Juwei Zhang

Kinetics and mechanism of direct reaction between CO2 and Ca(OH)2 in micro fluidized bed.

Even at present it is still difficult to characterize the reaction between CO2 and Ca(OH)2 at high temperature and atmospheric pressure using traditional instruments such as thermogravimetric analyzer and differential scanning calorimeter. This study was devoted to characterizing such a reaction in a newly developed micro fluidized bed reaction analyzer (MFBRA) under isothermal conditions in the temperature range of 773-1023 K. The results indicated that the MFBRA has not only a good adaptability for characterizing the above-mentioned reaction but enables as well a new insight into the mechanism of the reaction. An obvious time delay was identified for the release of the formed steam (H2O) in comparison with the onset of its CO2 absorption, which might be attributed to the formation of an unstable intermediate product Ca(HCO3)2 in the reaction process between CO2 and Ca(OH)2. The activation energy for forming Ca(HCO3)2 was found to be about 40 kJ/mol, which is much lower than that of the reaction between CO2 and CaO.

Flow instability in non-fluidized standpipe flow

Abstract Solids flow through an orifice at the bottom of a standpipe, against a counterflow of gas, becomes unstable as the gas flow is increased. In the unstable condition, the solids flow in the standpipe is jerky, reflecting the oscilllations between flow and no flow at the orifice, and resembles slip-stick flow. Practical operation of these systems requires a straightforward quantitative demarcation for distinguishing when flow behaviour changes from stable to unstable. This paper presents an analysis of flow stability in standpipes with gas counterflow, supported by small-scale experiments. For smaller orifices, solids flow instability occurs when a hemispherical bubble cap just spans the orifice, corresponding to a certain critical negative pressure gradient. For large orifices, stable solids flow is abruptly cut off when the counterflowing gas rate is high enough and no unstable solids flow region occurs.

Process simulation of a lignite-fired circulating fluidized bed boiler integrated with a dryer and a pyrolyzer

ABSTRACT A process model using Aspen Plus was established to simulate the performance of a lignite-fired 1,025 t/h circulating fluidized bed boiler integrated with a dryer and a pyrolyzer for improving the energy efficiency. The combustion module of the proposed model was first verified by design data and the system characteristics of the circulating fluidized bed boiler combined with the coal dryer and the pyrolyzer were then investigated. The most suitable case for the retrofit of existing 1,025 t/h CFB boilers is that 35% of the coal was dried and then pyrolyzed with the energy efficiency of 95.3% and exergy efficiency of 50.2%. The results demonstrate the lignite-fired circulating fluidized bed boiler integrated with a dryer and a pyrolyzer facilitates the system performances effectively.