• Energy Research
• engineering and technology close

Abstract Continuous steady-state gasification tests were performed, in which mixtures of lignite and solid recovered fuel (SRF) were fed to a bench-scale facility at atmospheric pressure, loaded with the bottom product of a high-temperature Winkler gasifier as the fluidized bed material. The O2/fuel and steam/fuel ratios were varied from 0.3 to 0.4 and from 0.25 to 0.35, respectively, and the effects of the temperature were examined at different levels (700, 750, and 800 °C). The objective of the experimental campaign was to evaluate the effects of above mentioned operating conditions on the (i) quality of syngas expressed in terms of gas yield ( Y gas ) , cold gas efficiency ( η CG ) , and carbon conversion ( X C ); (ii) effectiveness in tar reduction; and (iii) improvement of the H2/CO molar ratio. Characterization analyses (grain-size distribution, scanning electron microscopy and energy-dispersive X-ray spectroscopy) were performed on the materials before and after the tests. Pressure-fluctuation signals were acquired during the tests to monitor the fluidization quality and diagnose the correlated incipient sintering or agglomeration of the bed particles. At 800 °C, the obtained results ( Y gas = 1.53 ; η CG = 79 % ; X C = 92 % ; tar content = 7.35 g/Nm3; H2/CO molar ratio = 0.96) demonstrated the convenient feasibility of gasification with the SRF–lignite mixture as a fuel (SRF/lignite = 0.5 wt/wt) and helped define the operating conditions for future pilot tests aiming for liquid fuel synthesis, although the best results were obtained at 800 °C with SFR/lignite = 0.2 wt/wt ( Y gas = 1.79 ; η CG = 93 % ; X C = 102 % ; tar content = 0.92 g/Nm3; H2/CO molar ratio = 0.84).

Abstract Biomass gasification is a very promising process to produce energy from agricultural wastes; however, tar and particulate have to be removed in order to make the product gas exploitable. Catalytic filter candles inserted in the freeboard of a fluidized bed gasifier, perform hot syngas cleaning from particulate and promote the steam reforming reactions of tar and methane thanks to the addition of Ni-catalyst in their inner space. In this work, a 3D-CFD model is implemented in the FLUENT software, to simulate 6 catalytic candles housing in the freeboard of a pilot scale dual bubbling fluidized bed steam gasifier (100 kWth as biomass input). Main model parameters were derived from experimental results obtained with a bench scale gasification reactor, equipped with a ceramic filtering candle containing a commercial catalyst, to allow conditioning the raw syngas produced on site. Simulations were carried out to study the conversion of tar and methane in a real case application. The temperature drop along the gasifier freeboard causes very low conversion (≈15%) of tar produced inside the gasifier fluidized bed. It was found that small injections of O2 in the freeboard can rise the temperature and increase tar conversion up to 77%. Finally, simulations with raw syngas input subjected to primary tar removal by means of Fe-olivine as fluidized bed material resulted in high conversion rates and residual tar content lower than 1 g/Nm3.

The European research project CLARA (G.A. 817841) has studied pretreated residual biomasses for chemical looping gasification. This work investigated devolatilizations of wheat straw pellets (raw, torrefied, and torrefied-washed) at 700 °C, 800 °C, and 900 °C, performed in fluidized beds made of sand or three oxygen carriers (OCs): integral-average values (gas yield, H2/CO molar ratio, and carbon conversion) were calculated; instantaneous peaks of released syngas were evaluated by regression straight lines. For all biomasses and bed materials, the temperature increase (from 700 to 900 °C) was the dominant parameter, positively affecting all integral-average values. The OCs appeared more active at 900 °C. Biomass pretreatments improved the H2/CO molar ratio and decreased carbon conversion. SEM analyses showed that the purpose of washing (removal of low-melting elements) may be jeopardized by OCs' composition.

The European research project CLARA (chemical looping gasification for sustainable production of biofuels, G.A. 817841) investigated chemical looping gasification of wheat straw pellets. This work focuses on pretreatments for this residual biomass, i.e., torrefaction and torrefaction-washing. Devolatilizations of individual pellets were performed in a laboratory-scale fluidized bed made of sand, at 700, 800, and 900 °C, to quantify and analyze the syngas released from differently pretreated biomasses; experimental data were assessed by integral-average parameters: gas yield, H2/CO molar ratio, and carbon conversion. A new analysis of devolatilization data was performed, based on information from instantaneous peaks of released syngas, by simple regressions with straight lines. For all biomasses, the increase of devolatilization temperature between 700 and 900 °C enhanced the thermochemical conversion in terms of gas yield, carbon conversion, and H2/CO ratio in the syngas. Regarding pretreatments, the main evidence is the general improvement of syngas quality (i.e., composition) and quantity, compared to those of untreated pellets; only slighter differentiations were observed concerning different pretreatments, mainly thanks to peak quantities, which highlighted an improvement of the H2/CO molar ratio in correlation with increased torrefaction temperature from 250 to 270 °C. The proposed methods emerged as suitable straightforward tools to investigate the behavior of biomasses and the effects of process parameters and biomass nature.

Abstract The catalytic activity of a new catalytic filter of combined design consisting of a catalytic filter candle with an integrated catalytic ceramic foam was calculated under model gas conditions at 800 and 850 °C in the absence and presence of 100 ppmv H 2 S on the basis of separate and combined measurements of appropriate catalytic filter element and ceramic foam disk samples. Real gas validation of the determined model gas activity of the catalytic filter of combined design in a bench-scale gasifier was performed to check-up, if model gas conversions can be used as bases for the prediction of real tar conversions. A calculation of the model gas activity of the catalytic filter at a superficial velocity of 2 cm/s based on the disk measurements results to 99% naphthalene conversion at 850 °C in the absence of H 2 S and 94% in the presence of 100 ppmv H 2 S. At 800 °C, the calculated conversion is 95% in the absence and 70% in the presence of 100 ppmv H 2 S resulting in an estimated conversion of 85% at an H 2 S content of 40 ppmv. This value is comparable with the measured real tar conversion of 81% at 790 °C and a superficial velocity of 2.5 cm/s.

A bench-scale fluidized-bed biomass gasification plant, operating at atmospheric pressure and temperature within the range 800-820 degrees C, has been used to test an innovative gas cleaning device: a catalytic filter candle fitted into the bed freeboard. This housing of the gas conditioning system within the gasifier itself results in a very compact unit and greatly reduced thermal losses. Long term (22h) tests were performed on the gasifier both with and without the catalytic candle filter, under otherwise identical conditions. Analysis of the product gas for the two cases showed the catalytic filtration to give rise to notable improvements in both gas quality and gas yield: an increase in hydrogen yield of 130% and an overall increase in gas yield of 69% - with corresponding decreases in methane and tar content of 20% and 79%, respectively. HPLC/UV analysis was used to characterize the tar compounds.

Abstract The present paper deals with carbon dioxide capture from dry CO2/CH4 mixtures by means of zeolites produced from spent power plant fly ash. Commercial sorbents such as activated carbon, silica gel and zeolite 13X were also tested. At an operating pressure of 2 bar, the best zeolite synthesized in this work gave rise to recoveries of CH4 and CO2 of 95.2% and 98.1% respectively, at purities of 98 vol% and 99.6 vol% respectively. Among the commercial sorbents tested at 2 bar pressure, the best one was silica gel, with recoveries of CH4 and CO2 of 85.4% and 97.6% respectively at purities of 97.3 vol% and 94.9 vol% respectively. At this pressure, the CO2 adsorption rate was 0.402 moL/kg silica sorbent; at 6 bar this increased to 1.076 mol CO2/kg silica sorbent but at greatly reduced levels of both CH4recovery and CO2 purity. Three zeolites produced from fly ash also underwent PSA (Pressure Swing Adsorption) tests: after five adsorption-desorption cycles no loss in adsorption capacity of CO2 was observed, both activity and selectivity recovering completely after regeneration.