This study was conducted in major rice mill clusters (viz. Dinajpur, Naogaon, Bogra and Ishawrdi) of the northern part of Bangladesh. There are hundreds of rice mills in each cluster. An aerial view of a large rice mill cluster at Pulhat, Dinajpur district of Bangladesh. The different unit operations of rice food grain processing were critically analyzed to determine the energy consumption from different sources. Mainly there are three different unit operation of rice food processing, viz. i) parboiling, ii) drying and iii) milling. Out of these three unit-processes parboiling and drying need thermal energy.
The parboiling process is the partial boiling of rough rice (paddy) with steam. The parboiling process requires energy from steam and this is produced by using rice husk. The quantity of the husk fed to the furnace for parboiling purposes was measured directly by a weighing balance. The quantity of the paddy parboiled in a batch was recorded from the register of the rice mill. Then the energy required for parboiling of paddy is calculated using Equation (1):
Where:
- Eparboil = energy required for parboiling of paddy, kJ/tonne
- Whusk = weight of husk required for parboiling, kg
- Wpaddy = weight of paddy parboiled, tonne
- Chusk = calorific value of rice husk, kJ/kg
After parboiling the paddy is left to dry. Drying is the removal of moisture from parboiled paddy. The drying operation is mainly done on a floor under direct sunlight radiation. The drying process requires a huge amount of thermal energy for removing the moisture from paddy. The moisture content of the parboiled paddy was determined by digital moisture meter at field level as well as by oven dry method. The initial moisture content after parboiling was 32% and after drying the moisture content was 14%. To calculate the water removed from rough rice during drying process, the following Equation (2) is used. The thermal energy requirement for the sun drying of paddy is calculated using Equation (3):
where:
- Ww = water removed from paddy, kg
- Mw = moisture content in paddy, decimal
- Wd = weight of dry mass of paddy, kg
where:
- Esun_drying = energy required for sun drying, kJ
- Ww = water removed from paddy, kg
- L = latent heat of evaporation of water, kJ/kg
The energy required for mechanical drying comes from two sources viz. heat energy from rice husk combustion and electrical energy from grid for blowing hot air. The quantity of husk required for drying purposes was weighed by a balance. The quantity of electrical energy consumed for drying was recorded from the energy meter. The quantity of energy required for mechanical drying are calculated as follows Equation (4) & Equation (5):
where:
- Eheat_drying = heat energy required for mechanical drying of paddy, kJ/tonne
- Whusk = weight of husk required for mechanical drying, kg
- Wpaddy = weight of paddy dried, tonne
- Chusk = calorific value of rice husk, kJ/kg
where:
- Eelec_drying = energy required for mechanical drying, kWh/tonne
- EkWh = electricity consumed during mechanical drying operation, kWh
- Wpaddy = weight of paddy dried, tone
After drying the paddy is milled to get rice food grain. Rice husk is obtained as a by-product of milling that is ready for using as energy for parboiling. Electrical energy from the national grid is used for milling of paddy. The quantity of electrical energy was calculated from the energy meter reading and the monthly electricity bill. The electricity needed for milling of paddy is measured as following Equation (6):
where:
- Emilling = energy required for milling, kWh/tonne
- EkWh = electricity consumed during milling operation, kWh
- Wpaddy = weight of paddy milled, tonne
The energy demand in rice processing in the future and the potential global emissions from rice processing activities were analyzed by using energy planning and mitigation software ‘Long-range Energy Alternatives Planning System’ (LEAP). This software was downloaded from the Internet. It provides a free license for users from developing countries. A scenario was analyzed using LEAP software to see the energy consumption pattern and to determine the global warming potential from rice processing in Bangladesh. Energy demand is calculated as following Equation (7):
where:
- Edemand = energy demand, GJ
- A = activity level, tonne paddy processed
- EI = energy intensity for the activity, GJ/tonne
To address the emissions, LEAP includes a Technology and Environmental Database (TED) The Technology and Environmental Database. that provides extensive information describing the technical characteristics and environmental impacts of a wide range of energy technologies, including existing technologies, current best practices and next generation devices. To calculate the CO2 emission, chemical composition of rice husk and the intensity of husk consumption were provided into the LEAP software before running the scenario. To calculate the emission from electricity generation, the type of power plant such as Hydro, Gas turbine, CC gas turbine, thermal power plant in context of Bangladesh were considered in LEAP program. The rated capacity of different power plant as of Bangladesh was directed in LEAP program. The power generation plants were assessed based on their performance and the 220 MW Hydro power plant was found to be the most efficient one. The mathematical expression of the emission that references the chemical composition of the fuel is given by the following Equation (8):
where:
- LoadingCO2 = carbon dioxide emission, kg
- Ccontent = carbon content in specified fuel, kg
- FO = fraction oxidized
- CO2, C = constant
The emissions are calculated by the LEAP program based on the following Equation (9):
where:
- Econsumption = energy consumption
- Ef = emission factor