Jude A. Onwudili

Catalytic hydrothermal gasification of algae for hydrogen production: Composition of reaction products and potential for nutrient recycling

Chlorella vulgaris, Spirulina platensis and Saccharina latissima were processed under supercritical water gasification conditions at 500 °C, 36 MPa in an Inconel batch reactor for 30 min in the presence/absence of NaOH and/or Ni-Al(2)O(3). Hydrogen gas yields were more than two times higher in the presence of NaOH than in its absence and tar yields were reduced by up to 71%. Saccharina, a carbohydrate-rich macro-alga, gave the highest hydrogen gas yields of 15.1 mol/kg. The tars from all three algae contained aromatic compounds, including phenols, alkyl benzenes and polycyclic aromatic hydrocarbons as well as heterocyclic nitrogen compounds. Tars from Chlorella and Spirulina contained high yields of pyridines, pyrroles, indoles and pyrimidines. Up to 97% TOC removal were achieved in the process waters from the gasification of the algae. Analyses for specific nutrients in the process waters indicated that the process waters from Saccharina could potentially be used for microalgae cultivation.

Catalytic depolymerization of alkali lignin in subcritical water: influence of formic acid and Pd/C catalyst on the yields of liquid monomeric aromatic products

Alkali lignin was subjected to depolymerization in subcritical water at 265 °C, 6.5 MPa for reaction times between 1–6 h in a batch reactor and in the presence of formic acid (FA) and Pd/C catalyst. The oil products were extracted into diethyl ether and contained >90% of single-ring phenolic compounds. The reaction of lignin in subcritical water alone yielded 22.3 wt% oil containing 56% guaiacol as the main product. A maximum oil yield of 33.1 wt% was obtained when the lignin was reacted in the presence of formic acid alone. In the presence of FA, catechol became the predominant compound, with more than 80% of the ether extract after 6 h. The conversion of guaiacol to catechol in the presence of formic acid suggested the hydrolysis of O–CH3 ether bonds. In addition, the yields of alkyl guaiacols increased in the presence of FA. The use of 5 wt% Pd/C catalyst with FA slightly decreased the yields of oil products but led to compounds indicative of hydrogenolysis of aryl–O ether bonds as well as hydrogenation of CC bonds.

Reaction products from the subcritical water gasification of food wastes and glucose with NaOH and H2O2.

The gasification of some selected components of food wastes using H(2)O(2) as the oxidant and in the presence of NaOH has been investigated under subcritical water conditions. Hydrogen production was enhanced when both NaOH and H(2)O(2) were used compared to when either NaOH or H(2)O(2) alone was used or in their absence. Results indicated that the H(2)O(2) acted to partially oxidize the samples while NaOH significantly increased hydrogen gas yields by promoting the water-gas shift reaction with subsequent CO(2) capture. In the presence of NaOH, the main components were Na(2)CO(3), CH(3)COONa and CH(3)COONa.3H(2)O. Char and tar production were suppressed in the presence of NaOH.

Polycyclic aromatic hydrocarbons (PAH) formation from the pyrolysis of different municipal solid waste fractions

The formation of 2-4 ring polycyclic aromatic hydrocarbons (PAH) from the pyrolysis of nine different municipal solid waste fractions (xylan, cellulose, lignin, pectin, starch, polyethylene (PE), polystyrene (PS), polyvinyl chloride (PVC), and polyethylene terephthalate (PET)) were investigated in a fixed bed furnace at 800 °C. The mass distribution of pyrolysis was also reported. The results showed that PS generated the most total PAH, followed by PVC, PET, and lignin. More PAH were detected from the pyrolysis of plastics than the pyrolysis of biomass. In the biomass group, lignin generated more PAH than others. Naphthalene was the most abundant PAH, and the amount of 1-methynaphthalene and 2-methynaphthalene was also notable. Phenanthrene and fluorene were the most abundant 3-ring PAH, while benzo[a]anthracene and chrysene were notable in the tar of PS, PVC, and PET. 2-ring PAH dominated all tar samples, and varied from 40 wt.% to 70 wt.%. For PS, PET and lignin, PAH may be generated directly from the aromatic structure of the feedstock.

Hydrothermal reactions of sodium formate and sodium acetate as model intermediate products of the sodium hydroxide-promoted hydrothermal gasification of biomass

In this work, the hydrothermal reactions of ‘intermediate’ compounds of hydrothermal biomass decomposition have been investigated in relation to reaction temperature and time. Experiments were carried out in a batch Hastelloy-C reactor from 250 °C and 4 MPa up to 500 °C and 43 MPa. The results indicate that sodium formate decomposes to mainly H2 (up to 92% yield) with little yield of CO2. The reaction of sodium formate was complete at just above 400 °C, or after 60 min at 350 °C. On the other hand, sodium acetate was stable until around 380 °C, decomposing gradually to produce mainly CH4 (up to 75% yield) and some CO2. Increasing the reaction temperature beyond 400 °C, and increasing reaction time at 450 °C, led to the increasing production of both CH4 and H2 gases from sodium acetate. These results suggested two possible decomposition patterns for sodium acetate. The predominant mechanism is the reaction with water to produce CH4 and another possible oxidation mechanism that may involve the formation of sodium oxalate at a much higher temperature. The latter mechanism may explain the formation of H2 gas from the hydrothermal reaction of sodium acetate. Selected hydrothermal reactions of sodium oxalate were carried out between 350 and 500 °C. The results showed that at 450 and 500 °C, sodium oxalate yields H2 gas and CO2 almost exclusively. This suggests the possibility of H2 gas production from sodium oxalate as an intermediate product of the reaction of sodium acetate at high temperatures. Generally, this work suggests that sodium formate and sodium acetate may be the important intermediate species that lead to the production of H2 and CH4 gases during the sodium hydroxide-promoted hydrothermal gasification of biomass.

A parametric study on supercritical water gasification of Laminaria hyperborea: a carbohydrate-rich macroalga

The potential of supercritical water gasification (SCWG) of macroalgae for hydrogen and methane production has been investigated in view of the growing interest in a future macroalgae biorefinery concept. The compositions of syngas from the catalytic SCWG of Laminaria hyperborea under varying parameters including catalyst loading, feed concentration, hold time and temperature have been investigated. Their effects on gas yields, gasification efficiency and energy recovery are presented. Results show that the carbon gasification efficiencies increased with reaction temperature, reaction hold time and catalyst loading but decreased with increasing feed concentrations. In addition, the selectivity towards hydrogen and/or methane production from the SCWG tests could be controlled by the combination of catalysts and varying reaction conditions. For instance, Ru/Al2O3 gave highest carbon conversion and highest methane yield of up to 11 mol/kg, whilst NaOH produced highest hydrogen yield of nearly 30 mol/kg under certain gasification conditions.

Effect of interactions of PVC and biomass components on the formation of polycyclic aromatic hydrocarbons (PAH) during fast co-pyrolysis

The interactions of polyvinyl chloride (PVC) and biomass components (hemi-cellulose, cellulose and lignin) during fast pyrolysis were investigated at 800 °C in a fixed bed reactor. The interactions of PVC and biomass components decreased the HCl yield and increased the tar yield significantly. During the co-pyrolysis of PVC with the biomass components, most polycyclic aromatic hydrocarbon (PAH) components were decreased compared with the calculated proportion results. The mechanism of the interactions may be that in the fast pyrolysis process, the processes of dehydrochlorination and chain scission occur in a very short time. Biomass materials and/or bio-char can act as catalysts which inhibit the dehydrochlorination process or promote the chain scission of PVC. Therefore, the dehydrochlorination process might not be completed, resulting in the production of chlorinated oil compounds. Thus, the HCl yield is reduced and PAH concentrations are decreased during the co-pyrolysis of PVC and biomass.

PCDD/F formation from oxy-PAH precursors in waste incinerator flyash.

The yield of PCDD/F in relation to the presence of oxygenated PAH in model waste incinerator flyash has been investigated in a fixed bed laboratory scale reactor. Experiments were undertaken by thermal treatment of the model flyash at 250 and 350°C under a simulated flue gas stream for 2 h. After reaction, the PCDD/F content of the reacted flyash and the PCDD/F released into the exhaust gas, and subsequently trapped by XAD-II resin in a down-stream condensation system were analyzed. The PAHs investigated were, dibenzofuran and benzo[b]naphtho[2,3-d]furan and were spiked onto the model flyash as reactant precursors for PCDD/F formation. The results showed significant formation of furans from both of the PAH investigated, however except from some highly chlorinated dioxin congeners, the formation of dioxins was not so common. Benzonaphthofuran was significantly more reactive than dibenzofuran in PCDD/F formation, in spite of the fact that dibenzofuran is structurally more similar to that of PCDD/F. Thus, there was no clear attribution between the chemical structure of PAH used and the formation of PCDD/F. There were considerable differences between the yields of PCDD/F congeners in the gaseous species and those in the reacted flyash under the same operational conditions. The concentration of PCDD/Fs was reduced at the higher reaction temperature of 350°C; however, the higher temperature resulted in the majority of the PCDD/F formed on the flyash being released into the gas phase.

Effects of gaseous NH3 and SO2 on the concentration profiles of PCDD/F in flyash under post-combustion zone conditions

The influence of gaseous ammonia and sulphur dioxide on the formation of 2378-substituted PCDD/F on a reference flyash from a municipal waste incinerator has been investigated using a laboratory scale fixed-bed reactor. The reference flyash samples (BCR-490) was reacted under a simulated flue gas stream at temperatures of 225 and 375°C for 96h. The experiments were carried out in two series: first with simulated flue gas alone, and then with injection of NH(3) or SO(2) gas into the flue gas just before the reactor inlet. It was found that the injection of gaseous ammonia into the flue gas could decrease the concentration of both PCDD and PCDF by 34-75% from the solid phase and by 21-40% from the gas phase. Converting the results to I-TEQ values, it could reduce the total I-TEQ values of PCDD and PCDF in the sum of the flyash and exhaust flue gas by 42-75% and 24-57% respectively. The application of SO(2) led to 99% and 93% reductions in the PCDD and PCDF average congener concentrations, respectively in the solid phase. In the gas phase, the total reductions were 89% and 76% for PCDD and PCDF, respectively. Moreover, addition of SO(2) reduced the total I-TEQ value of PCDD and PCDF in the flyash and exhaust flue gas together by 60-86% and 72-82% respectively. Sulphur dioxide was more effective than ammonia in suppressing PCDD/F formation in flyash under the conditions investigated.

Removal potential of toxic 2378-substituted PCDD/F from incinerator flue gases by waste-derived activated carbons

The application of activated carbons has become a commonly used emission control protocol for the removal or adsorption of persistent organic pollutants from the flue gas streams of waste incinerators. In this study, the 2378-substituted PCDD/F removal efficiency of three types of activated carbons derived from the pyrolysis of refuse derived fuel, textile waste and scrap tyre was investigated and compared with that of a commercial carbon. Experiments were carried out in a laboratory scale fixed-bed reactor under a simulated flue gas at 275°C with a reaction period of four days. The PCDD/F in the solid matrices and exhaust gas, were analyzed using gas chromatography coupled with a triple quadrupole mass spectrometer. In the absence of activated carbon adsorbent, there was a significant increase in the concentration of toxic PCDD/F produced in the reacted flyash, reaching up to 6.6 times higher than in the raw flyash. In addition, there was a substantial release of PCDD/F into the gas phase, which was found in the flue gas trapping system. By application of the different commercial, refuse derived fuel, textile and tyre activated carbons the total PCDD/F toxic equivalent removal efficiencies in the exhaust gas stream were 58%, 57%, 64% and 52%, respectively. In general, the removal of the PCDDs was much higher with an average of 85% compared to PCDFs at 41%. Analysis of the reacted activated carbons showed that there was some formation of PCDD/F, for instance, a total of 60.6 μg I-TEQ kg(-1) toxic PCDD/F was formed in the refuse derived fuel activated carbon compared to 34 μg I-TEQ kg(-1) in the commercial activated carbon. The activated carbons derived from the pyrolysis of waste, therefore, showed good potential as a control material for PCDD/F emissions in waste incinerator flue gases.