Shaozeng Sun

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.

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.

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.

Numerical Simulation of the Gas-Solid Flow in a Square Circulating Fluidized Bed with Secondary Air Injection

The dynamic behavior of gas-solid flow in an experimental square circulating fluidized bed setup (0.25 m × 0.25 m × 6.07 m) is predicted with numerical simulation based on the theory of Euler-Euler gas-solid two-phase flow and the kinetic theory of granular flows. The simulation includes the operation cases with secondary injection and without air-staging. The pressure drop profile, local solids concentration and particle velocity was compared with experimental results. Both simulation and experimental results show that solids concentration increases significantly below the secondary air injection ports when air-staging is adopted. Furthermore, the flow asymmetry in the solid entrance region of the bed was investigated based on the particle concentration/velocity profile. The simulation results are in agreement with the experimental results qualitatively.

Two-dimensional temperature characteristics of syngas flames with nitrogen, carbon dioxide, and steam diluents by planar laser-induced fluorescence thermometry

ABSTRACT In this research, the effects of nitrogen, carbon dioxide, and steam diluents on the two-dimensional temperature structures and characteristics of syngas flames were investigated by two-line planar laser-induced fluorescence thermometry. The spatial resolution and the total uncertainty of the measured results were 41.7 µm and ±8.45%, respectively. The experimental results demonstrated that diluting with carbon dioxide in the syngas flame resulted in the lowest flame temperature, followed by steam diluent. The measured temperature fields for these three diluents exhibited a symmetrical but nonuniform structure. It was found that the high-temperature region exists in the downstream of flame front, and the temperature decreases gradually inside-out in the burned zone of syngas flame. The findings in this work will improve our understanding of the temperature characteristics for the syngas flames with different diluents.

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

Study on CO2 Absorption Enhancement by Adding Active Carbon Particles into MEA Solution

The chemical absorption of CO2 is generally recognized as the most efficient post-combustion technology of CO2 separation at present. A study on CO2 absorption enhancement by adding small particles of active carbon into MEA solution is investigated within a self-designed glass stirring tank. Experiments of different particle loadings and different particle sizes have been conducted. When active carbon particle concentration is fewer, compared to the absorption rate of CO2 gas absorbed by MEA aqueous solution, the role of active carbon adsorption CO2 gas is negligible. The enhancement efficiency of CO2 absorption could be improved by 10% to the upmost in this liquid-particle system.

Experimental Study of Rapid Brown Coal Pyrolysis at High Temperature

Rapid coal pyrolysis is a very important step in the early stage of combustion. Rapid pyrolysis experiments of a brown coal at high temperature have been studied on a laminar drop tube furnace. The volatile mass release measured in this study is high for low rank coal. The activation energy and pre-exponential factor of pyrolysis are 19901.22 kJ/mol and 102.71, respectively. The nitrogen distribution between volatile and char is 0.54. With the increase of temperature, the yields of NH3 decreases, while those of HCN increases, leading the value of HCN/NH3 to become larger. At high temperature, the main nitrogen- containing species of pyrolysis in volatile is HCN.