Effects of sewage sludge organic and inorganic constituents on the properties of pyrolysis products

Summarization: Pyrolysis of sewage sludge in the presence of humic acid (HA), CaO and kaolin was investigated in order to examine the effects of organic and inorganic constituents on the product properties. The presence of HA in the sludge increased the yields of H2 and CO compared to that from the raw sludge pyrolysis. Water evolved during decomposition of kaolin promoted the water-gas shift reaction resulting in less CO in the gas phase. The presence of CaO in the sludge had a major influence on CO2 sequestration reducing the volume of non-condensable gas by 18% compared to that from the raw sludge pyrolysis. In contrast, CO volume increased due to the induced Boudouard reaction. No significant impact on the distribution of the chemical compounds in the organic or aqueous phase was observed with the addition of HA to the sludge. Among the inorganic additives, CaO catalysed deoxygenation and ring opening reactions leading to an increment of aliphatic compounds in the organic phase of tar while kaolin did not show any significant impact. HA in sludge increased the carbon content in the derived char to 35.2% compared to that in the char derived from the raw sludge (31.0%). In contrast, kaolin and CaO resulted in higher char yields with lower carbon contents due to high ash content in the raw samples. Regarding heavy metals fate, the addition of HA and CaO to the sludge effectively immobilised heavy metals in the derived char while kaolin appeared to be less effective.Παρουσιάστηκε στο: Energy Conversion and Managemen

Thermodynamic analyses of synthetic natural gas production via municipal solid waste gasification, high-temperature water electrolysis and methanation

Summarization: The production of synthetic natural gas (SNG) from syngas derived through waste gasification is an attractive option for converting waste into useful fuel. However, the carbon conversion ratio of traditional SNG production is usually below 50% due to the limited amount of H2, while the excess CO2 needs to be removed. To realize a higher utilization of carbon, a novel SNG production system combining a municipal solid waste (MSW) gasifier, a solid oxide electrolyzer cell (SOEC) and a methanation unit is proposed and analyzed by thermodynamic modeling using ASPEN Plus®. Comparison is made with a similar system without the SOEC unit. The SOEC unit is used to electrolyze water into H2 required for higher CH4 production, thus eliminating the need of CO2 removal. Theoretical calculations show that the integration between SOEC with the gasifier produces H2-rich (greater than18 vol% H2) SNG without CO or CO2, allowing direct use of existing town-gas pipelines for easy transportation. With an equivalence air ratio of 0.3 for the gasification process, the carbon conversion ratio can achieve 98% with an efficiency of 67% when the ratio of H2O supplied to the SOEC unit to the carbon in the MSW reaches 2.75. In comparison, the system without SOEC unit can only achieve a carbon conversion ratio of 44% with an efficiency of 78%. Parameter selection of the proposed system in practical implementation is discussed considering the trade-off between the carbon conversion ratio and the efficiency. At the end, preliminary economic analysis also shows that the proposed system is financially attractive.Παρουσιάστηκε στο: Energy Conversion and Managemen

Nickel-based catalysts for steam reforming of naphthalene utilizing gasification slag from municipal solid waste as a support

Summarization: Nickel-based catalysts were synthesized using gasification slag as an abundant and inexpensive material with high mechanical strength. The synthesis procedure comprised of etching of gasification slag with hydrofluoric acid, aluminum hydroxide addition, impregnation with Ni and calcination. The naphthalene reforming activity was tested at 850 °C and 24,000 h−1 gas hourly space velocity in the presence of 50 ppmv H2S and 300 ppmv HCl in gas. The presence of aluminum hydroxide and an oxidizing environment during the calcination process were essential for preparation of active catalysts. The addition of 30% of aluminum hydroxide to gasification slag improved the dispersion of Ni oxide and increased the Brunauer–Emmett–Teller (BET) specific surface area determined by N2 adsorption at −196 °C from 4.5 to 15.0 m2 g−1 as compared to the catalyst without aluminum hydroxide. This resulted in the increased naphthalene conversion from 35 to 86%. The activity of Ni in synthesized slag catalyst was approximately 3.2 times higher than in commercial catalyst due to the deposition of Ni on the outer surface of catalyst particles. The deposited Al and Ni species were firmly attached to the slag particles during calcination at 500–1000 °C. While high calcination temperature was beneficial for high mechanical strength of catalyst, the procedure should be conducted under oxidizing environment. When calcination was carried out in N2 environment at 1000 °C, the sintering of catalyst particles and encapsulation of Ni occurred, which drastically decreased the BET specific surface area to 0.4 m2 g−1 and naphthalene conversion to 30%. These negative effects could be prevented when air was used during calcination, which facilitated earlier crystallization and inhibited excessive sintering of catalyst, thus, maintaining the high catalytic activity.Παρουσιάστηκε στο: Fue

Poisoning effects of H2S and HCl on the naphthalene steam reforming and water-gas shift activities of Ni and Fe catalysts

Summarization: H2S and HCl are common impurities in raw syngas produced during gasification of biomass and municipal solid waste. The purpose of this study was to investigate the poisoning effect of H2S and HCl on synthesized and commercial catalysts during steam reforming of naphthalene. Four synthesized catalysts with different loadings of Ni and Fe on alumina support and two commercial catalysts were selected and evaluated in a fixed bed reactor at 790, 850 and 900 °C. The obtained results revealed that reforming and water-gas shift (WGS) activities of catalysts did not benefit from the Fe addition. The activities were influenced differently by H2S and HCl indicating that the reactions were catalyzed by different active sites on the nickel surface. In the presence of H2S and HCl, the poisoning of naphthalene reforming activity was caused by H2S and was not affected by HCl when both compounds were present in the gas. H2S chemisorbs on nickel surface forming NiS and decreasing the accessibility of active sites to hydrocarbons. The poisoning effect was only partially reversible. On the contrary, the poisoning of WGS activity could be caused by both H2S and HCl, and the activity could be completely restored when H2S and HCl were removed from the gas. Unlike naphthalene reforming activity, which was comparable for catalysts with similar Ni loadings, WGS activity depended on the catalyst structure and was less susceptible to poisoning by H2S and HCl in case of the catalyst with strong NiO-support interactions.Παρουσιάστηκε στο: Fue