Effect of catalysts in the quality of syngas and by-products obtained by co-gasification of coal and wastes. 1. Tars and nitrogen compounds abatement

The aim of this work is to analyse the possibility of using co-gasification technology to process coal mixed with wastes to take profit of its energy content and at the same time to minimize the environmental impact associated with the use of wastes and to diminish the costs of flue gas treatment. The addition to coal of different types of materials, like: pine based waste, petcoke and polyethylene (PE), was not found to give rise to any operational problems, regarding both the feeding system and gasification process and led to higher energy conversions, however, the gas presented higher tars and hydrocarbons content. Several catalysts were tested, such as, dolomite, olivine, nickel and magnesium oxides, zinc oxides and cobalt and molybdenum oxides. Catalyst action was analysed in tars release and also in ammonia compounds reduction. The presence of catalysts allowed increasing hydrogen release, whilst there was a decrease in hydrocarbons and tars contents. A nickel-magnesium oxide was the catalyst that led to the highest reduction in hydrocarbons and tars. This catalyst also led to the lowest NH₃ content in the fuel gas produced, due to the catalyst efficiency in NH₃ destruction.     

Catalytic steam gasification of Miscanthus X giganteus in fluidised bed reactor on olivine based catalysts

The Miscanthus X giganteus (MXG) presents many advantages (high yield, perennial crop, easy harvesting…) so it can be considered as a good candidate in terms of renewable energy sources. Several works have been carried out and were devoted to the MXG, especially in the agricultural field, but this study is the first which deals with gasification in order to produce syngas. The catalytic steam gasification of MXG in a fluidised bed reactor into presence of olivine based catalysts was investigated. Three parameters were studied, the temperature (800 °C and 900 °C), the pellets size (6 mm and 8 mm) and the nature of catalyst (olivine and Ni/olivine). Noteworthy is the efficiency shown by the Ni/olivine at 800 °C, which leads to the production of 1.7 m³ kg^− 1 daf of gas, containing 50% of H₂. Ni/olivine catalyst was characterised by XRD, TPR and SEM-EDX in order to monitor its structural changes during the process. Moreover, a solvent system of tar recovery was tested, which allows to obtain a more representative set of the whole tars. Then, the tars composition was determined by GC/MS. The identification of different compounds shows the presence of different PAHs, in majority naphthalene.     

Steam gasification of Miscanthus X Giganteus with olivine as catalyst production of syngas and analysis of tars (IR, NMR and GC/MS)

Steam gasification of Miscanthus X Giganteus (MXG) at high heating rate in a fluidised bed reactor with the use of olivine as catalyst was investigated. The effects of temperature (815-880 °C) on the yields and the compositions of syngas and tars were determined. The experimental results show that the gas yields and the content of H₂ increase with the temperature, while the yields of tar, char and the content of CO, CO₂ and CH₄ in the product gas decrease. Noteworthy is that about 1.1 m³ of dry gas (at ambient conditions) per kg of dry ash free biomass were obtained with about 46% of H₂ and 24% of CO by volume at 880 °C.The tars composition was determined by FTIR, NMR and GC/MS. The identification of different compounds shows mainly the presence of simple molecules. This may be facilitating the possibility of complete tar reforming process (hot gas cleaning), to improvement of the syngas yield and the decrease of the formation of pollutants.     

The continuous self stirred tank reactor: measurement of the cracking kinetics of biomass pyrolysis vapours

The continuous self stirred tank reactor is a well known suitable device for studying the kinetics of high-temperature gas phase reactions. Temperature and composition are uniform at any point of the reactor, and for a given residence time, it is easy to calculate the values of kinetic constants from a very simple mathematical model. The aim of the present paper is to report the first experimental results obtained on the thermal cracking of vapours produced by the pyrolysis of biomass. The experiments are carried out between 836 and 1303 K and under mean residence times ranging from 0.3 to 0.5 s. The calculated activation energy and preexponential factor are in good agreement with those obtained by other authors operating in more usual devices, but needing the solving of more sophisticated models.     

Preparation of carbon fibre precursors from the pyrolysis and copyrolysis of Avgamasya asphaltite and Göynük oil shale: vacuum distillation and hexane extraction

Göynük oil shale (GOS) and Avgamasya asphaltite (AA) mixed in various ratios were used as raw materials for pitch precursors. Pure GOS and AA and their mixtures were pyrolysed at 550 °C and the resultant tars were vacuum distilled at 300 °C and 3 Torr for 60 min to produce pitch precursors. The pitch obtained in the highest yield was further modified by extraction with hexane. Structural characterisation by FTIR and ¹H and ¹³C NMR, and elemental analysis of the vacuum distillation pitches and the vacuum-distilled hexane-extracted pitch showed that vacuum distillation did not increase the softening point and toluene-insoluble (TI) content of the pitches. Further extraction with hexane removed significant amounts of aliphatic components from the pitch and increased the softening point to 136 °C. The vacuum-distilled, hexane-extracted pitch showed good spinnability into fibres of 15-20 μm diameter. The as-spun fibres were stabilised by nitric acid. The green fibres were pre-treated with air at low temperatures in order to complete stabilisation before the carbonisation process at 1000 °C. SEM images of the carbonised fibres showed surface defects due to pitch composition and/or insufficient stabilisation.     

Pyrolysis of Pinus pinaster in a two-stage gasifier: Influence of processing parameters and thermal cracking of tar

A new two-stage gasifier with fixed-bed has recently been installed on CIRAD facilities in Montpellier. The pyrolysis and the gasifier units are removable. In order to characterise the pyrolysis products before their gasification, experiments were carried out, for the first time only with the pyrolysis unit and this paper deals with the results obtained. The biomass used is Pinus pinaster. The parameters investigated are: temperature, residence time and biomass flow rate. It has been found that increasing temperature and residence time improve the cracking of tars, gas production and char quality (fixed carbon rate more than 90%, volatile matter rate less than 4%). The increase of biomass flow rate leads to a bad char quality.     

The impact of carbon formation on Ni-YSZ anodes from biomass gasification model tars operating in dry conditions

The combination of solid oxide fuel cells (SOFCs) and biomass gasification has the potential to become an attractive technology for the production of clean and renewable energy. However the impact of tars, formed during biomass gasification, on the performance and durability of SOFC anodes has not been well established experimentally. This paper reports on an experimental study of the effects of carbon formation on the anodes of SOFC button cells from synthetic model tars arising from the gasification of biomass material. Furthermore the paper evaluates appropriate model tars to study the effects of typical biomass gasification tars on SOFC operation. The anode material used in this work was a 60:40 wt.% NiO/YSZ cermet, which was tested in a 15% H₂ gas mixture containing a concentration of 15 g/Nm³ of different biomass gasification model tars. Model tars included benzene and toluene representing the simplest and most predominant of biomass gasification tars, and a tar mix consisting of higher molecular weight tars such as naphthalene, pyrene, and phenol. It was found that carbon formation in dry conditions significantly damaged the anode of the fuel cell resulting in decreased cell performance and excessive anode polarization resistances. The higher reactivity of benzene compared to other model tars led to higher levels of carbon formation on reduced Ni-O catalysts. Different types of carbon were formed depending on the operating temperature of the SOFC.     

The impact of steam and current density on carbon formation from biomass gasification tar on Ni/YSZ, and Ni/CGO solid oxide fuel cell anodes

The combination of solid oxide fuel cells (SOFCs) and biomass gasification has the potential to become an attractive technology for the production of clean renewable energy. However the impact of tars, formed during biomass gasification, on the performance and durability of SOFC anodes has not been well established experimentally. This paper reports an experimental study on the mitigation of carbon formation arising from the exposure of the commonly used Ni/YSZ (yttria stabilized zirconia) and Ni/CGO (gadolinium-doped ceria) SOFC anodes to biomass gasification tars. Carbon formation and cell degradation was reduced through means of steam reforming of the tar over the nickel anode, and partial oxidation of benzene model tar via the transport of oxygen ions to the anode while operating the fuel cell under load. Thermodynamic calculations suggest that a threshold current density of 365 mA cm⁻² was required to suppress carbon formation in dry conditions, which was consistent with the results of experiments conducted in this study. The importance of both anode microstructure and composition towards carbon deposition was seen in the comparison of Ni/YSZ and Ni/CGO anodes exposed to the biomass gasification tar. Under steam concentrations greater than the thermodynamic threshold for carbon deposition, Ni/YSZ anodes still exhibited cell degradation, as shown by increased polarization resistances, and carbon formation was seen using SEM imaging. Ni/CGO anodes were found to be more resilient to carbon formation than Ni/YSZ anodes, and displayed increased performance after each subsequent exposure to tar, likely due to continued reforming of condensed tar on the anode.     

Detailed kinetic study of anisole pyrolysis and oxidation to understand tar formation during biomass combustion and gasification

Anisole was chosen as the simplest surrogate for primary tar from lignin pyrolysis to study the gas-phase chemistry of methoxyphenol conversion. Methoxyphenols are one of the main precursors of PAH and soot in biomass combustion and gasification. These reactions are of paramount importance for the atmospheric environment, to mitigate emissions from wood combustion, and for reducing tar formation during gasification. Anisole pyrolysis and stoichiometric oxidation were studied in a jet-stirred reactor (673–1173 K, residence time 2 s, 800 Torr (106.7 kPa), under dilute conditions) coupled with gas chromatography–flame ionization detector and mass spectrometry. Decomposition of anisole starts at 750 K and a conversion degree of 50% is obtained at about 850 K under both studied conditions. The main products of reaction vary with temperature and are phenol, methane, carbon monoxide, benzene, and hydrogen. A detailed kinetic model (303 species, 1922 reactions) based on a combustion model for light aromatic compounds has been extended to anisole. The model predicts the conversion of anisole and the formation of the main products well. The reaction flux analyses show that anisole decomposes mainly to phenoxy and methyl radicals in both pyrolysis and oxidation conditions. The decomposition of phenoxy radicals is the main source of cyclopentadienyl radicals, which are the main precursor of naphthalene and heavier PAH in these conditions.