Steam gasification of Greek lignite and its chars by co-feeding CO2 toward syngas production with an adjustable H2/CO ratio

Low-rank lignite is among the most abundant and cheap fossil fuels, linked, however, to serious environmental implications when employed as feedstock in conventional thermoelectric power plants. Hence, toward a low-carbon energy transition, the role of coal in world's energy mix should be reconsidered. In this regard, coal gasification for synthesis gas generation and consequently through its upgrade to a variety of value-added chemicals and fuels constitutes a promising alternative. Herein, we thoroughly explored for a first time the steam gasification reactivity of Greek Lignite (LG) and its derived chars obtained by raw LG thermal treatment at 300, 500 and 800 °C. Moreover, the impact of CO₂ addition on H₂O gasifying agent mixtures was also investigated. Both the pristine and char samples were fully characterized by various physicochemical techniques to gain insight into possible structure-gasification relationships. The highest syngas yield was obtained for chars derived after LG thermal treatment at 800 °C, due mainly to their high content in fixed carbon, improved textural properties and high alkali index. Steam gasification of lignite and char samples led to H₂-rich syngas mixtures with a H₂/CO ratio of approximately 3.8. However, upon co-feeding CO₂ and H₂O, the H₂/CO ratio can be suitably adjusted for several potential downstream processes.     

Characterization of the physicochemical and structural evolution of biomass particles during combined pyrolysis and CO2 gasification

The combination of pyrolysis and CO₂ gasification was studied to synergistically improve the syngas yield and biochar quality. The subsequent 60-min CO₂ gasification at 800 °C after pyrolysis increased the syngas yield from 23.4% to 40.7% while decreasing the yields of biochar and bio-oil from 27.3% to 17.1% and from 49.3% to 42.2%, respectively. The BET area of the biochar obtained by the subsequent 60-min CO₂ gasification at 800 °C was 384.5 m²/g, compared to 6.8 m²/g for the biochar obtained by the 60-min pyrolysis at 800 °C, and 1.4 m²/g for the raw biomass. The biochar obtained above 500 °C was virtually amorphous.     

Effect of pore structure on CO2 gasification reactivity of biomass chars under high-temperature pyrolysis

The CO₂ gasification reactivity of pine sawdust chars (PS char) obtained from the different high-temperature pyrolysis is studied based on non-isothermal thermogravimetric method. Results show that the order of gasification reactivity is PS char-1073 > PS char-1273 > PS char-1473. Under the effect of high-temperature pyrolysis, the surface structure of biomass char is gradually destroyed and the pore structure parameters of specific surface area, total pore volume and average pore diameter increase. By means of the N₂ adsorption-desorption isotherms, it is seen that biomass char has more micro- and mesoporous at higher pyrolysis temperature. Besides, the PS char-1073 mostly has rich closed cylinder pores and parallel plate pores, and the PS char-1273 and PS char-1473 have plentiful open cylinder pores and parallel plate pores. An increase of pyrolysis temperature contributes to the development of porosity and improves diffusion path, which promotes the gasification reactivity. But, its effect on the decline of active site hinders the gasification reactivity. What's more, the kinetic model of distributed activation energy model (DAEM) is applied to calculate activation energy and pre-exponential factor with the integral and differential methods. The calculation results of integral method is more accurate and precise because the differential method is more sensitive than integral method for experimental noise. There is a compensation effect in the CO₂ gasification process.     

Syngas production by integrating CO2 partial gasification of pine sawdust and methane pyrolysis over the gasification residue

The aim of this work was to study syngas production by integrating CO₂ partial gasification (for CO production) of pine sawdust (PS) and methane pyrolysis (for H₂ production) over the gasification residue. Effect of the gasification conditions (including CO₂ flow rate, reaction temperature, mass ratio of PS:Ni and reaction time) was investigated on properties of the gasification residue. Besides CO-rich gas released from the gasification process with CO₂ conversion up to about 92%, the gasification residue could serve as robust catalyst for H₂ production by methane pyrolysis. Thanks to the nickel crystallites formed with high reduction degree and high dispersion on the surface after the gasification process, the gasification residue was competent for high and stable methane conversion (about 91%) at 850 °C. In addition to the flexible syngas output (in theory, with an arbitrary ratio of H₂/CO), valuable filamentous carbons can be achieved by regulating the process parameters.     

Kinetics of rice husk char gasification in an H2O or a CO2 atmosphere

Rice husk gasification has been attracting increasing attention in rice-producing countries, but the technology has not yet achieved optimal efficiency. Only a few studies have reported on the gasification kinetics of rice husk char, and the influence of some important parameters has not yet been investigated. This paper provides an experimental database and kinetic models of gasification of a rice husk char particle in an H₂O or a CO₂ atmosphere. A complete parametric study of rice husk char gasification was performed in a wide range of operating conditions, relevant to those that exist in industrial gasifiers. Two kinetic models were developed to predict the conversion of a particle, taking into account changes in the reactive surface. Results of this study could help researchers and engineers in the design, modeling or optimization of a new efficient rice husk gasifier.     

Effect of different gasifying agents (steam,H2O2, oxygen,CO2, and air) on gasification parameters

Two sensitivity analyses were performed in an Aspen simulation of fluidized bed gasification for five different gasifying agents such as steam, hydrogen peroxide (H₂O₂), pure oxygen (O₂), carbon dioxide (CO₂), and air. In the first sensitivity analysis, the modified equivalence ratio (MER) was varied (0.22-0.36). For the varied modified equivalence ratio (MER), %hydrogen, H₂/CO molar ratio, and hydrogen yield were the highest in steam-gasification, but %carbon monoxide, %methane, CO yield, and the lower heating values (LHV) were the highest in CO₂-gasification. In the second sensitivity analysis, the freeboard temperature was varied (500-900 °C). With increasing freeboard temperature, %hydrogen and %carbon monoxide increased while %carbon dioxide and %methane decreased for all the gasifying agents. Also, with increasing freeboard temperature, the LHV decreased and the hydrogen yield, CO yield, and the gas production rate increased for all the gasifying agents, but the H₂/CO molar ratio increased only in oxygen, air, and CO₂-gasification.     

Study of the flue gas characteristics and gasification reaction of pulverized coal combustion in O2/CO2/H2O atmosphere

The impacts of O₂ and H₂O on the combustion characteristics of pulverized coal in O₂/CO₂/H₂O atmosphere were studied. The gasification reaction ratio was calculated from the components of flue gas. The competition exists between C-CO₂ and C-H₂O reactions under rich CO₂ atmosphere. At various H₂O concentrations, the differences were found in generation amounts of CO and H₂ in flue gas. At O₂ concentrations <10%, C-CO₂ reaction decreased while C-H₂O reaction increased with increasing H₂O concentration; while at O₂ concentration >30%, H₂O showed no obvious specific patterns of effects. The gasification ratio was reduced as coal ranks increase.     

Production of H2- and CO-rich syngas from the CO2 gasification of cow manure over (Sr/Mg)-promoted-Ni/Al2O3 catalysts

The valorization of cow manure (CM), as bio-waste, under a CO₂ atmosphere could be an attractive strategy for tackling the environmental problems related to waste management and CO₂ emission and producing valuable syngas. For this purpose, highly loaded Ni–Al₂O₃ catalysts with alkaline-earth metals (Mg and Sr) were synthesized and applied to the gasification of CM under CO₂. The lowest yields of bio-oil (16.98 wt %) and coke (0.34 wt %) and the highest yield of syngas (55.09 wt %) were obtained from the catalytic decomposition of hydrocarbons when Sr was incorporated into Ni/Al₂O₃ (SN-AO). The highest selectivity for H₂ (34.23 vol %) and CO (37.16 vol %) were obtained applying SN-AO followed by Mg-promoted Ni/Al₂O₃ (MN-AO) and Ni/Al₂O₃ (N-AO) catalysts. With increasing gasification temperature from 750 °C to 850 °C, the syngas yield (from 55.09 to 70.17 wt %) and H₂ concentration (from 34.23 to 38.03 vol %) increased considerably because of the endothermic gasification process. The yield and selectivity of syngas (H₂ and CO) increased under CO₂ compared to those obtained under N₂, indicating the high potential of CO₂ for the thermal decomposition and dehydrogenation of the volatile matter.     

Effects of CO addition on laminar flame characteristics and chemical reactions of H2 and CH4 in oxy-fuel (O2/CO2) atmosphere

An experimental study is conducted to investigate the effect of CO addition on the laminar flame characteristics of H₂ and CH₄ flames in a constant-volume combustion system. In addition, one-dimensional laminar premixed flame propagation processes at the same conditions are simulated with the update mechanisms. Results show that all mechanisms could well predict the laminar flame speeds of CH₄/CO/O₂/CO₂ mixtures, when Z_CO is large. For mixtures with lower CO, the experimental laminar flame speeds are always smaller than the calculated ones with Han mechanism. For mixtures with larger or smaller Z_CO2, GRI 3.0, San diego and USC 2.0 mechanisms all overvalue or undervalue the laminar flame speeds. When CO ratio in the CH₄/CO blended fuels increases, laminar flame speed firstly increases and then decreases for the CH₄/CO/O₂/CO₂ mixtures. For H₂/CO/O₂/CO₂ mixtures, San diego, Davis and Li mechanisms all undervalue the laminar flame speeds of H₂/CO/CO₂/CO₂ mixtures. Existing models could not well predict the nonlinear trend of the laminar flame speeds, due to complex chemical effects of CO on CH₄/CO or H₂/CO flames. Then, the detailed thermal, kinetic and diffusive effects of CO addition on the laminar flame speeds are discussed. Kinetic sensitivity coefficient is far larger than thermal and diffusive ones and this indicates CO addition influences laminar flame speeds mainly by the kinetic effect. Based on this, radical pool and sensitivity analysis are conducted for CH₄/CO/O₂/CO₂ and H₂/CO/O₂/CO₂ mixtures. For CH₄/CO/O₂/CO₂ mixtures, elementary reaction R38H + O₂ ↔ O + OH and R99 OH + CO ↔ H + CO₂ are the most important branching reactions with positive sensitivity coefficients when CO ratio is relative low. As CO content increases in the CH₄/CO blended fuel, the oxidation of CO plays a more and more important role. When CO ratio is larger than 0.9, the importance of R99 OH + CO ↔ H + CO₂ is far larger than that of R38H + O₂ ↔ O + OH. The oxidation of CO dominates the combustion process of CH₄/CO/O₂/CO₂ mixtures. For H₂/CO/O₂/CO₂ mixtures, the most important elementary reaction with positive and negative sensitivity coefficients are R29 CO + OH ↔ CO₂ + H and R13H + O₂(+M) ↔ HO₂(+M) respectively. The sensitivity coefficient of R29 CO + OH ↔ CO₂ + H is increasing and then decreasing with the addition of CO in the mixture. Chemical kinetic analysis shows that the chemical effect of CO on the laminar flame propagation of CH₄/CO/O₂/CO₂ and H₂/CO/O₂/CO₂ mixtures could be divided into two stages and the critical CO mole fraction is 0.9.     

Identifying the roles of MFe2O4 (M=Cu,Ba, Ni,and Co) in the chemical looping reforming of char,pyrolysis gas and tar resulting from biomass pyrolysis

Biomass chemical looping gasification (BCLG), which employs oxygen carriers (OCs) as the gasification agent, is drawing more attention for its low cost and environmental friendliness. However, the complex products of biomass pyrolysis and the reactions between OCs and the pyrolysis products constrain its development. In this study, MFe₂O₄ (M = Cu, Ba, Ni and Co) ferrites synthesized via the sol-gel method were investigated as OCs in BCLG for hydrogen-rich syngas production. The properties of the as-prepared and spent OCs were characterized by X-ray diffraction (XRD), H₂-temperature programmed reduction (TPR), scanning electron microscopy (SEM), and automatic surface area porosimetry (BET). The three-phase products (char, pyrolysis gas and toluene) derived from biomass pyrolysis were employed as the reactants to investigate the reactivity of the ferrites. Then, BCLG experiments using biomass were conducted on the four ferrites to further determine their performance. The characterization results suggested that the four ferrites are all attractive for the chemical looping process, exhibiting good oxygen transferability and wide distributions of metal cations because of their metal synergistic effects in the spine structure. Reactions with pyrolysis gas and biomass char indicated that BaFe₂O₄ has a higher reactivity via a solid-solid reaction but a lower reactivity with pyrolysis gas, which make it very favorable for the production of hydrogen-rich syngas. Furthermore, BaFe₂O₄ showed excellent performance for toluene catalytic cracking with small amounts of carbon deposition. The synergetic effects between Ba and Fe metals considerably enhanced selective oxidation to produce 26.72% more H₂ than CoFe₂O₄ and 13.79% more H₂ than NiFe₂O₄ and CuFe₂O₄ for biomass gasification. The hydrogen yield produced by BaFe₂O₄ with the assistance of steam for biomass gasification can reach 41.8 mol/kg of biomass.     

Numerical analysis on influence of hydrogen peroxide (H2O2) addition on the combustion and emissions characteristics of NH3/CH4-air (O2/N2)/ H2O2 mixture

In this work, extensive chemical kinetic modeling is performed to analyze the combustion and emissions characteristics of premixed NH₃/CH₄–O₂/N₂/H₂O₂ mixtures at different replacement percentages of air with hydrogen peroxide (H₂O₂). This work is comprehensively discusses the ignition delay time, flame speed, heat release rate, and NOx & CO emissions of premixed NH₃/CH₄–O₂/N₂/H₂O₂ mixtures. Important intermediate crucial radicals such as OH, HO₂, HCO, and HNO effect on the above-mentioned parameters is also discussed in detail. Furthermore, correlations were obtained for the laminar flame speed, NO, and CO emissions with important radicals such as OH, HO₂, HCO, and HNO. The replacement of air with H₂O₂ increases flame speed and decreases the ignition delay time of the mixture significantly. Also, increases the CO and NOx concentration in the products. The CO and NOx emissions can be controlled by regulating the H₂O₂ concentration and equivalence ratios. Air replacement with H₂O₂ enhances the reactions rate and concentration of intermediate radicals such as O/H, HO₂, and HCO in the mixture. These intermediate radicals closely govern the combustion chemistry of the NH₃/CH₄– O₂/N₂/H₂O₂ mixture. A linear correlation is observed between the flame speed and peak mole fraction of OH + HO₂ radicals, and 2nd degree polynomial correlation is observed for the peak mole fraction of NO and CO with HNO + OH and HCO + OH radicals, respectively.     

九水合硝酸铝和八水合氢氧化钡导热系数的实验研究

用热敏电阻作加热元件和测温元件,首次测定了适合用作相变储能材料的九水合硝酸铝、八水合氢氧化钡导热系数的测定在10℃~80℃温度范围的导热系数,其实验值的不准确度分别为2.8%和3.2%。同时,还报道了九水合硝酸铝、八水合氢氧化钡实验中观测到的熔点依次为71℃和76℃,这些测量值在1℃~2℃的误差范围内与文献值相吻合。     

Effect of acid treatment on the catalytic performance of CuO/Cryptomelane catalyst for CO preferential oxidation in H2-rich streams

The effect of acid treatment on the catalytic performance of CuO/Cryptomelane (CuO/CR) for CO preferential oxidation (CO-PROX) in H₂-rich streams has been investigated. The CR supports are synthesized via the sol-gel approach. The hydrochloric acid or water is used to treat the CR support, and the corresponding CuO/CR catalysts are prepared by an initial wet impregnation method. Compared with the pristine CuO/CR and water-treated CuO/CR_W catalysts, the acid-treated CuO/CR_H exhibits the best catalytic activity with almost 100% of CO conversion at 110 °C, which can be maintained at least 100 h. The characterization results show that acid treatment decreases the K⁺ content in the CuO/CR_H catalyst, which is conducive to the formation of more oxygen vacancies, thereby promoting the reducibility of CuO/CR_H. This is the main reason for the high catalytic activity of the acid-treated CuO/CR_H catalyst. Moreover, the abundant Brönsted acid sites on CuO/CR_H are favorable for the desorption of acidic product CO₂, which also could result in the significant promotion of the catalytic activity for CO-PROX. This study sheds a light on the importance of acid treatment for cryptomelane and provides an efficient catalyst for hydrogen purification.     

Effect of O2 and temperature on the catalytic performance of Ni/Al2O3 and Ni/MgAl2O4 for the dry reforming of methane (DRM)

Al₂O₃ and MgAl₂O₄ supported 10% (w/w) Ni catalysts having a dispersion of 1.5 and 2.0% are active for DRM at 600 and 750 °C. High temperature reduction of both the calcined catalysts resulted in metallic Ni being formed, suggesting strong support metal interactions. The CH₄ and CO₂ conversion during DRM are relatively constant with time-on-stream, and are higher for Ni/MgAl₂O₄ than Ni/Al₂O₃. Carbon-whiskers are also detected on both catalysts. O₂ co-feed of 2.6% (v/v) and increasing reaction temperature to 750 °C helped in decreasing the amount of carbon deposited, except for Ni/MgAl₂O₄ at 600 °C. Furthermore, higher conversions and H₂/CO ratios are achieved. It appears that on spent Ni/MgAl₂O₄ a different type of carbon species was formed, and this carbon species was difficult to remove by oxygen at 600 °C. Thus, co-feeding O₂, using an appropriate temperature, and choosing a suitable support can reduce the carbon present on the nickel catalysts during DRM.     

Stress corrosion cracking behaviour of 316L stainless steel exposed to CO2–H2S–Cl− environments in the absence and presence of methyldiethanolamine (MDEA)

Stress corrosion cracking (SCC) behaviour of 316L stainless steel in CO₂–H₂S–Cl^? environments with and without methyldiethanolamine (MDEA) was investigated by slow strain rate testing and scanning electron microscopy (SEM). The results show that elongation ratio, reduction in area ratio (RAR) and time to failure ratio (TTFR) of 316L stainless steel were low in CO₂–H₂S–Cl^? environments. The corresponding fractography exhibited flat brittle fracture with quasi-cleavage pattern, indicative of high SCC susceptibility. Hydrogen penetration and corrosion pits could be responsible for the high SCC susceptibility of 316L stainless steel in this condition. For the CO₂–H₂S–Cl^? environments in the presence of MDEA, 316L stainless steel possessed high ER, RAR and TTFR (nearly 100%). High SCC resistance of 316L stainless steel could be associated with MDEA induced removal of H₂S/CO₂ and absorption on the steel surface.     

Optimisation of operating parameters on the performance characteristics of a free piston engine linear generator fuelled by CNG–H2 blends using the response surface methodology (RSM)

The free piston engine linear generator (FPELG) is a simple engine structure with few components, making it a promising power generation system. However, because the engine works without a crankshaft, the handling of the piston motion control (PMC) is the main challenge influencing the stability and performance of FPELGs. In this article, the optimal operating parameters of FPELG for maximising engine performance and reducing exhaust gas emissions were studied. Moreover, the influence of adding hydrogen (H₂) to compressed natural gas (CNG) fuel on FPELG performance was investigated. The influence of operating parameters on in-cylinder pressure was also analysed. The single-piston FPELG fuelled by CNG blended with H₂ was used to run the experiments. The response surface methodology (RSM), including the central composite design (CCD), was used. Then, adequacy models were developed and verified by ANOVA. Three independent factors on seven responses were utilised for optimisation. Results showed that the optimal operating conditions of lambda, ignition velocity, and injection position were 0.96, 0.53 m/s, and ?14.9 mm, respectively. The best-predicted values were as follows: indicated mean effective pressure (IMEP) of 7.6 bar, in-cylinder pressure of 27.87 bar, combustion efficiency of 39.64%, CO of 9531.41 ppm, CO₂ of 2.4%, HC of 551.75 ppm, and NO_X of 113.737 ppm. Furthermore, results showed that the experimental data could be fitted well with the predicted quadratic model.