Yijun Zhao

Experimental and numerical study of biomass flash pyrolysis in an entrained flow reactor

Experiments on flash pyrolysis of biomass (rice husk and sawdust) were performed in an entrained flow reactor at different temperatures (700-1000 degrees C). Results show that temperature has great impact on the biomass flash pyrolysis reactions. With the increase of temperature, the gas yield increased, while the yields of char and liquid decreased. After the early stage of pyrolysis, the gas yield increased gradually which could be attributed to the increase of CO and CH(4) yields at low temperature, while to the increase of CO and H(2) yields at high temperature. A first order kinetic model was employed to describe biomass flash pyrolysis characteristics and the relevant kinetic parameters were determined. Numerical simulation of pyrolysis process was carried out, the predicted yields of produced gases agreed reasonably with the experimental data.

Characteristics of rice husk gasification in an entrained flow reactor

Experiments were performed in an entrained flow reactor to better understand the characteristics of biomass gasification. Rice husk was used in this study. Effects of the gasification temperature (700, 800, 900 and 1000 degrees C) and the equivalence ratio in the range of 0.22-0.34 on the biomass gasification and the axial gas distribution in the reactor were studied. The results showed that reactions of CnHm were less important in the gasification process except cracking reactions which occurred at higher temperature. In the oxidization zone, reactions between char and oxygen had a more prevailing role. The optimal gasification temperature of the rice husk could be above 900 degrees C, and the optimal value of ER was 0.25. The gasification process was finished in 1.42 s when the gasification temperature was above 800 degrees C. A first order kinetic model was developed for describing rice husk air gasification characteristics and the relevant kinetic parameters were determined.

Determination of the intrinsic reactivities for carbon dioxide gasification of rice husk chars through using random pore model.

Rice husk is abundantly available and environmentally friendly, and char-CO2 gasification is of great importance for the biomass gasification process. The intrinsic reaction rates of carbon dioxide gasification with rice husk chars derived from different pyrolysis temperatures were investigated in this study by conducting thermogravimetric analysis (TGA) measurements. The effects of gasification temperature and reactant partial pressure on the char-CO2 gasification were investigated and the random pore model (RPM) was used to determine the intrinsic kinetic parameters based on the experimental data. The results obtained from this study show that the activation energy, reaction order and pre-exponential factor varied in the ranges of 226.65-232.28kJ/mol, 0.288-0.346 and 2.38×10(5)-2.82×10(5)1/sPa(n) for the rice husk chars pyrolyzed at 700-900°C, respectively. All the determination coefficients between the RPM predictions and experimental results were higher than 0.906, indicating the RPM is reliable for determining and evaluating the intrinsic reactivities of rice husk chars.

Experimental study on cyclone air gasification of wood powder

In this paper, effects of the equivalence ratio (ER) and the secondary air on the gasification system were studied. The results indicate that as the ER varies in the range of 0.20-0.26, the low heating value (LHV) of the producer gas is in the range of 3.64-5.76 MJ/Nm(3), the carbon conversion is 55%-67% and the cold gas efficiency of the gasification system is 33%-47%. In contrast to the gasification without the secondary air, air staged process is a gasification method capable of increasing the LHV of the producer gas from 4.63 to 5.67 MJ/Nm(3), the carbon conversion from 65.5% to 81.2%, and the cold gas efficiency of the gasifier from 42.5% to 56.87%, while the tar content of the producer gas decreases from 13.96 to 5.6g/Nm(3). There exists an optimum ratio of the secondary air.

Effects of volatile-char interactions on char during pyrolysis of rice husk at mild temperatures.

In order to understand the sensitivity of volatile-char interactions to mild temperatures (600-800°C), in-situ rice husk char was prepared from fast pyrolysis (>10(3)Ks(-1)) on a fixed-bed reactor. Retention of K in char, changes in char structure and char reactivity were determined. The results showed that volatile-char interactions did not cause obvious effect on the char yield but showed an inhibitory effect on char reactivity. The inhibition began only above 650°C and intensified with temperature rise, but kept almost unchanged at 700-800°C. Char structure and retention of K have a combined effect on char reactivity. The decreased reactivity was caused by additional volatilization of K from char matrix and transformation of relatively smaller aromatic ring systems to large ring systems (>6 benzene rings) above 650°C.

Effects of Water Molecule on CO Oxidation by OH: Reaction Pathways, Kinetic Barriers, and Rate Constants

The water dilute oxy-fuel combustion is a clean combustion technology for near-zero emission power; and the presence of water molecule could have both kinetic and dynamic effects on combustion reactions. The reaction OH + CO → CO2 + H, one of the most important elementary reactions, has been investigated by extensive electronic structure calculations. And the effects of a single water molecule on CO oxidation have been studied by considering the preformed OH(H2O) complex reacts with CO. The results show little change in the reaction pathways, but the additional water molecule actually increases the vibrationally adiabatic energy barriers (VaG). Further thermal rate constant calculations in the temperature range of 200 to 2000 K demonstrate that the total low-pressure limit rate constant for the water assisted OH(H2O) + CO → CO2 + H2O + H reaction is 1-2 orders lower than that of the water unassisted one, which is consistent with the change of VaG. Therefore, the hydrated radical OH(H2O) would actually slow down the oxidation of CO. Meanwhile, comparisons show that the M06-2X/aug-cc-pVDZ method gives a much better estimation in energy and thus is recommended to be employed for direct dynamics simulations.

Energy and exergy characteristics of syngas produced from air gasification of walnut sawdust in an entrained flow reactor

Energy and exergy are two important tools for evaluating energy sources. The energy and exergy of syngas produced from air gasification of walnut sawdust in an entrained flow reactor were detailed for typical reactor temperatures (700, 800, and 900°C) and equivalence ratios (0.22, 0.25, 0.28, 0.31, and 0.34). The results showed that the overall energy distribution was CO > (CH4, H2, N2, C2H4) > CO2 whereas the overall exergy distribution was CO > (CH4, H2) > C2H4 > N2 > CO2. The results that different component gases contributed differently to the energy and exergy of syngas were due to the fact that different component gases had different exergy/energy ratios. However, both the energy and exergy of syngas achieved the highest values at the same reactor temperature and equivalence ratio. The results obtained from this study can help understand the energy and exergy of syngas produced from biomass gasification as well as the effects of reactor temperature, equivalence ratio (ER), and fuel characteristics on the energy and exergy of syngas.

Experimental study of cyclone pyrolysis – Suspended combustion air gasification of biomass

Based on the original biomass cyclone gasifier, the cyclone pyrolysis-suspension combustion gasification technology was constituted with a bottom wind ring to build the biochar suspension combustion zone. This technology decouples the biomass pyrolysis, gasification (reduction reaction) and combustion (oxidation reaction) within the same device. With the feed amount and total air fixed, the effect of air rate arrangement on temperature distribution of the gasifier, syngas components and gasification parameters was studied. With the secondary air rate (0.20) and bottom air rate (0.50), the gasification efficiency was best, with gas heating value of 5.15MJ/Nm3, carbon conversion rate of 71.50%, gasification efficiency of 50.80% and syngas yield of 1.29Nm3/kg. The device with biochar for the tar catalytic cracking was installed at the gasifier outlet, effectively reducing the tar content in syngas, with a minimum value of 1.02g/Nm3.

Use of a Process Mass Spectrometer to Measure Rapid Changes of Gas Concentration

Abstract Experiments were conducted to investigate the suitability of a multistage in-situ reaction analyzer based on a micro fluidized bed (MFB-MIRA) for measuring rapid changes of gas concentration during gas-solid reactions. The results show that control of capillary temperature of a process mass spectrometer strongly influences the stability of on-line measurements. Based on observed regular patterns, the capillary temperature-control system was equipped with a precision controller that enabled a precision of ±0.2 °C to be achieved, and thereby guaranteed high stabilities of the sampling flow rate and chamber vacuum. Measurements using the modified gas-monitoring system showed that periodic fluctuations of the on-line measurement were eliminated and measurement stability significantly improved. The fluctuating range and relative standard deviation of the measured response to O 2 in air improved from 1.9% to 1.4% and 0.54% to 0.18%, respectively. A pressure-regulating device was also developed to control the absolute pressure at the gas sampling point. This achieved a control precision of ±0.02 kPa. The measured results showed that the response of the process mass spectrometer correlated positively with the absolute pressure at the sampling point, indicating the necessity for a pressure-regulating device. The accuracy and repeatability of the process mass spectrometer were improved. This work has enhanced the suitability of MFB-MIRA for studying rapid gas-solid reactions and broadened the scope of reliable applications of MFB-MIRA combined with a process mass spectrometer.

Process Simulation and Energy Analysis of Oxy-Coal Combustion Steam Systems (OCCSS) With Near-Zero Emissions Based on Coal Gasification

Oxy-coal combustion steam system with near-zero emissions (OCCSS) is a new type of power generation system, which is able to satisfy the demands for high efficiency utilization of coal and control of CO2 emissions. It is necessary to study the characteristics of the new system, since both the combustion mode and the working medium are different to the power systems utilizing conventional combustion. In the present paper, we report the process simulation of a conceptual OCCSS based on coal gasification, as well as comparisons with a conventional combustion and an oxy-fuel combustion. The model of the system was built and parametric studies showed the pressure of burners and ratios of fuel have effects on the performance of the system. The net efficiency of power generation increases quickly with the increasing pressure of the gas generator followed by a slower decrease. The net efficiency of the system increases with the increasing fuel ratio to a burner which is set before any burners with decreasing ratio. Otherwise, it decreases. The energy analysis shows the net efficiency of OCCSS is higher than the conventional combustion system and the oxy-fuel combustion system.Copyright