Two-stage Reactor Research Articles

Aromatic recovery from distillate oil of oily sludge through catalytic pyrolysis over Zn modified HZSM-5 zeolites

As a potential energy resource with high content of crude oil, hydrocarbon recovery from oily sludge has attracted intensive interests. In this paper, a catalytic pyrolysis experiment of oily sludge has been conducted using HZSM-5 and Zn loaded HZSM-5 zeolites in a two-stage tubular fixed-bed reactor to recover aromatics. The physicochemical properties of HZSM-5 zeolites before and after Zn loading were characterized by XRD, FTIR, N2-adsorption and NH3-TPD. The detailed compositions of oil product samples obtained from pyrolysis at differene residence time and over modified HZSM-5 were analyzed by gas chromatography-mass spectrometry (GC–MS). Results showed that the total aromatic hydrocarbons (TAH) yield increased from 48.7% to 92.2% as residence time changed from 1.0 s to 7.6 s, indicating longer contact time led to a much higher aromatics production. However, under 7.6 s, aromatics were more concentrated on polyaromatics with three or more benzene rings. By contrast, 3.8 s was regarded as the optimum residence time for a higher aromatic yield (84.8%) and the highest selectivity (67.4%) of naphthalenes yield. The catalytic effects of Zn modified HZSM-5 were investigated under 1.9 s. The incorporation of 3% Zn enhanced the TAH yield from 58.7% to 81.0%, as well as the naphthalenes yield (from 31.5% to 67.5%). The promotion of dehydrogenation and deoxygenation over Zn modified zeolite was confirmed by a remarkable increase of the H2 and CO2 yield. Coke depositon on the catalysts was further studied by thermogravimetric (TG) and X-ray photoelectron spectroscopy (XPS) analysis. The coke was determined as polyaromatics coke due to the polycondensation reaction over HZSM-5. Moreover, longer residence time promoted the formation of coke while the loading of Zn slightly reduced it.

Influence of silica-alumina support ratio on H2 production and catalyst carbon deposition from the Ni-catalytic pyrolysis/reforming of waste tyres.

The influence of catalyst support alumina–silica in terms of different Al2O3 to SiO2 mole ratios containing 20 wt.% Ni on the production of hydrogen and catalyst coke formation from the pyrolysis-catalysis of waste tyres is reported. A two-stage reactor system was used with pyrolysis of the tyres followed by catalytic reaction. There was only a small difference in the total gas yield and hydrogen yield by changing the Al2O3 to SiO2 mole ratios in the Ni-Al2O3/SiO2 catalyst. The 1:1 ratio of Al2O3:SiO2 ratio produced the highest gas yield of 27.3 wt.% and a hydrogen production of 14.0 mmol g-1tyre. Catalyst coke formation decreased from 19.0 to 13.0 wt.% as the Al2O3:SiO2 ratio was changed from 1:1 to 2:1, with more than 95% of the coke being filamentous-type carbon, a large proportion of which was multi-walled carbon nanotubes. Further experiments introduced steam to the second-stage reactor to investigate hydrogen production for the pyrolysis-catalytic steam reforming of the waste tyres using the 1:1 Al2O3/SiO2 nickel catalyst. The introduction of steam produced a marked increase in total gas yield from ~27 wt. % to ~58 wt.%; in addition, hydrogen production was increased to 34.5 mmol g-1 and there was a reduction in catalyst coke formation to 4.6 wt.%.

Combustion of solid fuel in a hybrid porous reactor

One of the most significant human-made methane emission sources is the municipal solid waste (MSW), deposited on sanitary landfills and open dumps [1,2]. Within this work, an alternative MSW treatment concept is presented, which could provide a relatively clean waste/biomass-to-energy transformation. The proposed procedure comprises of a combustion and a gasification (or pyrolysis) step, which are consecutively taking place in a two-stage hybrid porous reactor system. The core of the system are two packed bed reactors, in which solid fuel (waste or biomass) is mixed with inert ceramic particles of similar size.This paper overviews the initial experimental investigation of the combustion step of a hybrid mixture, composed of wood pellets (fuel) and alumina balls (inert ceramic particles) in a 250 mm-high batch reactor. The temperature profile along the reactor, the concentration of CO and the flame front propagation velocity were measured as a function of the ceramic particle size (11 and 20 mm), the inert-to-fuel mass ratio (0:1, 2:1, 3:1) and the airflow rate (30, 42, 60 l/min).Experiments indicate that an increase of the mass ratio of inert-to-fuel material and a decrease of the inert ceramic particles size lead to a decrease of the maximum temperature of the packed hybrid bed. Measured CO concentrations showed strong dependence on the inert ceramic particle size, i.e. the particle size reduction from 20 to 11 mm resulted in a significant reduction of CO-emission peaks. The maximum flame front propagation velocity of 0.2 mm/sec was detected for the airflow of 42 l/min, the particle size of 20 mm and the mass ratio of 3:1.

Biogas production from chicken manure pretreated with wood decay fungi Trametes versicolor and Pleurotus ostreatus

The concept of year-round biogas production to increase the capacity factor of anaerobic digestion (AD) plants in sugarcane biorefineries was investigated for the first time in semi-continuous feeding mode. To simulate the use of sugarcane vinasse during the sugarcane season and sugarcane filter cake (SFC) during the off-season period, a two-stage reactor system based on an acidogenic continuous stirred-tank reactor (1st stage) followed by solid–liquid separation and an upflow anaerobic sludge blanket (UASB) reactor (2nd stage) to convert the COD-rich liquid fraction into biogas was operated. Additionally, to optimize the biogas production from SFC, the effects of its thermo-chemical pre-treatment on AD were investigated in a parallel reactor set-up. The saponification effect provided by autoclaving the substrate with sodium hydroxide improved the hydrolysis/fermentation of SFC in the acidogenic reactor, which in turn resulted in a 28% higher volumetric methane production in the methanogenic reactor (p < 0.05). However, the methane yields observed during operation of the two-stage reactor system were markedly lower than previously found in biochemical methane potential tests using SFC. In this case, the feed-in with low suspended solids required by UASB reactors prevented the utilization of the non-hydrolyzed/fermented solid fraction of SFC (> 60% of the substrate’s methane potential). Nevertheless, the capacity factor of the AD plants in sugarcane biorefineries could be increased from 0.55 up to 0.69 when considering a 200 d a−1 sugarcane season (0.66–0.83 for a longer season of 240 d a−1), representing an increase of 25.7%. The average capacity factor for biogas combined heat and power and upgrading units of around 0.91 (8000 h a−1) would be reached if further developments could improve the solubilization of non-hydrolyzed/fermented solids or alternatively allow their direct use in the methanogenic reactor.

Parametric study of two-stage hydropyrolysis of lignocellulosic biomass for production of gaseous and light aromatic hydrocarbons

Non-catalytic hydropyrolysis of pinewood and its components was carried out using a two-stage reactor. The main aim of this work is to investigate the hydrodeoxygenation and hydrogenation of volatile matter in the post hydrocracking reactor for oriented production of gaseous and light aromatic hydrocarbons. A portion of volatile matter, which evolved from hemicellulose, neutral extractives and lignin below 275°C, was found to be thoroughly hydrodeoxygenated preventing the release of CO2 and CO. Increasing hydrocracking temperature from 600°C to 750°C and pressure from 1.0MPa to 5.0MPa strongly facilitated the hydrogenation reactions to target products. The summed yield of CH4 and C2H6 (dry biomass basis) reached up to 33.2% at a hydrocracking temperature of 750°C and 5.0MPa, with a concomitant 5.1% yield of BTX. All components in pinewood significantly contributed to the production of CH4 and BTX by hydropyrolysis, differing from the case of pyrolysis.

An integrated O/A two-stage packed-bed reactor (INT-PBR) for total nitrogen removal from low organic carbon wastewater

Total nitrogen (TN) removal from low organic carbon wastewater is challenging. Traditional heterotrophic denitrification requires additional organics as electron donors, potentially increasing the operational challenges and costs of wastewater treatment. Especially for decentralized domestic sewage in rural areas, it is essential to develop an economical and efficient integrated reactor to satisfy high TN removal demand. This study presents an integrated O/A two-stage packed-bed reactor (INT-PBR) composed of two concentric zones, with total effective volume of 30.6L. Influent is pumped into the inner oxic (O) zone for nitrification, then flows through the outer anoxic (A) zone for denitrification. Iron–carbon micro–electrolysis carriers (IC-MECs), which provide H2/[H] and Fe2+ as inorganic electron donors for autotrophic denitrifiers, and luffa cylindrical sponge (LCS), which provides slow–release organic carbon for heterotrophic denitrifiers, were placed into the A zone to achieve combined heterotrophic and autotrophic denitrification (HAD) without requiring organic carbon in influent. Thus, the INT-PBR involves no return sludge and internal recycle. Using organic-free synthetic wastewater with 39.36±0.74mg/L NH4+-N, approximately 98% of ammonia in influent was transferred to nitrate in the O zone, and approximately 93% of TN was removed by HAD process at 30±2°C in the A zone. Microbial community analysis showed that autotrophic denitrifiers (e.g., Thiobacillus) and heterotrophic/mixotrophic denitrifiers (e.g., Hydrogenophaga) were combined in the reactor, consistent with the performance findings. The results suggest a promising technology for total nitrogen removal from wastewater with low organic carbon.

Carbon and nutrient-limiting conditions stimulate biodegradation of low concentration of phenol

Biological removal of phenol in wastewater is kinetically constrained if its initial concentration is low (<5mg/L) making compliance of environmental norms (≤0.35mg/L) difficult. Taking a cue from the soil matrix where bio-remediation of very low concentrations phenol is possible under nitrogen (N), phosphorus (P) and carbon (C) limiting conditions, the present study was carried out to stimulate phenol biodegradation in a low-phenolic (<5mg/L) refinery wastewater. Experiments with nutrient limiting conditions showed that N-limited and P-supplemented conditions were necessary to stimulate phenol uptake if the initial concentration of phenol is <1mg/L or is between 2.5–5.8mg/L. Further studies revealed that instead of manipulating nutrients, enforcing the C-limited condition on the microorganisms through a two-staged reactor process, not only did the phenol removal efficiency improve but also eliminated the need to externally manipulate N and P concentrations in the system. The high abundance of hydrocarbon degrading gram-negative bacteria and Pseudomonas species in the second stage of the two-staged process, further confirmed that C-limitation was a better option than the nutrient manipulation. The study thus illustrates methods of acclimatizing microorganisms to remove very low concentration of target substrate (here phenol) in low strength wastewater.

Separate production of hydrogen and methane from biodiesel wastewater with added glycerin by two-stage anaerobic sequencing batch reactors (ASBR)

The objective of this study was to investigate hydrogen and methane production from biodiesel wastewater with added glycerin by using two-stage anaerobic sequencing batch reactors (ASBR) under 37 °C with a recycle ratio of the effluent from the methane ASBR unit-to-the feed flow rate of 1:1. Hydrogen ASBR unit was operated at a constant pH of 5.5 whereas the pH of methane ASBR unit was not controlled. Glycerin was added into biodiesel wastewater having 2330 mg/l COD to obtain a constant feed chemical oxygen demand (COD) of 45,000 m/l. At optimum COD loading rate 11.3 kg/m3d (based on feed COD load and methane ASBR volume), the system provided the highest overall hydrogen and methane production performance (75.15 ml H2/g glycerin removed) which was consistent with the maximum overall COD removal of 76.7% and the highest overall glycerin uptake of 90%. Most of added glycerin was converted to gaseous products with small amounts of organic acids and 1,3 propanediol. The low volumetric ratio of hydrogen ASBR unit-to-methane ASBR unit of 1:6 resulted in low hydrogen yields with high methane yields. Process performance of the two-stage ASBR system was limited by the inhibition of total volatile organic acids produced in both bioreactors.

Methane production and characteristics of the microbial community in a two-stage fixed-bed anaerobic reactor using molasses

Molasses is a typical feedstock for fermentation, but the effluent is hard to treat. In this study, molasses containing a high concentration of organic matter was treated by a two-stage Fix-bed reactor system with an increased organic loading rate (OLR). The results indicated at high molasses loading rate, the two-stage system was more efficient (i.e. organic matter removal, the COD of effluent and biogas production) than the single-stage system. The relative abundance of Anaerolineaceae and W5_norank was higher in the first stage (R1), where these organisms digest carbohydrates, while the second stage (R2) had higher relative abundance of Synergistaceae and SB-1_norank, which digest VFAs and decomposition-resistant compounds to produce compounds used by hydrogen methanogens. The qPCR analysis demonstrated that the Methanosaetaceae dominated the archaeal community in the first stage (R1), while Methanomicrobiales and Methanobacteriales were predominant in the second stage (R2), where they were involved in hydrogen production.

Experimental study on biomass (eucalyptus spp.) gasification in a two-stage downdraft reactor by using mixtures of air, saturated steam and oxygen as gasifying agents

In this work, biomass (eucalyptus spp.) gasification has been investigated in a two-stage downdraft reactor by using different gasifying agents such as: air, air+saturated steam, and O2+saturated steam. The utilization of low quality saturated steam for gasification in different mixtures with air and O2 is the main novelty of this paper. Thus, the influence of different gasifying agents was assessed on the concentrations of CO, H2 and CH4 and hence, on the lower heating value (LHV), the syngas power and the cold gasification efficiency. Since the type and flow of gasifying agent control the biomass consumption in downdraft gasifiers, the equivalence ratio (ER) and the steam-to-biomass ratio (SBR) were calculated by means of a mass balance. The results showed that low quality steam can increase remarkably the H2 concentration and the LHV of syngas despite its low temperature. Particularly, when O2+saturated steam is fed to the gasifier, the H2 concentration varied between 27.4 and 35.9 vol.%, while the LHV was between 7.13 and 8.35MJ/Nm3, i.e. almost twice the produced when only air or air+saturated steam is used. The optimum ER seems to be below 0.412, while the SBR is found between 0.495 and 1.032. Under this conditions, the cold gasification efficiency was between 70 and 75%. Moreover, the O2 seems to mitigate in a better way the negative effect of the low temperature of the saturated steam given the lower tar concentration in syngas (58.74–64.18mg/Nm3) respect to those found with air+saturated steam (91.41–91.95mg/Nm3). This paper concludes that saturated steam can be used blended either with air or O2 (being better with O2) for biomass gasification in two-stage downdraft reactors and therefore, the requirements of superheated steam are not so important as in other gasification processes.

Characteristics of Denitrification Inhibiting Sulfate Reducing Process

In this study, a set of two-stage UASB reactor was used to study the characteristics of denitrification inhibiting sulfate reduction in oil field gathering and transportation system and some high salt wastewater, and the characteristics of granular sludge and microbial characteristics were studied after the stable operation of the process. The results showed that the addition of NaNO2 increased the number of denitrifying bacteria (DNB) from 7.0×103 CFU·(100 mL)-1 to 7.3×105 CFU·(100 mL)-1 and remained stable in the process from start to mature. The competitive inhibition caused by DNB decreased the number of SRB from 8.0×105 CFU·(100 mL)-1 to 7.6×104 CFU·(100 mL)-1. Meanwhile, the sulfate reduction was inhibited, and the inhibition rate increased and finally stabilized to 82%. The study on mass ratio of microbial mass to nitrite effecting on denitrification efficiency showed that the inhibition rate of S2- was the highest, reaching 92%, when the mass ratio was 1200. The inhibition rate of the process could remain at about 92%, with good stability. The particle size and sedimentation rate of granular sludge were measured to determine whether the process conditions were favorable for the formation of granular sludge. The study showed that the denitrifying granular sludge formed was brown, basically ellipsoidal and spherical, with smooth surface and high density. Before the denitrification inhibition, the particle size of granular sludge was 1.0-1.4 mm and the average diameter was 1.17 mm, and after denitrification inhibition, the particle size distribution was 1.2-1.6 mm, the average particle size was 1.21 mm, which indicated that the denitrification inhibition increased the size of granular sludge. The average sedimentation velocity of denitrifying granular sludge was 47.6 m·h-1, which revealed the good settling performance of the granular sludge. The results of PCR-DGGE analysis showed that the denitrification inhibition reduced the number of microbial species from 18 to 14 and the number of dominant species decreased from 4 to 3, indicating the decreasing microbial diversity. The similarity of microbial population before and after nitrification was 62.6% and the population structure changed greatly from SRB to DNB. The number of dominant SRB species decreased from 4 to 2 after denitrification inhibition and the abundance of every species decreased obviously. The main functional bacterial species of the inhibition process was uncultured Sulfurimonas sp., which is a kind of autotrophic denitrifying bacteria. It dominated and competed with SRB for electrons, inhibiting the sulfate reduction and the production of sulfide.

Decomposition of benzene using wire-tube AC/DC discharge reactors

Decomposition of benzene was investigated using single and two-stage stainless steel reactors with application of high-voltage AC and DC currents. Benzene removal, CO, and CO2 were measured as a function of reactor diameter, applied voltage, and stage of reactor configuration. The results showed that, benzene removal and CO2 selectivity improved in two-stage configuration and reached up to 59% and 97.8% respectively, where the gap size was 8.5 mm. Formation of OH, N-H and N=O function groups were seen in the exhaust gas. Subsequently, organic acids, phenols, and alcohols were identified as the main byproduct of benzene decomposition considering GC-MS analysis.

Important role of volatile–char interactions in enhancing PM1 emission during the combustion of volatiles from biosolid

A three-stage pyrolysis/combustion reactor was used to demonstrate the importance of volatile–char interactions in inorganic particulate matter (PM) emission from the combustion of biosolid volatiles. It consists of a two-stage quartz reactor (including an inner drop-tube/fixed-bed pyrolyser as Stage I and an outer fixed-bed as Stage II) cascaded into a large drop-tube furnace (DTF, Stage III). The unique reactor design enables the volatiles that are produced in situ from the fast pyrolysis of cellulose, polyethylene or acid-washed biosolid in Stage I to pass through a preloaded bed of slow-pyrolysis biosolid char in Stage II then be immediately combusted (achieving complete combustion) in the DTF as Stage III at 1300°C. Limited by quartz maximum working temperature (in Stages I and II), two temperatures (800 or 1000°C) were considered for preparing the bed of char and generating the in situ volatiles. The results clearly show that volatile–char interactions lead to significant changes in the particle size distributions (PSDs) of PM emitted from the combustion of volatiles produced in situ from cellulose, polyethylene or acid-washed biosolid pyrolysis. The volatile–char interactions increase the yield of PM1 (i.e. PM with aerodynamic diameter<1µm), dominantly PM0.1 (i.e. PM with aerodynamic diameter <0.1µm). The results show that small non-oxygenated reactive species (especially H free radicals) in the fresh volatiles can react with the chars to enhance the release of alkalis (Na and K) as well as P and S in the chars. The released Na, K, P and S can react to form alkali metaphosphate and sulphate which subsequently form PM1 during volatiles combustion. It is also evident that volatile–char interactions convert some of Pb and Cr in the biosolid chars into volatile forms which are released and then contribute to PM1 emission.

Biomass Torrefaction in a Two-Stage Rotary Reactor: Modeling and Experimental Validation

A mechanistic model of torrefaction was developed for a two-stage rotary reactor, and it was verified with experimental results. Mass and energy balances for each phase are considered in the model. A kinetic model that considers the progressive decomposition of biomass into volatiles and char released simultaneously from the raw biomass was coupled to the balances. Mathematical expressions for residence time, heat transfer coefficient, and bed height inside the kiln were taken from the literature for model calculations. Release of condensable and noncondensable volatiles from biomass during the process was considered in the gas phase, while the solid phase included raw and torrefied biomass. The model can predict different output parameters of torrefaction in a rotary continuous torrefier, such as final amounts of solids, gas yields, and temperatures, for different operational conditions. Properties for torrefied solid, such as high heating value, fixed carbon, and volatile matter, can also be predicted b...

Product distribution and interactive mechanism during co-pyrolysis of a subbituminous coal and its direct liquefaction residue

Compared with individual pyrolysis of Shendong coal or the direct coal liquefaction residue, co-pyrolysis has an improved tar yield. However, the synergistic mechanism in the integrated process is still controversial. In order to explore the interactive effect in co-pyrolysis of coal and the direct coal liquefaction residue, elemental analysis, gas chromatography, Fourier – transform infrared spectrometer, thermogravimetric simulation distillation, nuclear magnetic resonance and gas chromatograph – mass spectrometer were employed to analyze the structure and chemical composition of products (gas, semi-coke, and tar) obtained in co-pyrolysis at 600°C. The results show that the actual product yields are different from the calculated values and elemental redistribution occurs in co-pyrolysis. Instruments characterization suggests an increased loss of aromatic hydrogen and carbon in co-pyrolysis, in comparison with individual pyrolysis. The increasing tar is mainly composed of heavy components with boiling points higher than 400°C. Co-pyrolysis favors the minimization of the dealkylation of phenolic species. Furthermore, the synergistic mechanism analysis using two-stage reactor indicates that the increase in tar yield should be attributed to the change in the free radical reaction during the pyrolysis of Shendong coal and the direct coal liquefaction residue. In co-pyrolysis, more radicals generated from coal pyrolysis are used to stabilize the radicals generated from the pyrolysis of tetrahydrofuran soluble fraction of the direct coal liquefaction residue.

Effect of Si/Al Ratios on the Structure and Catalytic Properties of Alkaline-Treated ZSM-5 Catalysts

A series of hierarchical HZSM-5 catalysts were synthesized by alkaline (denoted as HZSM-5-At) and/or alkaline-acid (denoted as HZSM-5-At-acid) treatments, and they as catalyst pairs were employed for the synthesis of glycerol and ammonia toward pyridine bases in a series-connected two-stage reactor. The characterization was analysed by means of N2-physorption and NH3-TPD techniques. The catalytic evaluation showed that the lower Si/Al ratio (Si/Al = 25 before treatment) in catalyst pairs (HZSM-5-At + ZnO/HZSM-5-At-acid) possessed the highest total yield of pyridine bases among all the catalyst pairs, but the suitable Si/Al ratio in alkaline-treated ZSM-5 (Si/Al = 38 before treatment) in the first stage of reactor and ZnO/HZSM-5-At-acid (Si/Al = 25 before treatment) in the second stage of reactor had the stronger stability relative to other catalyst pairs. The characterization revealed that the amount of acid sites play a vital role in the formation of pyridine bases whereas the appearance of mesoporous structure and the declined concentration of acid sites were main reasons for promoting the stability.

Iterative learning fault-tolerant control for differential time-delay batch processes in finite frequency domains

This paper develops a fault-tolerant iterative learning control law for a class of differential time-delay batch processes with actuator faults using the repetitive process setting. Once the dynamics are expressed in this setting, stability analysis and control law design makes use of the generalized Kalman–Yakubovich–Popov (KYP) lemma in the form of the corresponding linear matrix inequalities (LMIs). In particular, sufficient conditions for the existence of a fault-tolerant control law are developed together with design algorithms for the associated matrices. Under the action of this control law the ILC dynamics have a monotonicity property in terms of an error sequence formed from the difference between the supplied reference trajectory and the outputs produced. An extension to robust control against structured time-varying uncertainties is also developed. Finally, a simulation based case study on the model of a two-stage chemical reactor with delayed recycle is given to demonstrate the feasibility and effectiveness of the new designs.

Anaerobic treatment of wastewater using a two-stage packed-bed reactor containing polyvinyl alcohol gel beads as biofilm carrier

Anaerobic treatment of wastewater is a viable alternative to aerobic treatment with the advantage of low net energy requirement and less sludge production. Successful establishment of a two-stage anaerobic treatment process resulted in better performance and improved tolerance to perturbations in operating conditions such as high organic loading rate and higher methane production. In this study we have reported process development, microbial enrichment and validation of performance of a two-stage anaerobic packed-bed reactor comprising of an acidogenic reactor followed by a methanogenic reactor, each containing porous polyvinyl alcohol (PVA)-gel beads as biofilm carrier, harboring different consortia of microorganisms for treating wastewater at mesophilic temperature. The stage-separated reactor system demonstrated chemical oxygen demand (COD) removal efficiency up to 89% with overall methane productivity of 0.08L methane/(L reactor.d). During steady-state operation, the acidogenic reactor produced an effluent with concentration ratio of propionic acid to acetic acid as low as 0.2, which indicated the stability of the reactor system against environmental shock-related failure. During an event of deliberately imposed low-temperature shock period of 60days, the COD removal efficiency decreased from 89% at steady-state to as low as 33% with concomitant decline in methanogenic productivity. The predominance in microbial community changed from steady-state characteristics of Gram positive to Gram negative during the temperature shock. After the shock was withdrawn, the reactor system regained its original performance within 28days; the Gram positive bacteria also switched back to the predominance.

Absorption-enhanced steam gasification of biomass for hydrogen production: Effects of calcium-based absorbents and NiO-based catalysts on corn stalk pyrolysis-gasification

Effects of several calcium-based absorbents and NiO-based catalysts on absorption-enhanced pyrolysis-gasification of corn stalk for H2 production were investigated in a two-stage fixed-bed reactor. The results indicated that the addition of calcined CaCO3, calcined limestone, and calcined dolomite significantly improved the concentration and yield of H2 in the product gas because of in situ CO2 absorption and the catalytic effects of the CaO species. The magnesium species in calcined dolomite further catalyzed the gasification process and improved CO2 absorption to increase the concentration and yield of H2. NiO/CaO bi-functional catalysts/absorbents had a more significant catalytic effect on the gasification process than calcined dolomite: although the H2 concentration in the product gas was lower, the yields of H2, CO, and CO2 were higher. A large amount of CO2 generation which could not be sufficiently absorbed resulted in increased concentration and yield of CO2 in the product gas. The addition of the NiO/γ-Al2O3 catalyst together with calcined dolomite further increased the concentration and yield of H2 in the product gas. A product gas with a maximum H2 concentration of 85.1 vol.% and negligible amounts of CO2 and C2+ was obtained at a NiO loading of 15 wt.%.

Behaviors of coking and radicals during reaction of volatiles generated from fixed-bed pyrolysis of a lignite and a subbituminous coal

The reaction of volatiles generated from coal during pyrolysis determines the yield and quality of products. To understand this reaction in detail particularly on the condensation reaction that forms coke, this paper studies the reaction of volatiles generated from a lignite and a subbituminous coal using a two-stage fix-bed reactor, with the first stage for coal pyrolysis and the second stage for reaction of volatiles. The yields of tar, gas and coke formed on the wall and presented in tar at a second-stage temperature range of 440–700°C and a residence time range of 1.5–6.9s are quantified. The concentration, g value and linewidth of radicals in tars, mainly in coke presented in tars, are measured by electron spin resonance (ESR) and discussed to explore the transformation of coke in structure. It is found that the reactions of volatiles generated from the lignite and the subbituminous coal are different in the temperature range studied but the structure of coke formed in these two volatiles differs mainly at temperatures lower than 600°C. The proportion of coke formed on the reactor wall is small at 440°C, but large at 650 and 700°C, close to 50% in 4.2s. The increase in concentration and decreases in g value and linewidth of radicals with increasing temperature over time suggest that the coke undergoes condensation, leading to loss in oxygen and increase in aromaticity.