No abstract

A thermosyphon-fixed bed reactor was designed and constructed to investigate the temperature distribution of the cassava rhizome and its decomposition behavior. To study the properties of torrefied char obtained from this reactor, cassava rhizome was torrefied in five different configurations, including the thermosyphon-fixed bed reactor, a laboratory reactor in compact bulk arrangement with N2 as the purge gas and without any purge gas, and another one in a hollow bulk arrangement with and without purge gas. It was found that the use of thermosyphons with a fixed bed reactor improved the uniform temperature distribution. The average heating rate to the cassava rhizome bed was 1.40 °C/min, which was 2.59 times higher than that of the fixed bed reactor without thermosyphons. Compared to the other configurations, this reactor gave the highest higher heating value (HHV) and the lowest mass yield of 23.97 MJ/kg and 47.84%, respectively. The water vapor produced in this reactor played an autocatalyst role in the decomposition reaction. Finally, the thermosyphon-fixed bed reactor gave an energy yield in the range of 70.43% to 86.68%. The plot of the HHV ratio–mass yield diagram indicated the difference of torrefied char obtained from different reactors. The thermosyphon-fixed bed reactor produced torrefied biomass with the highest HHV ratio compared to that of other reactors at the same energy yield.

The heating rate, firepower, and thermal efficiency of a modified rocket stove using sawdust and liquid petroleum gas (LPG) as co-firing fuel were investigated. Three modified rocket stoves with a height of 400 mm and outside diameters of 225, 385, and 550 mm were tested. It was found that there was an insignificant difference in heating rate and firepower when stoves were tested without co-firing with LPG. In this case, the stove heating rate was in the range of 1.49–1.55 °C/min. When LPG was used, the heating rate tended to linearly increase with the increase of LPG flow rate. The heating rate was in range of 2.42–2.80, 2.63–3.27, and 3.07–4.22 °C/min when LPG consumption rates were 2.38 × 10−5, 3.33 × 10−5, and 5.00 × 10−5 kg/s, respectively. The slight increase of stove heating rate and firepower was seen when the stove diameter was increased from 225 to 385 mm. The increase of stove diameter from 385 to 550 mm resulted in a huge increase of heating rate and firepower. Thermal efficiency of the sawdust stove without LPG decreased from 17.90% to 9.97% when the stove diameter was increased from 225 to 550 mm. For co-firing of sawdust and LPG, the increase of LPG flow rate from 2.38 × 10−5 to 5 × 10−5 kg/s caused the linear increase of thermal efficiency from 20.27% to 33.80%, 29.36% to 38.89%, and 25.25% to 36.39% for the stove with diameters of 225, 385, and 550 mm., respectively.

The small-scale rotating drum reactor (SS-RDR) was designed and constructed without using purge gas for the purpose of household application. The thermal and torrefaction characteristics of SS-RDR were studied and compared with other reactor types. It was found that the heat loss at the reactor wall and heat loss from exhaust gas of the SS-RDR were in the range of 6.3–12.4% and 27.9–42.8%, respectively. The increase of flue gas temperature resulted in the decrease of heat loss at the reactor wall and the increase of heat loss from exhaust gas. The heating rate of the SS-RDR was in the range of 7.3–21.4 °C/min. The higher heating value (HHV) ratio, mass yield, and energy yield ofthe SS-RDR were in the range of 1.2–1.6, 35.0–81.0%, and 56.2–96.5%, respectively. A comparison of torrefaction characteristics of various reactor types on HHV ratio-mass yield-iso-energy yield diagram indicated that the torrefaction characteristics of the SS-RDR were better than that of the rotating drum reactor with purge gas.

This work investigated quality properties of pellets of raw cassava rhizome (P-RC), pellets of pelletized cassava rhizome followed by torrefaction (T-CP), and pellets of torrefied cassava rhizome followed by pelletizing (P-TC). Torrefaction was conducted at temperatures of 230, 250, and 280 °C for 30 min. Pyrolysis characteristics of T-CP and P-TC at torrefied temperatures of 230 and 250 °C were studied using thermogravimetric analysis. It was found that at the similar torrefied temperature, P-TC had a higher bulk density, energy density, and pellet durability than that of T-CP and P-RC while T-CP had a higher HHV and moisture absorption than P-TC and P-RC. The bulk density of P-TC was 1.13–1.19 and 1.33–1.52 times higher than that of P-RC and T-CP, respectively. The HHV of T-CP was 1.07 and 1.29 times higher than P-TC and P-RC, respectively. The energy density of P-TC was 1.24–1.56 and 1.20–1.41 times higher than that of P-RC and T-CP. In terms of Pellet Fuel Institute (PFI) standard, the durability index of P-RC, P-TC, and T-CP at torrefied temperatures of 230 and 250 °C was acceptable. However, dramatically low and unacceptable durability index was found in case of T-CP at torrefied temperature of 280 °C. The moisture absorption of P-TC was lower than that of P-RC and T-CP. Finally, T-CP had a lower pyrolysis temperature and had a much lower solid yield than that of P-TC. Variation of pyrolysis characteristics indicated the difference in chemical composition between T-CP and P-TC.