Grindability and combustion behavior of coal and torrefied biomass blends

Biomass samples (pine, black poplar and chestnut woodchips) were torrefied to improve their grindability before being combusted in blends with coal. Torrefaction temperatures between 240 and 300 °C and residence times between 11 and 43 min were studied. The grindability of the torrefied biomass, evaluated from the particle size distribution of the ground sample, significantly improved compared to raw biomass. Higher temperatures increased the proportion of smaller-sized particles after grinding. Torrefied chestnut woodchips (280 °C, 22 min) showed the best grinding properties. This sample was blended with coal (5–55 wt.% biomass). The addition of torrefied biomass to coal up to 15 wt.% did not significantly increase the proportion of large-sized particles after grinding. No relevant differences in the burnout value were detected between the coal and coal/torrefied biomass blends due to the high reactivity of the coal. NO and SO2 emissions decreased as the percentage of torrefied biomass in the blend with coal increased.This work was part of the subcontracted contribution of INCAR to a project carried out by EDP Spain with the financial support from the European Regional Development Fund (ERDF) and acting IDEPA (Economic Development Agency of the Principality of Asturias) as research funding agency (Ref.: IDE/2013/000233). The authors thank A. J. Martín, member of the PrEM group at INCAR-CSIC, for his contribution.Peer reviewe

Kinetics of CO2 adsorption on cherry stone-based carbons in CO2/CH4 separations

Most practical applications of solids in industry involve porous materials and adsorption processes. A correct assessment of the equilibrium and kinetics of adsorption is extremely important for the design and operation of adsorption based processes. In our previous studies we focused on the evaluation of the equilibrium of CO2/CH4 adsorption on cherry stone-based carbons. In the present paper the kinetics of adsorption of CO2 on two cherry stone-based activated carbons (CS-H2O and CS-CO2), previously prepared in our laboratory, has been evaluated by means of transient breakthrough experiments at different CO2/CH4 feed concentrations, at atmospheric pressure and 30 °C. A commercial activated carbon, Calgon BPL, has also been evaluated for reference purposes. Three models have been applied to estimate the rate parameters during the adsorption of CO2 on these carbons, pseudo-first, pseudo-second and Avrami´s fractional order kinetic models. Avrami´s model accurately predicted the dynamic CO2 adsorption performance of the carbons for the different feed gas compositions. To further investigate the mechanism of CO2 adsorption on CS-H2O, CSCO2 and Calgon BPL, intra-particle diffusion and Boyd´s film-diffusion models were also evaluated. It was established that mass transfer during the adsorption of CO2 from CO2/CH4 is a diffusion-based process and that the main diffusion mechanisms involved are intra-particle and film diffusion. At the initial stages of adsorption, film diffusion resistance governed the adsorption rate, whereas intra-particle diffusion resistance was the predominant factor in the following stages of adsorption.This work has received financial support from the Spanish MINECO (Project ENE2011-23467), co-financed by the European Regional Development Fund (ERDF), and from the Gobierno del Principado de Asturias (PCTI 2013-2017 GRUPIN14-079). N.A-G. also acknowledges a fellowship awarded by the Spanish MINECO (FPI program), and co-financed by the European Social Fund.Peer reviewe

Production of fuel-cell grade H2 by sorption enhanced steam reforming of acetic acid as a model compound of biomass-derived bio-oil

Fuel-cell grade H2 has been produced by the sorption enhanced steam reforming (SESR) of acetic acid, a model compound of the bio-oil obtained from the fast pyrolysis of biomass. A Pd/Ni–Co catalyst derived from a hydrotalcite-like material (HT) with dolomite as CO2 sorbent was used in the process. A fixed-bed reactor with three temperature zones was employed to favor the catalytic steam reforming reaction in the high-temperature segment, the SESR reaction in the intermediate-temperature part, as well as the water-gas shift (WGS) and CO2 capture reactions in the low-temperature segment. Different conditions of pressure, temperature, steam/C molar ratio and weight hourly space velocity (WHSV) in the feed were evaluated. Higher steam/C molar ratios and lower WHSV values facilitated the production of H2 and reduced the concentrations of CH4, CO and CO2 in the produced gas. A fuel-cell grade H2 stream with a H2 purity of 99.8 vol.% and H2 yield of 86.7% was produced at atmospheric pressure, with a steam/C ratio of 3, a WHSV of 0.893 h−1 and a temperature of 575 °C in the intermediate part of the reactor (675 °C in the upper segment and 425 °C in the bottom part). At high pressure conditions (15 atm) a maximum H2 concentration of 98.31 vol.% with a H2 yield of 79.81% was obtained at 725 °C in the intermediate segment of the reactor (825 °C in the upper segment and 575 °C in the bottom part). Under these conditions an effluent stream with a CO concentration below 10 ppm (detection limit) was obtained at both low and high pressure, making it suitable for direct use in fuel cell applications.This work was carried out with financial support from the Spanish MINECO (Project ENE2014-53515-P), co-financed by the European Regional Development Fund (ERDF) and the Principado de Asturias (PCTI 2013-2017, GRUPIN14-079)Peer reviewe

Biomass devolatilization at high temperature under N2 and CO2: Char morphology and reactivity

Oxy-fuel combustion is usually performed in pf reactors under an enriched O2 atmosphere of CO2 to obtain a high CO2 content in the flue gases. The effect of the differences in thermal properties of N2 (conventional air combustion) and CO2 (oxy-fuel combustion) on the devolatilization process needs to be evaluated. The morphology and reactivity of biomass chars obtained by devolatilization in an EFR (entrained flow reactor) at 1300 °C under N2 and CO2, simulating air and oxy-fuel combustion atmospheres, were studied. Four biomasses were selected: PIN (pine sawdust), OW (olive waste), OS (olive stones) and CW (coffee waste). The apparent volatile yield under CO2 was greater than under N2. The morphology of the chars was assessed using SEM (scanning electron microscopy). The higher mass loss and the lower char particle size obtained during CO2 devolatilization indicate that a char-CO2 reaction occurred. The reactivity indices indicate a lower reactivity of the CO2-chars than the N2-chars. The devolatilization atmosphere had a significant effect on the biomass chars, suggesting that gasification had occurred during CO2 devolatilization. The OW, OS and CW chars showed a very high reactivity up to intermediate conversion levels, probably due to the catalytic effect of inherent alkali metals.This work was carried out with financial support from the Principado de Asturias (PCTI 2013-2017, GRUPIN14-079), co-financed by the European Regional Development Fund (ERDF). Financial support from the CSIC (Project PIE 201380E064) is also gratefully acknowledgedPeer reviewe

Kinetic models comparison for non-isothermal steam gasification of coal–biomass blend chars

The non-isothermal thermogravimetric method (TGA) was applied to a bituminous coal (PT), two types of biomass, chestnut residues (CH) and olive stones (OS), and coal–biomass blends in order to investigate their thermal reactivity under steam. Fuel chars were obtained by pyrolysis in a fixed-bed reactor at a final temperature of 1373 K for 30 min. The gasification tests were carried out by thermogravimetric analysis from room temperature to 1373 K at heating rates of 5, 10 and 15 K min−1. After blending, no significant interactions were detected between PT and CH during co-gasification, whereas deviations from the additive behaviour were observed in the PT–OS blend. However, for the two coal–biomass blends, the gasification behaviour resembled that of the individual coal, as this component constituted the larger proportion of the blend. The temperature-programmed reaction (TPR) technique was employed at three different heating rates to analyze noncatalytic gas–solid reactions. Three nth-order representative gas–solid models, the volumetric model (VM), the grain model (GM) and the random pore model (RPM) were applied in order to describe the reactive behaviour of the chars during steam gasification. From these models, the kinetic parameters were determined. The best model for describing the reactivity of the PT, PT–CH and PT–OS samples was the RPM model. VM was the model that best fitted the CH sample, whereas none of the models were suitable for the OS sample.This work was carried out with financial support from the Spanish MICINN (Project PS- 120000-2006-3, ECOCOMBOS), and co-financed by the European Regional Development Fund, ERDF.Peer reviewe

Production of fuel-cell grade hydrogen by sorption enhanced water gas shift reaction using Pd/Ni–Co catalysts

It has been demonstrated both thermodynamically and experimentally that fuel-cell grade hydrogen can be produced by a one-step sorption enhanced water gas shift (SEWGS) process at about 500 °C, where the water gas shift (WGS) catalyst and the CO2 sorbent are highly integrated. A synthetic CaO-based mixed oxide sorbent was also assessed, which showed a good CO2 capture capacity and stability in the cyclic operation of the SEWGS reaction. Catalysts play a significant role in CO conversion via WGS, methanation and methane steam reforming reactions. A Pd promoted Ni–Co catalyst (1%Pd/20%Ni–20%Co) derived from hydrotalcite-like material (HT) showed a high activity for WGS and methane steam reforming. The methanation activity was further reduced on 30%Ni–10%Co. There exists an optimum temperature (500 °C) for hydrogen production by the SEWGS process, where it is kinetically limited by the WGS reaction at lower temperatures (425–475 °C) and it is thermodynamically unfavorable at higher temperatures (475–550 °C). The challenges for hydrogen production by SEWGS at high CO pressures were also demonstrated, where CO pressure has shown a negative influence on WGS activity. An induction period was observed, which can be reduced by improving catalyst activity and by adding hydrogen to the reactant mixture.T. Noor would like to acknowledge Research Council of Norway for financial support under its KOSK project 10305300. M.V. Gil acknowledges funding from the CSIC JAE-Doc program, Spain, co-financed by the European Social Fund, and support from the Research Council of Norway under the Yggdrasil programme.Peer reviewe

Coal and biomass cofiring: fundamentals and future trends

Biomass cofiring is a promising technology to decrease the use of fossil fuels for energy generation and hence mitigate greenhouse gas emissions. Coal and biomass cofiring accounts for the relevant advantages of a relative ease of implementation and an effective reduction of CO2 and other pollutant (SOx, NOx) emissions to the atmosphere. Cofiring biomass with coal may record no loss in total boiler efficiency after adjusting combustion output for the new fuel mixture. However, the guarantee of a stable and cheap supply of biomass, together with an optimum delivery system, is a key parameter for the success of biomass cofiring. Standardization in the characterization and treatment of biomass is also lacking. Future research on thermal, chemical, and mechanical characteristics of biomass; technical problems in burning biomass; and optimum cofiring conditions have to be addressed. Finally, incentives and favorable regulatory and environmental policies will be the major factors encouraging the development of the cofiring technology.Authors are grateful to the Gobierno del Principado de Asturias (PCTI-GRUPIN14-079) and to the CSIC (PIE-201780E057) for funding

Syngas Fermentation: Cleaning of Syngas as a Critical Stage in Fermentation Performance

The fermentation of syngas is an attractive technology that can be integrated with gasification of lignocellulosic biomass. The coupling of these two technologies allows for treating a great variety of raw materials. Lignin usually hinders microbial fermentations; thus, the thermal decomposition of the whole material into small molecules allows for the production of fuels and other types of molecules using syngas as substrate, a process performed at mild conditions. Syngas contains mainly hydrogen, carbon monoxide, and carbon dioxide in varying proportions. These gases have a low volumetric energy density, resulting in a more interesting conversion into higher energy density molecules. Syngas can be transformed by microorganisms, thus avoiding the use of expensive catalysts, which may be subject to poisoning. However, the fermentation is not free of suffering from inhibitory problems. The presence of trace components in syngas may cause a decrease in fermentation yields or cause a complete cessation of bacteria growth. The presence of tar and hydrogen cyanide are just examples of this fermentation’s challenges. Syngas cleaning impairs significant restrictions in technology deployment. The technology may seem promising, but it is still far from large-scale application due to several aspects that still need to find a practical solution.Peer reviewe

Anaerobic Codigestion of Sludge: Addition of Butcher’s Fat Waste as a Cosubstrate for Increasing Biogas Production

Fat waste discarded from butcheries was used as a cosubstrate in the anaerobic codigestion of sewage sludge (SS). The process was evaluated under mesophilic and thermophilic conditions. The codigestion was successfully attained despite some inhibitory stages initially present that had their origin in the accumulation of volatile fatty acids (VFA) and adsorption of long-chain fatty acids (LCFA). The addition of a fat waste improved digestion stability and increased biogas yields thanks to the higher organic loading rate (OLR) applied to the reactors. However, thermophilic digestion was characterized by an effluent of poor quality and high VFA content. Results from spectroscopic analysis suggested the adsorption of lipid components onto the anaerobic biomass, thus disturbing the complete degradation of substrate during the treatment. The formation of fatty aggregates in the thermophilic reactor prevented process failure by avoiding the exposure of biomass to the toxic effect of high LCFA concentrations.This work was financially supported by the Ministerio de Economía y Competitividad through the project CTQ2015-68925-R (http://www.mineco.gob.es/). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.Peer reviewe

H2 production by sorption enhanced steam reforming of biomass-derived bio-oil in a fluidized bed reactor: An assessment of the effect of operation variables using response surface methodology

High-purity H2 was produced by the sorption enhanced steam reforming (SESR) of acetic acid, a model compound of bio-oil obtained from the fast pyrolysis of biomass, in a fluidized bed reactor. A Pd/Ni–Co hydrotalcite-like material (HT) and dolomite were used as reforming catalyst and CO2 sorbent, respectively. The hydrogen yield and purity were optimized by response surface methodology (RSM) and the combined effect of the reaction temperature (T), steam-to-carbon molar ratio in the feed (steam/C) and weight hourly space velocity (WHSV) upon the sorption enhanced steam reforming process was analyzed. T was studied between 475 and 675 °C, steam/C ratio between 1.5 and a 4.5 mol/mol and WHSV between 0.893 and 2.679 h−1. H2 yield, H2 selectivity and H2 purity, as well as the CH4, CO and CO2 concentrations in the effluent gas, were assessed. The operating temperature proved to be the variable that had the greatest effect on the response variables studied, followed by the WHSV and the steam/C ratio. The results show that the H2 yield, H2 selectivity and H2 purity increased, while the CH4, CO and CO2 concentrations decreased, concurrently with the temperature up to around 575–625 °C. Higher values of the steam/C ratio and lower WHSV values favored the H2 yield, H2 selectivity and H2 purity, and reduced the CH4 concentration. It was found that the SESR of acetic acid at atmospheric pressure and 560 °C, with a steam/C ratio of 4.50 and a WHSV of 0.893 h–1 gave the highest H2 yield of 92.00%, with H2 purity of 99.53% and H2 selectivity of 99.92%, while the CH4, CO and CO2 concentrations remained low throughout (0.04%, 0.06% and 0.4%, respectively). The results also suggested that a slow CO2 capture rate led to a poor level of hydrogen production when the SESR process was carried out at low temperatures, although this can be improved by increasing the sorbent/catalyst ratio in the fluidized bed.The financial support from the Research Council of Norway (RCN) is gratefully acknowledged. The authors thank Franefoss Miljøkalk A/S (Norway) for supplying Arctic dolomite. M.V. Gil acknowledges funding from the CSIC JAE-Doc program, Spain, co-financed by the European Social Fund, and support from the Research Council of Norway through the Yggdrasil program.Peer reviewe