Techno-economical evaluation of a rice husk ash (RHA) based sand–cement block for reducing solar conduction heat gain to a building

This paper reports on the performance of a rice husk ash (RHA) based sand–cement block. Its performance is compared with that of a standard commercial clay brick. The RHA-cement block reduces solar heat gain in buildings and the comparisons include an evaluation of room temperature, solar conduction heat transfer and economics. An appraisal of the two was conducted using two small rooms (floor area of 5.75 m²). One of the rooms was constructed using the RHA based sand–cement block wall; the other, which served as the reference, used a commercial clay brick wall. Experiments were performed throughout a period of one summer month (March) in Thailand. The results showed that the RHA based sand–cement block reduced solar heat transfer by 46 W. An economic analysis indicates that the payback period of the RHA block in tandem with a 1 ton, split-type air conditioner depends on the indoor set-point temperature. The payback period is 4.08 years when the indoor set-point temperature of 26 °C is taken.     

Effect of temperature and relative humidity on yellowing rate of paddy

The effects of temperature and relative humidity (or water activity) in storage chambers on yellowing rate of paddy were investigated and then an empirical equation for predicting the yellowing rate was developed. Paddy was conditioned using saturated salt solutions at relative humidities ranging from 0.80 to 0.95 and temperatures of 35, 45, 55, 60 and 65°C. The yellowing rate was found to follow the zero order kinetics. The yellowing constant value (k) increased exponentially with temperature and increased linearly with water activity. The magnitude of apparent activation energy varied from 130–145 kJ/mol. A predictive equation for determining yellowing rate was ln k=−δa_w−/T+(γa_w)/T where a_w was water activity (valid from 0.80 to 0.95), T was absolute temperature (valid from 308 to 338 K) and , δ, and γ were constants. The results of variance analysis showed that temperature, water activity and their interaction significantly influenced the yellowing rate of paddy.     

Influences of coal size and coal-feeding location in co-firing with rice husks on performance of a short-combustion-chamber fluidized-bed combustor (SFBC)

This study examined the combustion characteristics and performance of rice husks co-fired with coal in a short-combustion-chamber fluidized-bed combustor (SFBC) with a 225 kW_th capacity. Rice husks were the main fuel, and coal was a supplementary fuel in the experiments. The effects of coal size (< 5 mm and 5-10 mm) and coal-feed location (above or below a recirculating ring) on combustion performance were investigated. Various co-combustion tests of rice husks with coal were performed, with different thermal percentages (10, 15, 20, and 25%) of coal. The results were compared to firing 100% rice husks alone. With the assistance of a stirring blade and a recirculating ring, good combustion was feasible without using any inert materials mixed into the bed.Combustion efficiency in an excess of 98% was readily achievable. CO and SO₂ emissions (at 6% O₂) were in the range 64-104 and 10-22 ppm, respectively, while NO_x emissions were in the range of 208-281 ppm. Although the CO and SO₂ emissions were acceptable, combustion of 100% rice husks and co-combustion with < 20% coal failed to comply with Thai NO_x emission limits. Therefore, to minimize NO_x emissions (208-244 ppm, at 6% O₂), coal of both sizes was introduced below the recirculating ring. The results demonstrated that the thermal percentage of coal in the fuel mixture should be 20-25%.     

Development of Cross-Flow Fluidized Bed Paddy Dryer

ABSTRACT

A cross-flow fluidized bed paddy dryer with a capacily of 200 kgh was designed, fabricated and tested. Experimental results showed that final moisture content of paddy should not be lower than 23 % dry-basis if quality is to be maintained. Drying air temperature was keot constant at 115°C according to the recommendation of previous work. Results obtained from the mathematical model developed in this study indicatedthat optimum operating parameters should be as follows : air speed of 2.3 m/s, bed thickness of 10 cm and fraction of air recycled of 80 %. At this condition, energy consumption was close to the minimum while drying capacity was near the maximum. A prototype fluidized bed dryer with a capacity of 1 t/h was designed, fabricated and installed with the collaboration of a private company. The unit has been used for almost the whole past harvesting season in 1994 at a paddy merchant sile with preference compared to conventional column continuous dryers. More than 300 tons of paddy were dried without any problems.     

Rice husk co-firing with coal in a short-combustion-chamber fluidized-bed combustor (SFBC)

This study encompassed the characteristics and performance of co-firing rice husk, a by-product of rice-milling process, with coal in a short-combustion-chamber fluidized-bed combustor (SFBC). Bed phenomena investigated in a cold-flow model combustor showed that with the different mixes of materials, the anticipated offshoot of combustion, the minimum fluidizing velocity (U_mf) was 0.4-0.8 m/s. In concord with axial temperature profiles, axial gas concentration profiles implied that a recirculating ring was able to circumscribe CO within the short-main chamber. The formation, decomposition, and eventual maturity of NO_x characterized the NO_x evolution, inferred from concentration profiles. The impacts of fluidizing velocity and blending ratio on gas emissions and combustion efficiency (E_c) are described. The fluidizing velocity had consequential effect on gas emissions, except NO_x. Surprisingly, NO_x did not hinge much on increased N-content of the mixtures with coal. As expected, increased SO₂ was relevant to increased coal mass. Increased fluidizing velocity adversely affected E_c while increased coal fraction enhanced E_c, mostly >97%.     

Effects of fluidized bed drying temperature and tempering time on quality of waxy rice

Waxy rice, which is soft and sticky in nature, can be used as a raw material to produce many food products. After being harvested, high-moisture waxy paddy must be dried to appropriate moisture content to prolong its storage life and to achieve higher head rice yield. Fluidized bed dryer could be used to dry waxy rice at high-temperature. However, due to the high heat and mass transfer rates during drying, stresses are generated in a rice kernel, leading to crack and low head rice yield. Tempering is thus recommended to reduce the moisture-induced stresses in the kernel after rapid drying. In this study, the effects of fluidized bed drying temperature (90, 110, 130 °C) and tempering time (30–120 min) on the quality of waxy rice, i.e., head rice yield, thermal properties, pasting properties, color, translucent kernel and microstructure, were investigated. The results showed that head rice yield of waxy rice after drying was significantly lower than that of the reference sample even when tempering was performed. Higher drying temperatures led to higher head rice yield while the tempering time did not have any effect on the head rice yield except when the drying temperature of 90 °C was used. Drying at higher temperatures also affected the starch granule morphology and the pasting properties. Waxy rice changed its appearance from opaque white to translucent when being dried at 130 °C.     

Accelerated aging of jasmine brown rice by high-temperature fluidization technique

After being cooked, newly harvested brown rice becomes a pasty mass and swells only slightly. To modify these undesirable brown rice properties, the paddy needs to be stored for at least 3–6 months. However, problems arise since brown rice has short shelf-life (3–6 months) due to accumulation of free fatty acids (FFA) leading to rancidity during storage. In the present study, high-temperature fluidized-bed drying technique in combination with tempering step was tested to alleviate the above-mentioned problems. The quality of rice dried at temperatures of 130 and 150 °C and tempered for 30 up to 120 min was compared to that of brown rice stored at ambient temperature (approximately 30 °C) for 7 months. The experimental results showed that the cooking and eating properties of the fluidized bed dried brown rice, i.e., hardness, solid loss, volume expansion and elongation ratio, changed in a similar fashion to those of the conventionally aged brown rice. The drying temperature and tempering time affected significantly the brown rice properties. The texture of cooked thermally treated brown rice was significantly firmer than that of the conventionally aged brown rice. In addition, it was found for the thermally treated brown rice that the contents of free fatty acids increased only slightly during storage while the glycemic index reduced from high to low-medium level.     

A MATHEMATICAL MODEL FOR CONTINUOUS DRYING OF GRAINS IN A SPOUTED BED DRYER

A mathematical model for a continuous spouted bed dryer has been presented to predict moisture content, air and grain temperatures as well as energy consumption. To better understand the interactive influence of processes in each region of the spouted bed, solution schemes for the spout and downcomer were treated separately. The behavior of dryer was investigated experimentally and found that the dryer behaved differently from an ideal plug flow. The drying rate as simulated by the model is almost constant during grain movement in the dryer. Absence of airflow in the downcomer leads to a tempering process that takes place in the downcomer while intense heat and mass transfer occurs mainly in the spout due to the high airflow rate there. Furthermore, by considering the predicted grain temperature history as one of the indicators of product quality, one can, in principle, design appropriate successive processes in a continuous spouted bed dryer to minimize product damage.     

Modal analysis of free vibration of liquid in rigid container by the method of fundamental solutions

Modal analysis of free vibration of liquid in a rigid container having arbitrary shape requires a numerical method. The method of fundamental solutions has an advantage over the finite element method and the boundary element method in that it does not require either volume mesh generation or surface mesh generation. This paper presents the formulation of the method of fundamental solutions for determining natural frequencies and mode shapes of free vibration of liquid in rigid container. Modal analyses are performed for cylindrical container, cylindrical quadrant container, cylindrical equilateral triangle container, hemispherical container, and cylindrical container with baffle. It is shown that natural frequencies of certain modes obtained by the method of fundamental solutions agree with analytical results and other numerical results.     

EFFECT OF TEMPERING ON SUBSEQUENT DRYING OF PADDY USING FLUIDISATION TECHNIQUE

Drying and tempering models for paddy drying by a fluidised bed technique have been developed to describe the moisture movement inside a single paddy kernel. The grain shape was considered as a finite cylinder. The internal diffusion is an important contribution to control the drying rate of paddy. The dependence of effective diffusion coefficient on drying temperature can be adequately explained based on Arrhenius form. The parameters of this equation were evaluated in the range of temperatures between 110°C and 170°C by using the regression analysis with 189 experimental drying data. As compared with no tempering, the faster drying rate can be obtained by tempering treatment between drying stages. The effect of degrees of tempering on determining the moisture reduction in the second stage has also been explored. According to the simulation results, a prediction equation of the required tempering time for the tempering index of 0.95 has been established in which the drying air temperature, initial moisture content and drying time are taken into account. The tempering time for 35 min is recommended for the continuous fluidised bed dryers being operated in rice mills.