Experimental Performance Analysis of a Pilot-Scale Biomass-Assisted Recirculating Mixed-Flow Dryer for Drying Paddy

A large amount of heat energy is required for paddy drying processes to evaporate water from paddy grains. Currently, fossil fuels are being used as an energy source to heat air during the drying process. However, fossil fuels cause air pollution, climate change, and disruption of ecological balance. In this study, to reduce the dependence on fossil fuels for paddy drying, a pilot-scale biomass-assisted recirculating mixed-flow drying system (PSBA-RMFD) for drying paddy was designed, installed, and tested. In this PSBA-RMFD, the heat energy required for heating the drying air was provided only by biomass. The PSBA-RMFD comprises a biomass furnace, drying column, vibratory feeder, bucket elevator, and blower. This study is aimed at evaluating the performance of the PSBA-RMFD with a drying capacity of 400 kg/h. The performance metrics of the PSBA-RMFD were specific energy consumption (SEC), specific thermal energy consumption (STEC), specific moisture evaporation rate (SMER), thermal efficiency of the PSBA-RMFD, exergy efficiency of the drying section, and improvement potential of the dryer. From the experiments conducted in this study, the values of the aforementioned performance parameters were as follows: 0.806-8.656 kW h/kg of water evaporated; 0.385-4.136 kW h/kg of water evaporated; 0.122-1.308 kg of water evaporated/kW h; 7.82-83.99%; 15.28-25.64%; and 858.90-1355.62 W, respectively. The paddy moisture content was reduced from 20.90% wet basis (initial weight of 400 kg) to 13.30% wet basis (final weight of 364 kg) in 270 min, with an average temperature of 78.15°C and average relative humidity of 8.55%. The percentage of biomass energy used in the drying system was approximately 47.77% of the overall energy. In addition, the payback period of the PSBA-RMFD was 1.9 years.


Introduction
Indonesia is an agricultural economy-based country with a vast agricultural land and is one of the world's leading paddy producers. Its paddy production in 2019 reached approximately 84 million t. Indonesia is also one of the world's largest paddy consumers, with an annual per capita consumption of approximately 111.58 kg [1]. In Indonesia, the paddy after being harvested must be dried immediately to avoid mold growth and yellowing of grains, as Indonesia's climate is characterized by high humidity. The harvested paddy contains a high degree of moisture, ranging from 20% to 27% (wet basis). Therefore, to secure a long shelf life, the paddy is typically dried to a moisture content of 14% (wet basis) [2]. Traditionally, paddy is dried using open sun drying, where the product is exposed directly to the sun. Drying with open sunlight has several advantages, including free and abundant supply of energy from the sun, simplicity of the method, and low initial and maintenance costs; however, its disadvantages include, the requirement of a large area, which is a function of the solar radiation, long drying times, lowered quality of the dried products, and significant loss of paddy during the drying process.
To overcome the limitations of open sun drying for drying paddy, a mixed-flow dryer can be used, which offers significant advantages, such as suitability for drying of grains, short drying time, good quality of the dried products, and smaller losses in the quantity of paddy during the drying process [3].
Mixed-flow dryers have been used to dry high-moisture grain products, such as rice [4], maize [5], wheat [6][7][8][9][10][11], soybeans [12,13], barley, oats [14], and corn [15]. However, fossil fuels are still being used as energy sources for heating the drying air during the drying process. Fossil fuels suffer from disadvantages, such as generation of significant amounts of CO 2 , SO 2 , and Nox emissions. They cause air pollution, climate change, and disruption of ecological balance. Moreover, their prices are high and will continue to rise, while their resources are limited [16,17].
Biomass energy serves as an alternative to fossil fuels and has some advantages, such as low emissions and sustainable renewable energy sources. Moreover, it is locally available in abundance and is relatively low-priced.
However, to the best of our knowledge, the performance of a pilot-scale biomass-assisted recirculating mixed-flow dryer (PSBA-RMFD) for drying paddy using biomass energy as a source of heat energy has not yet been reported. In addition, Indonesia produces significantly large amounts of biomass energy every year (approximately 236 million t; 756.083 million GJ), and this can be used as a source of heat energy in the drying process [52]. Therefore, this study is aimed at designing, installing, and evaluating the performance of a PSBA-RMFD for drying paddy using biomass as a source of heat energy.

Experimental Set-Up.
A PSBA-RMFD, which comprised a biomass furnace, drying column, vibratory feeder, bucket elevator, and blower, was designed and installed. Photographs and a schematic of the PSBA-RMFD are presented in Figures 1 and 2. The biomass furnace consisted of the following key parts: a furnace chamber, heat exchanger pipes, chimney, and blower. The wall of the furnace chamber was fabricated from Sk-34 fire brick, Sk-34 cement mortar, and AISI 1030 steel plate materials. Sk-34 fire brick has advantages such as high mechanical strenght, high temperature resistance, and high resistance to abrasion; Sk-34 cement mortar has advantages such as high temperature resistance and high adhesiveness. Meanwhile, AISI 1030 steel plate has advantages such as high mechanical strenght and high temperature resistance. The pipes of the heat exchanger were fabricated from mild steel. The drying column had three main sections, namely, the storage, drying, and discharge sections, and its dimensions are shown in Figure 3. The discharge section consisted of a motor, gear box, sprocket, chain, roller, and support structure. The vibratory feeder consisted of a motor, pulley, bearing, trough, and support structure. The bucket elevator consisted of a pulley, belt, buckets, gear box, motor, feed hopper, discharge sprout, and casing. The drying section of the drying column had a cross-sectional area of 95 × 95 cm 2 and was equipped with inlet and outlet air ducts arranged horizontally. The inlet and outlet air ducts consisted of four rows each. Each inlet air duct row had six full-air ducts, and each outlet air duct row had five full and two half-air ducts. The crosssectional area of each air duct was 56.25 cm 2 . The inlet and outlet air ducts were wedge-shaped. The photographs of air duct and drying section are shown in Figure 4. Meanwhile, the arrangement of air ducts and their dimensions (in the drying section) are shown in Figures 5 and 6.
The working procedure of the PSBA-RMFD is presented in Figure 2. At first, the dryer is filled with moist paddy by a bucket elevator into the hopper at top the dryer and flowed vertically downwards by gravity, while the discharge roller at the bottom of the dryer is turned off (discharge device is closed). The discharge roller operated on the principle of a rotary valve to allow a constant product mass flow rate. When the moist paddy in the dryer reach the desired height, then the ambient air is passed through the biomass furnace heat exchanger by a centrifugal blower to increase its temperature. The ambient air is heated up to a prescribed temperature with the heat generated by burning the biomass fuel (coconut shell charcoal) in the combustion chamber of the biomass furnace. Then, the heated air is passed to the drying section, from where the heated air is forced through the paddy grains outwards carrying the moisture from drying material. Then, the moist air is discharged into the environment. In the drying section, the moist grains crosses the drying section nearly vertically while heated air (drying air) flows horizontally. Then, the discharge roller is turned on (discharge device is opened). Subsequently, the dried paddy falls into the vibratory feeder, then flow to the bottom of the bucket elevator. From here, the dried paddy is moved or circulated to the top of dryer by the bucket elevator for further drying process. This process continues until the desired moisture content of the paddy grains is achieved.

Experimental Procedure.
A drying experiment was conducted to evaluate the performance of the PSBA-RMFD, with a holding capacity of 400 kg. Fresh paddy was purchased from a farmer, and during the drying experiment the ambient temperature, the air temperatures at the entry and exit of the biomass furnace, drying section, and the temperatures of the paddy at various points inside the drying section were measured using thermocouples (type T, accuracy: ±0.1°C, operating range: -200 to 400°C, Japan). An anemometer (model HT383, accuracy: ±0.2 ms -1 , operating  3 International Journal of Food Science of the paddy were recorded every 30 min. The uncertainty of measurements was determined using the following equation [53,54]: where W R is the total uncertainty in the result measurement; x 1 , x 2 , ⋯x n are the independent variables; and w 1 , w 2 , ⋯w n are the corresponding uncertainties in the aforementioned independent variables.

Performance Analysis.
The performance of the PSBA-RMFD, in terms of the drying rate ð _ m water Þ, specific energy consumption (SEC), specific thermal energy consumption (STEC), specific electrical energy consumption (SEEC), specific moisture evaporation rate (SMER), thermal efficiency of the dryer ðη th Þ, exergy efficiency of the drying sections ðη Ex Þ, improvement potential (IP), and efficiency of the biomass furnace ðη BF Þ was determined using the equations presented in Table 2. The data obtained from the performance analysis of the PSBA-RMFD during the drying experiment of the paddy are presented in Table 3.   For evaluating the economy of a drying system, two important aspects should be considered: first, the investment cost (I C ); second, the production cost (C pr ), profit (PR), return on capital (ROC), payback period (PP), and net present value (NPV). I C of the PSBA-RMFD includes the costs of the biomass furnace, drying column, vibratory feeder, bucket elevator, air duct, motor, blower, and labor for construction and installation. The components of I C are listed in Table 4. C pr , PR, ROC, PP, and NPV of the PSBA-RMFD were calculated using the equations presented in Table 5.

Results and Discussion
3.1. Performance Evaluation. Table 6 summarizes the uncertainties in the parameters during the paddy drying experiment. Referring to Table 3, the ambient temperature (T1) and ambient relative humidity ranged between 30.30 and 35.60°C and 50.76% to 65.37%, respectively, with   International Journal of Food Science Specific energy consumption Specific thermal energy consumption Specific electrical energy consumption [18,34] Specific moisture evaporation rate Paddy moisture content (wet basis) Paddy moisture content (dry basis)  The efficiency of the biomass furnace as well as the temperatures of air entering and leaving the biomass furnace with time is presented in Figure 7. These temperatures varied over 30.60-35.50°C and 78.20-88.90°C, with average values of 32.73 and 83.51°C, respectively. The efficiency of biomass furnace is the ratio of the extraction of heat energy from biomass furnace or heat energy used for drying process to the heat energy produced from burning of biomass fuel, and it was calculated using Eq. (18). The efficiency of the biomass furnace was found to be 70.63-87.70%, with an average value of 79.53% and an air mass flow rate of 0.1084 kgs -1 .
The air temperature and relative humidity values of air entering and leaving the drying section with the drying time are presented in Figure 8. The temperatures of air entering and leaving the drying section were in the ranges of 75.40-81.40°C and 46.30-54.70°C, respectively, with corresponding average values of 78.15°C and 50.14°C. The relative humidity values of air entering and leaving the drying section were in the ranges of 6.77%-9.57% and 39.75%-57.45%, with corresponding average values of 8.55% and 50.35%. As shown in Figure 8, the air temperature value leaving the drying section increased with increasing in drying time, while the air relative humidity value decreased with increasing in drying time.
These, due to the heat and mass transfer coefficients, decreased with increasing in the drying time.
The paddy temperatures at the inlet, center, and outlet of the drying section with the drying time are illustrated in Figure 9. These temperatures varied from 34 Figure 9, the paddy temperature at the center of the drying section was observed to be higher than the paddy temperatures at the inlet and outlet of the drying section due to high heat transfer rate in the drying section.
The variation in the paddy moisture content and drying rate with the drying time is presented in Figure 10. The paddy moisture content inside the drying section was reduced from 20.90% wet basis (initial weight 400 kg) to 13.30% wet basis (final weight 364 kg) in 270 min, for a mass flow rate of 0.1084 kg/s, average temperature of 78.15°C, and average relative humidity of 8.55%. The drying rate is defined as the mass of moisture removed from the product per unit time and it was estimated using Eq. (6). The drying rate was found to be 1.688-18.126 kg/h, with an average of 7.792 kg/h. As shown in Figure 10, paddy drying occurred in the falling rate period, and constant drying rate period was not observed. In the falling rate period, the material surface is no longer saturated with water, and the drying rate is controlled by diffusion of moisture from the interior of solid to the surface. During the falling rate period, the drying rate decreased with increasing in drying time, and this is due to the decreased mass transfer rate with increasing in drying time.
The variation of the moisture ratio (MR) with the drying time is illustrated in Figure 11. It is defined as the ratio of the initial moisture content of paddy to the instantaneous moisture content of paddy during the drying process, and it was calculated using Eq. (9). The moisture ratio of paddy in PSBA-RMFD decreased exponentially with increasing drying time. The continuous decrease in the moisture ratio indicates that diffusion has governed the internal mass transfer.
The variation of the SMER with the drying time is presented in Figure 12. It is defined as the ratio of the moisture evaporated from wet product to the energy input to the drying system and it was calculated using Eq. (5). The SMER was in the range of 0.122-1.308 kg/kW h, with an average of 0.562 kg/kW h. Figure 12 also shows the variation of the SEC, STEC, and SEEC with the drying time. The SEC is the ratio of the total energy input (thermal energy + electrical energy) to the drying system to the moisture evaporated from wet material, the STEC is the ratio of the thermal energy input to the drying system to the moisture evaporated from wet material, the SEEC is the ratio of the electrical energy input to the drying system to the moisture evaporated from wet material, and these were calculated using Eqs. (2)-(4), respectively. The SEC ranged between 0.806 and 8.657 kW h/kg, with an average of 4.119 kW h/ kg, while the STEC and SEEC ranged between 0.385 and 4.136 kW h/kg and 0.421 and 4.521 kW h/kg, respectively, with corresponding average values of 1.968 and 2.151 kW h/kg. As seen from Figure 12, the SMER decreased with increasing in drying time, while the SEC, STEC, and SEEC increased with increasing in drying time, and these is due to the decreased the moisture removal rate with increasing in drying time.
The inflow, outflow, and loss of exergy for the drying section during the drying experiment versus the drying time are presented in Figure 13, and these were calculated using Eqs. (13)-(15), respectively. These three parameters were found to be over 1439.  The thermal dryer and exergy efficiencies of the PSBA-RMFD versus the drying time are presented in Figure 14. The thermal efficiency of the drying system is the ratio of the energy used for moisture evaporation to the energy input to the drying system, the exergy efficiency is the ratio of exergy use in the drying of the product to exergy of the drying air supplied to the drying chamber, and these were  Figure 14, the thermal dryer and exergy efficiencies were found to be high at the beginning of the drying process and low at the end of the drying process. The high values of the thermal dryer and exergy efficiencies at the beginning of the drying process due to rapid evaporation of the surface moisture of the paddy or the drying rate is very high and vice versa. Furthermore, they decreases exponentially during the drying from inside the paddy until the end of the drying process. Figure 14 also shows the improvement potential of the dryer with the drying time and it was calculated using Eq. (17), which was in the range of 858.90-1355.62 W, with an average of 1114 W. A summary of the performance evaluation of the PSBA-RMFD is presented in Table 7.

Evaluation of the Effect of Drying Air
Temperature on the Quality of Drying Products. In the drying process, beside performance of drying system and drying time, the quality of drying product must be taken into consideration, and the quality of drying product is greatly influenced by drying air temperature. The drying air temperature may cause the quality degradation of the drying product. In the paddy drying process, the quality of the drying product can be expressed in terms of color of paddy grains and percentages of head rice yield, broken rice, rice grouts, and rice of whitewash. The percentages of head rice, broken rice, rice grouts and rice of whitewash obtained by milling the paddy. In PSBA-RMFD, the paddy grains is dried with a drying air temperature of 75.40-81.40°C, and the moisture content of the paddy grains in the dryer is reduced from 20.90% wet basis to 13.30% wet basis in 270 minutes. The values of head rice yield, broken rice, rice grouts and rice of whitewash were 67.37 ±0.69%, 26.34 ±0.80%, 3.55 ±0.51%, and 2.72 ±0.75%, respectively. Based on the Indonesian national rice quality standard (SNI No.01-6128-2015) as shown in Table 8, the quality of rice obtained by drying paddy grains using the PSBA-RMFD at a drying air temperature of 75.40-81.40°C is belong to in the third medium class. It can be stated that the quality of rice is low or there is a decrease in the quality of paddy during drying process due to the low percentage of head rice yield and high percentages of broken rice, rice groats, and rice of whitewash. The color of the paddy grain samples after successive drying time intervals is shown in Figure 15. From the figure, it can be seen that the color of the paddy grain sample during the drying process at a drying air temperature of 75.40-81.40°C changed significantly. This is due to the high drying Table 5: Equations used to determine the production cost, profit, payback period, and net present value of the PSBA-RMFD.

Economic parameter Equation
Eq. no. Ref.
Production cost C Pr = C fm + C ec + C lb + C ad (21) [54,56] Energy consumption cost C ec = C el + C bmf (22) [54] Additional cost C ad = C m + C dp (23) [54] Profit PR = TS − I C − C Pr (24) [54] Return of capital ROC = PR/I C (25) [54] Payback period PP = I C /PR (26) [57] Net present value Discounted present value (S)   9 International Journal of Food Science air temperature and longer drying time to achieve a paddy moisture content of around 13.3%.

Economic
Evaluation. An economic analysis was performed for the PSBA-RFMD to calculate the PP and NPV, which are strongly determined by I C , C pr , total sale of the dried product, and PR. I C of the PSBA-RMFD included the costs of buying the construction material and equipment and the labor cost for the construction and installation. The

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International Journal of Food Science components of I C of the PSBA-RFMD are listed in Table 4. The annual C pr (drying cost) was classified into direct and fixed costs. The direct costs included the cost of buying the fresh paddy, labor cost, electricity cost, and biomass fuel cost; these costs were found to be 110288.87, 5036.02, 2337.77, and 1208.64 USD/year, respectively. The fixed costs included the maintenance cost and depreciation, and these were found to be 76.94 and 346.23 USD/year, respectively. Then, the annual C pr (drying cost) was found to be 119294.47 USD/year. On the contrary, the annual PR and the I C were 2003.34 and 3846.95 USD, respectively. Therefore, PP and NPV were found to be approximately 1.9 years and 7472.34 USD, respectively. A summary of the economic evaluation of the PSBA-RMFD is presented in Table 9.

Conclusions
A PSBA-RMFD with a drying capacity of 400 kg/h was tested and evaluated for its performance. The results showed that the PSBA-RMFD reduced the paddy moisture content from 20.90% (wet basis) to 13  Mass of wet of paddy (kg) Rh amb : Ambient relative humidity (%) Rh i,ds and Rh o,ds : Relative humidity entering and leaving the drying section, respectively (%) T1 = T amb ðdbÞ : Dry bulk ambient temperature (°C) T2 = T amb ðwbÞ : Wet bulk ambient temperature (°C) T3 = T ai,bf : Air temperature at the inlet of the biomass furnace (dry bulk) (°C) T4 = T ao,bf : Air temperature at the outlet of the biomass furnace (dry bulk) (°C) T5 = T ai,ds ðdbÞ : Temperature of dry bulk air entering the drying section (°C) T6 = T ai,ds ðwbÞ : Temperature of wet bulk air entering the drying section (°C) T7 = T pdi,ds : Paddy temperature at the inlet of the drying section (°C) T8 = T pdc,ds : Paddy temperature at the center of the drying section (°C) T9 = T pdo,ds : Paddy temperature at the outlet of the drying section (°C) T 10= T ao,ds ðdbÞ : Dry bulk air temperature at the outlet of the drying section (°C) T 11= T ao,ds ðwbÞ : Wet bulk air temperature at the outlet of the drying section (°C) V: Voltage (V) Δt: Interval of drying time (h).

Data Availability
The data from drying experiment can be access at the following link: https://drive.google.com/file/d/ 1pgSOLhNxs63UI785y-gMhXwg6Oe_a_vF/view?usp= sharing. And also the data from drying experiment is already in the manuscript and can be seen in Table 3.

Additional Points
Highlights. (i) A pilot-scale biomass-assisted recirculating mixed-flow drying system for drying paddy (PSBA-RMFD) was tested. (ii) The average specific energy consumption (SEC), specific thermal energy consumption (STEC), and specific thermal energy consumption (SEEC) were 4.119, 1.968, and 2.151 kW h/kg of water evaporated, respectively. (iii) The average thermal dryer efficiency was 36.11%. (iv) The average exergy efficiency and payback period of the dryer were 19.46% and 1.9 years, respectively. (v) The heat energy required in the drying process was supplied only by biomass energy

Conflicts of Interest
The authors declare that they have no conflicts of interest.