N₂ = 52.2/19.5/3.0/9.4/15.9, v/v). The highest carbon conversion of 42.5% and the maximum alcohol yield of 0.18 kg/kg_cat h with selectivity of 53.8 wt% were obtained over the Cu/ZnO/Al₂O₃(2.5)//Cu₂₅Fe₂₂Co₃K₃/SiO₂(2.5) catalyst combination system. The mechanism and evaluation for lower alcohols synthesis from model bio-oil derived syngas and model mixture gas were also discussed. The integrative process of hydrogen rich bio-oil derived syngas production and downstream lower alcohols synthesis, potentially providing a promising route for the conversion of organic wastes into high performance fuels and high value-added chemicals.     

Bio-oil production from hydrogenation liquefaction of rice straw over metal (Ni,Co, Cu)-modified CeO2 catalysts

This paper describes catalytic hydrogenation liquefaction of rice straw over metal (Ni, Co, and Cu)-modified CeO₂ catalysts for bio-oil production. The results show that the highest rice straw conversion (89.08%) and bio-oil yield (66.7%) were obtained over Ni/CeO₂ catalyst. The bio-oil contains mainly phenols, high-value-added, and widely used chemicals. Furthermore, metal-modified CeO₂ catalysts can significantly influence the components of bio-oil with the highest percentage of C7-C10 compounds. This work thus demonstrates that metal/CeO₂ catalysts can be effective in improving the bio-oil yield and selectivity in hydro-liquefaction of rice straw into bio-oil.     

Effect of ZnO nanomaterials associated with Ca(OH)2 as anode material for Ni–Zn batteries

Prism ZnO nanomaterials coated with Ca(OH)₂ were prepared by direct precipitation. TEM micrographs showed that dendritic Ca(OH)₂ seemed to attach on the surface of nanosized ZnO. The XRD patterns indicated that the coating was Ca(OH)₂·2Zn(OH)₂·2H₂O. The nanosized ZnO coated with Ca(OH)₂ as the anode materials were investigated by the charge–discharge cycle measurement and EIS. The combination of ZnO nanomaterials and Ca(OH)₂ prevented the discharge product ZnO from dissolving in the electrolyte. Therefore, the Ca(OH)₂-coated ZnO nanomaterials exhibited higher electrochemical activity than the pure nanosized ZnO, including high the discharge capacity and discharge middle voltage, low the charge middle charge voltage. Although Ca(OH)₂ resulted in more difficult activation, EIS showed that the charge-transfer resistance was lower than that of the pure ZnO nanomaterials.     

A comparative study of the behavior of Chlamydomonas reinhardtii and Spirulina platensis in solar catalytic pyrolysis

The aim of this study was to investigate the behavior of two distinct microalgae species during solar catalytic pyrolysis and the influence of their chemical composition and the process variables (biomass charge, reaction time, and catalyst percentage) on the product yields and bio-oil composition. For this purpose, solar catalytic pyrolysis of Spirulina platensis and Chlamydomonas reinhardtii was performed using hydrotalcite-derived mixed oxides as the catalyst. To gain more insight into the effect of composition on pyrolysis behavior, the biomasses were analyzed using various analytical techniques. The results indicated that a high percentage of catalyst (47.1%) culminated in liquid yields of 42.48% and 21.31% for Chlamydomonas pyrolysis and Spirulina pyrolysis, respectively. Additionally, Spirulina pyrolysis resulted in higher solid yields compared with Chlamydomonas pyrolysis. The results also showed that Spirulina bio-oil was rich in oxygenated compounds, probably due to its high carbohydrate content, whereas Chlamydomonas bio-oil was rich in nitrogenated compounds because of its higher protein content. The microalgae composition (lipids, protein, carbohydrates) exerted a large influence on the catalytic pathways and led to differences in yield and product distribution. A high percentage of catalysts preferentially promoted a deoxygenation of the bio-oil obtained from Spirulina solar pyrolysis compared with that obtained from Chlamydomonas pyrolysis.