A.E. Ghaly

Thermal degradation of rice husks in nitrogen atmosphere

Abstract Rice husk, a renewable by-product of rice milling operations, is an energy source that can be utilized to meet the increasing demands for energy. Its utilization can also help rice millers meet increasing costs of rice husk disposal because of increasing restrictions on disposal. However, industrial utilization of rice husk in thermochemical conversion systems to liberate energy requires the knowledge of its thermal characteristics for the proper design and modelling of these systems. The thermal degradation of four rice husk varieties (Lemont, ROK 14, CP 4, and Pa Potho) were investigated at three heating rates (10, 20 and 50°C/min) in nitrogen atmosphere using the technique of thermogravimetric analysis between ambient temperature and 700°C. The thermal degradation rate in active and passive pyrolysis zones, the initial degradation temperature, and the residual weight at 700°C were determined. Increasing the heating rate increased both the thermal degradation rate and the residual weight at 700°C, and decreased the initial degradation temperature. The higher the cellulosic content of the rice husk, the higher the thermal degradation rate and the initial degradation temperature. Also, higher ash content in the rice husk resulted in higher residual weight at 700°C. Rice husk could be degraded to the extent of 56–64%.

Agglomeration of silica sand in a fluidized bed gasifier operating on wheat straw

Abstract The effects of straw feed rate and ash composition on the agglomeration characteristics of the bed material (silica sand) in a fluidized bed gasification system were investigated. By changing the air flow rate and/or straw feed rate, the effects of four equivalence ratios (0.26, 0.38, 0.57 and 0.76) on agglomeration were investigated. The effect of straw ash content (0.0, 3.3, 27.8 and 43.5%) on the agglomeration of the bed material was also investigated at various temperatures (620, 740 and 850°C) using a high temperature furnace. It was found that a fluidized bed of silica sand agglomerated at around 800°C in the presence of straw ash, resulting in serious channelling and defluidization. A very hard and brittle structure was formed at 850°C. Changes in air velocity and/or fuel rate did not improve agglomeration characteristics. K 2 O, which is present at a high weight percentage in the straw ash and has a relatively low melting temperature, was the major contributor to the agglomeration process.

A comparative study of anaerobic digestion of acid cheese whey and dairy manure in a two-stage reactor.

Abstract The performance of a two-stage, two-phase, unmixed anaerobic digester of 155 l working volume operating on acid cheese whey and dairy manure at various temperatures and hydraulic retention times was investigated. The effect of controlling the pH of the methanogonic stage of cheese whey digestion on the biogas production rate and pollution potential reduction was also investigated. The digester was designed to act as a liquid-solid separator, in order to maximize the microbial mass in the reactor, and was operated at three hydraulic retention times (10, 15 and 20 days) and two temperatures (25 and 35°C). It operated as a single-phase reactor under both the cheese whey and dairy manure feeding conditions when the pH was not controlled (similar pH values in the inlet and outlet chambers) and as a two-stage, two-phase reactor when the pH of the whey was controlled in the methanogenic stage (different pH values in the inlet and outlet chambers). The results indicated that production of biogas from cheese whey without pH control is not feasible as the digester experienced acid-phase digestion in both stages. However, controlling the pH of the methanogenic stage increased the biogas production rate and methane yield, as well as the reductions in COD and solids concentrations of the cheese whey by a factor of 2.7-3.0. Although the pH was maintained at 5.7-6.0, the results are comparable to those obtained with dairy manure of a similar solids concentration and a pH of 7 ± 0.2. A further increase in the pH (to 7.0 ± 0.2) would, therefore, increase the biogas production rate and methane yield from cheese whey.

Kinetic modelling of continuous production of ethanol from cheese whey

Abstract A kinetic model for continuous fermentation of ethanol from cheese whey was developed. The model accounts for substrate limitation, substrate inhibition, ethanol inhibition and cell death. Three bioreactors of 5 L volume each were operated at different hydraulic retention times (HRT) ranging from 18 to 42 h and initial lactose concentrations ranging from 50 to 150 g/L. The experimental data were used to validate the model. The model predicted the cell, lactose and ethanol concentrations with high accuracy ( R 2 = 0.96–0.99). The cell concentration, lactose utilization and ethanol production were significantly affected by hydraulic retention time and initial substrate concentration. Lactose utilizations of 98, 91 and 83% were obtained with 50, 100 and 150 g/L initial lactose concentrations at 42 h HRT. The highest cell concentration (5.5 g/L), highest ethanol concentration (58.0 g/L) and maximum ethanol yield (99.6% of theoretical) were achieved at 42 h HRT and 150 g/L initial lactose concentration.

Effect of micro-aeration on the growth of Candida pseudotropicalis and production of ethanol during batch fermentation of cheese whey

Abstract Four pilot-scale batch reactors of 5 l volume each were used to study the effect of micro-aeration (240–720 ml/min or 0·05-0·15 v/v/min) on the cell growth rate, substrate utilization and ethanol production during cheese whey batch fermentation using the yeast Candida pseudotropicalis at lactose concentrations ranging from 50 to 200 g/l. The results indicated that micro-aeration had a significant effect on the maximum cell concentration and the specific growth rate. It improved the viability of the yeast cells and enhanced the substrate utilization rate and the ethanol yield. The optimum micro-aeration level was found to be 0·1 v/v/min; further increases in the rate of micro-aeration resulted in an increased biomass concentration and decreased ethanol production as more lactose was utilized for biomass synthesis. The maximum ethanol yield (98·3%) was obtained with the micro-aeration rate of 0·1 v/v/min and the 150 g/l initial substrate concentration. The ethanol productivity (g/l/h) under these conditions was 35% higher than that obtained at 150 g/l initial substrate concentration with no aeration.

Kinetics of batch production of ethanol from cheese whey

Abstract A kinetic model for ethanol fermentation was developed. The model accounted for substrate limitation, substrate inhibition, ethanol inhibition and cell death and performed satisfactorily for predicting the transient responses of cell growth, ethanol production and substrate utilization during the batch fermentation process of cheese whey ( R 2 = 0.96 to 0.99). The maximum specific growth rate ( μ m ), the saturation constant ( K 5 ), the ethanol inhibition constant and the substrate inhibition constant were found to be 0.051 h −1 , 1.9 g/l, 20.65 g/l and 112.51 g/l, respectively. The maximum ethanol concentration above which Candida pseudotropicalis does not grow was found to be 100 g/l. The maximum ethanol production occurred at about 150 g/l initial substrate concentration after about 62 h. High initial substrate concentrations reduced both the specific growth rate and the substrate utilization rate due to the substrate inhibition phenomenon.

Quality of gas produced from wheat straw in a dual-distributor type fluidized bed gasifier

Abstract A 400 kW, dual distributor type fluidized bed gasifier was used to investigate the production rate and composition of the gas produced from wheat straw at various equivalence ratios (0.17, 0.20, 0.25, 0.35) and fluidization velocities (0.28, 0.33 and 0.37 m s − ). The results showed that the equivalence ratio was the major parameter affecting the gas composition. The equivalence ratio of 0.25 appeared to be the optimum with respect to the quality of the gas. The mole fractions of the combustible components reached their maximum values at this equivalence ratio. A typical gas composition at the equivalence ratio of 0.25 was 7% H 2 , 7% hydrocarbons (CH 4 , C 2 H 2 , C 2 /H 4 and C 2 H 6 ), 14% C0 2 , 22% CO and 50% N 2 . The higher heating value of the produced gas (6.3–7.3 MJ Nm −3 ) obtained at this equivalence ratio appeared to be higher than most values reported in the literature for several types of biomass fuels.

Study of agglomeration characteristics of silica sand-straw ash mixtures using scanning electronic microscopy and energy dispersion X-ray techniques

Abstract The effects of temperature and straw ash content on the agglomeration characteristics of the silica sand-straw ash mixture were investigated using a scanning electron microscope and an energy dispersive X-ray analyser. Four levels of temperature (620, 740, 850 and 1000°C) and four levels of straw ash content (0.0, 3.3, 27.8 and 43.5%) were used. The results showed that a low melting temperature eutectic was formed at elevated temperatures (850–1000°C) due to the chemical interaction between the silica (SiO 2 ) and potassium oxide (K 2 O) present, in a relatively high concentration, in the straw ash.

Agglomeration characteristics of alumina sand-straw ash mixtures at elevated temperatures

Abstract The agglomeration characteristics of alumina sand-straw ash mixtures were investigated at various levels of ash content (0.0, 3.3, 27.8, and 43.5%) and temperature (620, 740, 850, and 1000°C) using scanning electron microscopy and energy dispersive X-ray analysis techniques. Agglomeration of alumina sand did not take place below the temperature of 850°C at all levels of ash content, but at the temperature of 850°C a weak bonding of particles was observed. However, at the temperature of 1000°C, the alumina particles agglomerated in the presence of straw ash at all levels of ash content as a layer of ash melt bonded the particles together. The temperature at which agglomeration occurred was within the range of the initial deformation (921°C) and softening (1054°C) temperatures of straw ash and the liquidus temperature of potash feldspar (990 ± 20°C). The softening of straw ash and the formation of low melting temperature eutectic (potash feldspar) are two possible mechanisms for the agglomeration of alumina sand.

Determination of the kinetic parameters of oat straw using thermogravimetric analysis

Abstract Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) techniques were used to study the thermochemical behaviour of three varieties of oat straw (Sentinel, Shaw, and Tibor). The thermal degradation of oat straw was studied in an oxidizing atmosphere (15% oxygen and 85% nitrogen) from ambient temperature to a temperature of 700°C using a heating rate of 20°C min−1. Two distinct reaction zones were observed on the TGA and DTA curves. The kinetic parameters (order of reaction, activation energy, and pre-exponential factor) were determined for each zone separately by applying thermo-analytical techniques to the reaction kinetics. Higher thermal degradation rates were observed in the first reaction zone due to rapid release of volatiles as compared to those in the second reaction zone. The activation energies were in the range of 83–102 and 58–75 kJ mol−1 for the first and the second reaction zones, respectively.