Soot suppression by nonthermal plasma in coflow jet diffusion flames using a dielectric barrier discharge

The effect of nonthermal plasma on diffusion flames in coflow jets has been studied experimentally by adopting a dielectric barrier discharge (DBD) technique. The plasma reactor had wire-cylinder-type electrodes with AC power supply operated at 400 Hz. The effect of flame on the behavior of electrical discharge was first investigated to identify the regime of plasma generation, discharge onset voltage, and delivered power to the plasma reactor. The generation of streamers was enhanced with a flame by the increase in the reduced electric field intensity due to high-temperature burnt gas and by the abundance of ions in the flame region. The effect of streamers on flame behavior reveals that the flame length was significantly decreased as the applied voltage increased. The yellow luminosity by the radiation of soot particles was also significantly diminished. The temperature of burnt gases, the concentrations of major species, and the spatial distribution of OH radical, PAH, and soot have been measured. The formation of PAH and soot was influenced appreciably by the nonthermal plasma, while the flame temperature and the concentration of major species were not influenced much with the plasma generation. The results demonstrated that the application of nonthermal plasmas can effectively suppress PAH and soot formation in the flames with low power consumption even in the order of 1 W.     

Catalytic gasification of rice hull by dielectric barrier discharge non-thermal plasma over potassium catalyst

Dielectric barrier discharge (DBD) non-thermal plasma was used for gasification of rice hull. And the gasification efficiency, gas production performance and gasification productions of rice hull were studied in the different atmosphere of the DBD gasification. The effect of the potassium metal on DBD gasification of rice hull was investigated and the catalytic mechanism of potassium was studied by density functional theory calculation. It was found that the gasification efficiency of rice hull by DBD in CO₂ atmosphere is up to 87.14%, which is higher than that in the N₂ atmosphere. The implantation of the potassium metal can also improve the DBD gasification efficiency, and promote the generation of H₂ and CH₄. The rice hull loaded with 3% potassium had the best gasification in the CO₂ atmosphere. Almost no tar was found during the DBD gasification of rice hull. N₂ adsorption and desorption, scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) were used to investigate the properties of the carbon materials, which was generated from the rice hull gasification. The carbon material which was produced by DBD gasification in CO₂ atmosphere had large specific surface area and abundant functional groups. It can be used as catalyst carrier and other fields.     

Recent developments in catalyst synthesis using DBD plasma for reforming applications

The catalyst has a significant role in gas processing applications such as reforming technologies for H₂ and syngas production. The stable catalyst is requisite for any industrial catalysis application to make it commercially viable. Several methods are employed to synthesize the catalysts. However, there is still a challenge to achieve a controlled morphology and pure catalyst which majorly influences the catalytic activity in reforming applications. The conventional methods are expansive, and the removal of the impurities are major challenges. Nevertheless, it is not straightforward to achieve the desired structure and stability. Therefore, significant interest has been developed on the advanced techniques to take control of the physicochemical properties of the catalyst through non-thermal plasma (NTP) techniques. In this review, the systematic evolution of the catalyst synthesis using NTP technique is elucidated. The emerging DBD plasma to synthesized and effective surface treatment is reviewed. DBD plasma synthesized catalyst performance in reforming application for H₂ and syngas production is summarised. Furthermore, the status of DBD plasma for catalyst synthesis and proposed future avenues to design environmentally suitable and cost-effective synthesis techniques are discussed.     

Production of hydrogen from hydrogen sulfide assisted by dielectric barrier discharge

Hydrogen production by non-thermal plasma (NTP) assisted direct decomposition of hydrogen sulfide (H₂S) was carried out in a dielectric barrier discharge (DBD) reactor with stainless steel inner electrode and copper wire as the outer electrode. The specific advantage of the present process is the direct decomposition of H₂S in to H₂ and S and the novelty of the present study is the in-situ removal of sulfur that was achieved by operating DBD plasma reactor at ∼430 K. Optimization of various parameters like the gas residence time in the discharge, frequency, initial concentration of H₂S and temperature was done to achieve hydrogen production in an economically feasible manner. The typical results indicated that NTP is effective in dissociating H₂S into hydrogen and sulfur and it has been observed that by optimizing various parameters, it is possible to achieve H₂ production at 300 kJ/mol H₂ that corresponds to ∼3.1 eV/H₂, which is less than the energy demand during the steam methane reforming (354 kJ/mol H₂ or ∼3.7 eV/H₂).     

Steam enhanced carbon dioxide reforming of methane in DBD plasma reactor

Considering the inevitable high energy input to implement the CO₂ reforming of methane under high-temperature operation using conventional catalysis method, the low temperature conversion of CO₂ and methane in the coaxial dielectric barrier discharge (DBD) plasma reactor was investigated in this work. Steam was introduced to enhance the CO₂ reforming of methane with synergetic catalysis effect by cold plasma and catalyst. The experimental results showed that a certain percent of steam could promote the conversion of both CH₄ and CO₂. Meanwhile, the carbon deposition was evidently reduced compared with the dry reforming of methane. With the increase of steam input, the steam reforming occurred predominantly. As a result, the hydrogen volume percentage in the product gases increased. In this way, the products with different H₂/CO ratio could be achieved by changing the mole ratio of CH₄/CO₂/H₂O at the reactor inlet. In particular, when the mole ratio of H₂O/CH₄ increased to almost 3 corresponding to the pure steam reforming process, the conversion of CH₄ reached almost 0.95 and the selectivity to H₂ was almost 0.99 at 773K.     

Steam reforming of toluene as model biomass tar to H2-rich syngas in a DBD plasma-catalytic system

Biomass tar is one of the most troublesome issues limiting the further development of biomass pyrolysis and gasification. In this study, a plasma enhanced catalytic steam reforming technology was applied for biomass tar removal. Toluene was selected as biomass tar surrogate. The nano-sized alumina-supported nickel and iron catalysts with different molar ratios of M/Al (M: Ni or Fe, 0:1, 1:3, 1:1, 3:1, 1:0) were prepared for catalytic steam reforming of toluene in a non-thermal plasma reactor featuring dielectric barrier discharge (DBD). The results showed that syngas was the dominant gas product of toluene decomposition. The conversion efficiency of toluene and energy efficiency using Ni-Al and Fe-Al catalysts both followed a sequence: M1Al3 > M1Al1 > M3Al1, which is in line with the BET surface area and pore volume. However, the selectivity of H₂ and CO catalysed by Ni-Al and Fe-Al catalysts follows the order of M1Al3 < M1Al1 < M3Al1. Presumably, toluene dissociation is a process composed of adsorption-reaction-desorption. The formation of syngas is supposed to proceed as a series of ionic and free radical reactions occurring preferably in the gas phase. Ni1Al3 catalyst shows the largest potential in converting biomass tar into H₂-rich syngas, with a maximum toluene conversion of 96% and a largest H₂ yield of 2.18 mol/mol-toluene. Besides, the results showed that this hybrid plasma-catalysis system was potential in anti-carbon deposition.     

Pyrolysis-catalytic steam reforming of agricultural biomass wastes and biomass components for production of hydrogen/syngas

The pyrolysis-catalytic steam reforming of six agricultural biomass waste samples as well as the three main components of biomass was investigated in a two stage fixed bed reactor. Pyrolysis of the biomass took place in the first stage followed by catalytic steam reforming of the evolved pyrolysis gases in the second stage catalytic reactor. The waste biomass samples were, rice husk, coconut shell, sugarcane bagasse, palm kernel shell, cotton stalk and wheat straw and the biomass components were, cellulose, hemicellulose (xylan) and lignin. The catalyst used for steam reforming was a 10 wt.% nickel-based alumina catalyst (NiAl₂O₃). In addition, the thermal decomposition characteristics of the biomass wastes and biomass components were also determined using thermogravimetric analysis (TGA). The TGA results showed distinct peaks for the individual biomass components, which were also evident in the biomass waste samples reflecting the existence of the main biomass components in the biomass wastes. The results for the two-stage pyrolysis-catalytic steam reforming showed that introduction of steam and catalyst into the pyrolysis-catalytic steam reforming process significantly increased gas yield and syngas production notably hydrogen. For instance, hydrogen composition increased from 6.62 to 25.35 mmol g^?1 by introducing steam and catalyst into the pyrolysis-catalytic steam reforming of palm kernel shell. Lignin produced the most hydrogen compared to cellulose and hemicellulose at 25.25 mmol g^?1. The highest residual char production was observed with lignin which produced about 45 wt.% char, more than twice that of cellulose and hemicellulose.     

Integrated process of coal pyrolysis and CO2 reforming of methane with and without using dielectric barrier discharge plasma

The integrated processes of Shenmu subbituminous coal pyrolysis and CO₂ reforming of methane over catalyst (Ni/SiO₂) with and without using dielectric barrier discharge plasma (ICCP and ICCC) were carried out to check the effectiveness of the integrated process on improving the tar yield of coal pyrolysis. The effects of the pyrolysis temperature and time on product yields were investigated. The results indicate that both the ICCC and ICCP have an effect on increasing the tar yield compared with coal pyrolysis under N₂ or H₂. The tar yield increases with the increase of pyrolysis temperature and time in the ICCC, while relatively lower pyrolysis temperature and shorter pyrolysis time is preferable in the ICCP. The highest tar yield is 24.8 wt% at 600°C for 22 min in the ICCC and that is 23.7 wt% at 500°C for 7 min in the ICCP.     

Steam reforming of gasification-derived tar for syngas production

In this study, the steam reforming of tar was catalyzed by dolomite, Ni/dolomite, and Ni/CeO₂ for syngas production under different reaction temperature and weight hourly space velocity (WHSV, h⁻¹). The tar was the major side product from the biomass gasification.     

Hydrogen production from partial oxidation of methane over dielectric barrier discharge plasma and NiO/γ-Al2O3 catalyst

Hydrogen production from partial oxidation of methane under the combination of dielectric barrier discharge (DBD) plasma and NiO/γ-Al₂O₃ catalyst with cordierite honeycomb monoliths as substrate was investigated. The results showed that obvious synergistic effect was generated between DBD plasma and catalyst. Compared with the DBD plasma reactor without catalyst, the CH₄ conversion and H₂ yield increased from 60.1% and 21.3% to 83.6% and 28.4%, respectively. When the discharge power is above 70 W, the combination of DBD plasma and NiO/γ-Al₂O₃ catalyst promotes partial oxidation of methane. The catalyst was characterized by X-ray diffraction (XRD). NiO on the surface of catalyst was reduced to Ni because of the introduction of DBD plasma. The activity of catalyst at low temperature was improved, and the generation of oiliness by-products was significantly reduced.     

Production of renewable hydrogen by steam reforming of tar from biomass pyrolysis over supported Co catalysts

Co catalyst supported on BaAl₁₂O₁₉ (BA) showed higher activity in the steam reforming of tar from the pyrolysis of biomass than those supported on Al₂O₃, ZrO₂, SiO₂, MgO, and TiO₂. Characterization results indicate that the Co metal particles supported on BA had high dispersion, although the surface area of Co/BA was small. High dispersion of Co metal particles on BA can account for the high steam reforming activity, and this high dispersion is related to the strong basicity of the BA surface. Strong basicity of BA and high dispersion of Co metal particles on BA are connected to high H₂O reactivity to form H₂, probably at the interface between Co metal and BA. In addition, the Co/BA catalyst exhibited higher reusability through the coke combustion and the subsequent reduction treatment than the Co/Al₂O₃ catalyst. This is attributed to the suppression of the solid reaction between the oxidized Co and BA.     

High hydrogen content syngas for biofuels production from biomass air gasification: Experimental evaluation of a char-catalyzed steam reforming unit

This work investigates the performance of a reformer reactor for the upgrading of syngas and char derived from a pilot-scale air gasifier. The proposed setup represents a circular approach for the production of hydrogen-rich syngas from air gasification. Specifically, the reforming-unit was operated under a range of temperatures (from 700 °C to 850 °C) and steam flow rates and for each the improvement in producer gas composition and reducing species yield is evaluated. The results highlight that an increase in hydrogen concentration is obtained at higher temperature, moving from 16.2% to 21.3%, without using steam, and to 45.6%, with steam injection on the char-bed, while CO concentration did not follow a monotonic behavior. Moreover, the gas quality index, defined as a ratio between reducing species and inert species, delivered the highest values at the highest temperatures and steam flow rates. These results provide a guide for future gas quality optimization studies.     

Liquid phase reforming of woody biomass to hydrogen

This work concentrates on the production of H₂ directly from raw biomass through liquid phase reforming in the presence of a liquid base and a solid catalyst. Both precious metal and base metal catalysts were found to be active for the liquid phase hydrolysis and reforming of wood. Pt-based catalysts, particularly Pt–Re, were shown by atomistic modeling to be more selective toward breaking C–C bonds, resulting in a higher selectivity to hydrogen versus methane. Ni-based catalysts were found to prefer breaking C–O bonds, favoring the production of methane. The results showed that at a constant wood concentration, increasing the concentration of base (base to wood ratio) in the presence of Raney Ni catalysts resulted in greater selectivity toward hydrogen. The amount of wood converted to gas was lower due to increased production of undesirable organic acids from the wood at higher base concentrations. It was shown that by modifying Ni-based catalysts with dopants, it was possible to reduce the base concentration while maintaining the selectivity toward hydrogen and increasing wood conversion to gas versus organic acids.     

Plasma synthesis of ammonia in a tangled wire dielectric barrier discharge reactor: Effect of electrode materials

Plasma synthesis of NH₃ from N₂ and renewable H₂ under mild conditions is very attractive for decentralised sustainable green ammonia production using intermittent renewables. In this study, NH₃ synthesis was performed under ambient conditions in a dielectric barrier discharge (DBD) plasma reactor. Different tangled wire internal electrodes were employed to understand the influence of electrode materials on plasma ammonia synthesis. Compared with a rod electrode, a tangled wire electrode substantially enhanced the NH₃ concentration and reduced the energy cost for ammonia production, which can be attributed to the expanded surface area and the chemisorption properties of the tangled electrodes. The influence of the N₂/H₂ molar ratio and total flow rate on the reaction performance was also evaluated. The lowest energy cost (59.0 MJ mol⁻¹) for ammonia production was achieved using a Cu tangled electrode at a total flow rate of 250 ml min⁻¹ and a discharge power of 20 W. The electrical diagnostics of the plasma process showed that the tangled wire electrodes decreased the breakdown voltage of the DBD and enhanced charge deposition, which enhanced the NH₃ production. The reaction mechanism was discussed for the process optimisation of ammonia synthesis in a tangled wire DBD system.     

A novel process simulation model for hydrogen production via reforming of biomass gasification tar

Process modeling and simulation are very important for new designs and estimation of operating variables. This study describes a new process for the production of hydrogen from lignocellulosic biomass gasification tars. The main focus of this research is to increase hydrogen production and improve the overall energy efficiency of the process. In this study, Aspen HYSYS software was used for simulation. The integration structure presented in this research includes sections like tar reforming and ash separation (Ash), combined heat and power cycle (CHP), hydrogen sulfide removal unit (HRU), water-gas shift (WGS) reactor, and gas compression as well as hydrogen separation from a mixture of gases in pressure swing adsorption (PSA). It was found that the addition of CHP cycle and the use of the plug flow reactor (PFR) model, firstly, increased the overall energy efficiency of the process by 63% compared to 29.2% of the base process. Secondly it increased the amount of hydrogen production by 0.518 kmol (H2)/kmol Tar as compared with 0.475 of the base process. Process analysis also demonstrated that the integrated process of hydrogen production from biomass gasification tars is carbon neutral.     

An alternative atmospheric-pressure cold plasma method for synthesizing Pd/P25 catalysts with the assistance of ethanol

Atmospheric-pressure (AP) cold plasma has experienced a growing interest recently due to its simplicity and high efficiency for preparing supported metal catalysts by using the active hydrogen species generated from explosive H₂ gas. In this communication, we report an alternative AP cold plasma method for synthesizing supported Pd catalysts using Ar gas and ethanol instead of explosive H₂ gas and toxic reducing agents. The TOF value of the prepared Pd/P25 catalyst at 120 °C is 3.1 times of that prepared by cold plasma using H₂ gas due to the protection of the formed carbon species. AP cold plasma using ethanol as the origin of active hydrogen species is safer and more efficient than that using H₂ gas, and may also have great potentials to synthesize other supported metal catalysts.     

Hydrogen production from ammonia by the plasma membrane reactor

A new plasma membrane reactor (PMR) was developed to efficiently produce hydrogen from NH₃ with the use of atmospheric pressure plasma and a hydrogen separation membrane. The generation of high-purity hydrogen from NH₃ was also examined. First, hydrogen gas flowing into the PMR revealed the effect of the PMR on hydrogen separation. Hydrogen separation depends on the partial pressure of hydrogen gas supplied (P_{in, H2}) and permeated (P_{out, H2}) when P_{in, H2}^0.5 − P_{out, H2}^0.5 > 0. Second, NH₃ gas flowing into the PMR revealed its hydrogen production characteristics: the maximum hydrogen conversion rate of a typical plasma reactor (PR) is 13%, whereas the PMR converted 24.4%. Hydrogen obtained by hydrogen separation was approximately 100% pure. A hydrogen generation rate of 20 mL/min was stably obtained.     

Kinetic study of the catalytic reforming of biomass pyrolysis volatiles over a commercial Ni/Al2O3 catalyst

An original kinetic model has been proposed for the reforming of the volatiles derived from biomass fast pyrolysis over a commercial Ni/Al₂O₃ catalyst. The pyrolysis-reforming strategy consists of two in-line steps. The pyrolysis step is performed in a conical spouted bed reactor (CSBR) at 500 °C, and the catalytic steam reforming of the volatiles has been carried out in-line in a fluidized bed reactor. The reforming conditions are as follows: 600, 650 and 700 °C; catalyst mass, 0, 1.6, 3.1, 6.3, 9.4 and 12.5 g; steam/biomass ratio, 4, and; time on stream, up to 120 min. The integration of the kinetic equations has been carried out using a code developed in Matlab. The reaction scheme takes into account the individual steps of steam reforming of bio-oil oxygenated compounds, CH₄ and C₂-C₄ hydrocarbons, and the WGS reaction. Moreover, a kinetic equation for deactivation has been derived, in which the bio-oil oxygenated compounds have been considered as the main coke precursors. The kinetic model allows quantifying the effect reforming conditions (temperature, catalyst mass and time on stream) have on product distribution.