In the present investigation, two units of PCFB (one cold and one hot) of similar dimensions have been developed. The bed hydrodynamics along the height of the riser have been investigated in the cold PCFB unit. The heat transfer characteristics have been investigated in both the PCFB units at the upper splash region of the riser. A novel biomass feeding system was also developed for feeding of biomass in the PCFB units and performance was analyzed. The bed hydrodynamics and heat transfer characteristics have been investigated at different operating conditions such as operating pressures, solid inventories, superficial velocities, and at different blends of biomass in sand and at two different weight compositions. The biomass used for the study has been characterized. The thermogravimetric (TG) analysis and differential scanning calorimetry (DSC) have been used to study the influence of heating rate on the degradation of biomass. The kinetic parameters viz. pre-exponential factor, activation energy and order of reaction were evaluated for both first and second reaction zones which were categorized based on the degradation of cellulose, hemicelluloses and lignin content present in the biomass. The degradation of mass with temperature obtained from TG curve was validated numerically. The thermal response of biomass undergoing decomposition has also been modelled by using one dimensional (1-D) transient thermal model with an nth order approximation for the rate of decomposition. Gasification study has been carried out at an equivalence ratio of 0.27 in the hot PCFB unit. The product gas composition has been investigated by using a gas chromatograph and a flue gas analyzer at different blending ratios of biomass and at three different operating pressures. The enhancement of heat transfer by using twisted tape inserts at the upper splash region of the riser has been studied at three different solid inventories and at three different operating pressures. The key research findings are summarized in the following subsections.
8.1.1 Results of characterization and thermal analysis of biomass
Kinetic parameters of both first and second reaction zones of locally available biomass such as rice husk and sawdust were evaluated by using multi variable regression analysis. These kinetic parameters were utilized in the numerical technique in order to predict the mass degradation with temperature in both the reaction zones at three heating rates of 10, 30 and 80 °C/min. From the TG and DTG analysis, it has been found that, the major weight loss due to the degradation of volatiles starts at around 180-205 °C for all the biomasses at three different heating rates. Up to 400 °C, weight loss is about 85 % of the total volatile matter. After a rapid pyrolysis, a relatively slow pyrolysis occurred over 400 °C. Finally, bio-char is formed with the removal of most of the volatile matters. It is also observed that, with the increase of rate of heating, the degradation temperature increases. The predicted degradation results are well comparable with the experimental data in both the reaction zones for all the three heating rates. A transient one dimensional heat conduction model was formulated to investigate the transient response without and with considering the heat of formation. Kinetic parameters, properties of biomass, temperature range for both the reaction zones are used as input to the model. Results of the model agree well with the results obtained by using Heisler Chart without considering heat of formation.
8.1.2 Results of cold bed studies
In this investigation, the effect of superficial velocity, operating pressures, particle size and solid inventory on hydrodynamics and wall-to-bed heat transfer characteristics in a PCFB unit has been studied. The experiments were carried out at four different superficial velocities such as 5, 6, 7 and 8 m/s. At each superficial velocity, the experiments were performed at three different operating pressures of 1, 3 and 5 bar. The hydrodynamics and wall-to-bed heat transfer characteristics at four different proportions blending of sawdust (2.5, 7.5, 12.5, 15.0 and 20.0 %) and at two different sets of weight (inventories) composition ratio were investigated. Effects of recirculation rate have also been studied. From the study, it has been found that flattened S- Shaped bed voidage profile appeared in all the percentage mixing of sawdust in sand at all the operating conditions. With an increase in pressure, the bed voidage increases at the lower splash region of the riser and decreases at the upper splash region. The solid circulation rate increases with an increase in operating pressures and an increase in bed inventories. The suspension
density increases with the decrease in particle size and increases with an increase in operating pressure. The suspension density is also found to increase towards the riser exit with an increase in pressure. It is also seen that with the increase in both superficial velocity and system pressure, the heat transfer coefficient increases along the height of the heat transfer probe. The heat transfer coefficient is found to be maximum (145.17 W/m2-K) at 12.5 % biomass blending with sand and at 5 bar operating pressure and at a superficial velocity of 8 m/s. However, at this superficial velocity, the suspension density along the height of the riser is found to be higher at higher pressure except at the riser exit. The increase in suspension density results in an increase of heat transfer coefficient, which indicates a lower consumption of blower power. The radial heat transfer coefficient decreases from the wall to the bed at all the operating conditions and operating pressures. Similar profiles have been observed in all the percentage mixing of biomass in sand. The heat transfer coefficient decreases with the increase in particle size. More homogenous fluidization and uniform heat transfer coefficient has been found as the operating pressure increases. Sawdust blend in sand (7.5 – 15.0 %) is observed to be optimum for obtaining higher heat transfer coefficient at both the sets of weight compositions. Prominent results of the present investigation are comparable to the published results under similar operating conditions.
8.1.3 Results of hot bed studies
Experiments were performed at three different heat inputs, and hence, at three different controller temperatures. At each temperature, the effect of pressure was studied at three different operating pressures of 1, 3 and 5 bar. At each pressure condition, the bed temperature distributions along the height of the riser were investigated. The temperature distributions at two different biomass blending ratios of 12.5 % and 20.0 %, and at two different weight composition ratios were investigated and compared. It has been observed that, the bed temperature first increases up to a height of 0.5 m from the distributor, and then decreases along the height of the riser before it increases at the exit. The product gas composition was also investigated at each pressure condition and at an equivalence ratio of 0.27 and at two different biomass blending ratios. With an increase in operating pressure, % composition of H2 in the product gas decreases, however, the % composition of CO increases with pressure. The effect of twisted tape inserts on heat transfer at the upper splash region of the riser was studied at three different operating
pressures and at three different solid inventories of 400, 600 and 800 g. The heat transfer coefficient at the upper splash region increases with the increase in operating pressures as well as with the used of twisted tape having twist ratio of 4.
8.1.4 Performance of biomass feeding system
Performance of a biomass feeding system has been evaluated at four different biomass inventories of 250, 500, 750 and 1000 g and at three different particle sizes of 732, 853, and 921 µm. Absolutely no choking has been observed at all the inventories having biomass particle size
≤ 853 µm. The bridge formation and shocking has been observed for the biomass particles having particle size ≥ 921 µm. The developed system is a very flexible one, depending on the requirement of the feed, the appropriate voltage may be supplied in order to control the speed and hence flow. The feeding range of the developed feeding system is found to be 1-12 kg/hr, which is capable of supplying feed to a reactor of power generating capacity of 1-5 kW-hr. The developed feeding system is very much suitable for feeding of low bulk density biomass such as sawdust having particle size ≤ 853 µm to the circulating fluidized bed unit. Feeding of biomass without porous rod has also been tried, however, a continuous flow could not have been achieved.
In conclusion, this research gives an approach for an effective utilization of biomass under specific range of operating parameters in a PCFB unit where the reaction kinetics of biomass, bed hydrodynamics, and heat transfer behaviour along the riser have been thoroughly studied.
Further, the findings of the present investigation will give sufficient guidance for the design of an efficient PCFB unit suitable for combustion as well as gasification applications.