When rice husks arrive at the project, they will be taken to the indoor storage awaiting being fed to the furnace. The maximum rate of conveying rice husks to the furnace is set to be 20 tons per hour while the fuel consumption rate is noted to be 471 tons per day. In the first step, a belt conveyor will be used to carry rice husks to a grinder. Then the ground rice husks will be carried by a pneumatic conveyor to be kept at the 170 cubic meter capacity service silo before they are further fed to the furnace. Filters are installed to trap rice husks and particulates from escaping out of the silo.

The production process starts with the continuous conveying of rice husks in a sequential order as follows: from the storage yard to the indoor storage, to the grinder, to the service silo and finally to the furnace chamber of a boiler. Rice husks will be burned at a temperature of 800-900 degrees C. The resulting thermal energy will be used to boil water to generate steam of 480 degrees C. This steam will be used to move the turbines of a generator to generate electricity. Steam which have passed the electricity generation process will be cooled down by a condenser as a condensate. This condensate will be transported as drops of water to a boiler to be recycled as steam. Meanwhile, the cooling water used for condensing steam, which now become hotter will be cooled in a cooling tower for reuse. This system is thus a closed-circuit type cooling system.

Boiler or Fuel Combustion System
After the combustion chamber is started up by fuel oil until reaching a temperature of 700-800 degrees C, then ground rice husks will be fed from a service silo while the use of fuel oil is gradually decreased until all rice husks ignite continuously, then a complete stop of fuel oil can be carried out.
The combustion system in the combustion chamber is a suspension-fired combustion, where ground husks from a service silo are fed to a fuel-air mixing system and are compressed by air from a primary air fan which is injected through a burner and then the mixture is carried into the boiler furnace on the front portion about the furnace's middle height. The burner has adjustable vanes to force the fuel-air mixture to circulate for proper combustion in the combustion chamber at temperature around 800-900 degrees C. At the lower side of the combustion chamber, there is a compression port where outside air is drawn by a forced draft fan and blown through an economizer in the exhaust port to preheat the air. Subsequently, the hot air will be compressed through this port in a quantity of 30 percent in excess of the need of the system to prevent incomplete combustion which will result in the occurrence of carbon monoxide (CO) and, at the same time, the air will suspend the husks in the suspension firing zone. Husks subjected to grinding will be broken into small lightweight particles, thus enabling faster combustion while those with more weight and mass will become bottom ash after burning and fall to the inclined bottom of the furnace and flow out through the ash port to be swept by a screw conveyor for removal. Lightweight husks, after burring, will be mixed in the flue gases and are carried out of the combustion chamber through the flue port. These so-called "fly ashes" will be collected by an electrostatic precipitator (ESP) before the flue is discharged to tht' atmosphere. Air which is being compress~d into a combustion chamber by a forced draft fan will be controlled by inlet guide vanes. When the pressure inside the combustion chamber is in balance with outflowing flue gases, the heat obtained from flue gases will be used to vaporize water to become steam having temperature of 480 degrees C. Then the flue gases used in heating water will be released through an economizer to exchange remaining heat for efficient heat recovery. An induced draft fan will draw the flue gases to an ESP to trap fly ash in flue gases before they are released from the stack.

Steam Turbine and Generator
Hot steam from a boiler will be transferred to a steam turbine where the thermal energy of the steam is converted to mechanical energy. Inside the boiler, a governor is fitted to regulate the steam flow rate and the speed of rotors to be at 15,000 rpm and subsequently this speed will be reduced to 3,000 rpm by a reduction gear unit in order to turn a 11.5 KV turbine generator. Then the mechanical energy will be converted to electric energy which is transmitted by a generator breaker and a step-up transformer to generate a voltage of 115 KV for subsequent distribution through the transmission line of the Provincial Electricity Authority (PEA).

Condenser
Steam that has been used in the process will be sent to a condenser where it is liquefied to become condensate flowing into a holding reservoir. The resultant water which is still hot will be recycled to the boiler for generating steam.

Cooling Tower
This cooling system removes heat from water coming out off a condenser where the cooling water has cooled steam used in turning a turbine. Heat will be transferred from steam to the cooling water (the temperature of the incoming cooling water is 37.5 degrees C) and the heat in the cooling water is removed by a fan blower and will be dissipated together with evaporated water into the atmosphere. The outgoing cooling water now will a temperature of 32 degrees C will be combined with madeup water supplied from the a water reservoir to compensate for evaporating water and subsequently it will be fed to a condenser to remove heat from steam in the process circuit for another cycle. In this regard, water from this system will be recycled at a daily rate of about 1,922 cubic meters and use in the cooling system for not less than 10 times.

A.T.Biopower Company Limited has selected suspension - fired combustion technology which provides high efficiency in combustion, resulting in economy in fuel consumption for power generation and better control of consistent quality of ash which is a by - product so that ash with more quality benefits is produced.

Generally, in a suspension - fired combustion system, the boiler furnace is designed for use with solid fuel, but employing the principle of liquid fuel combustion such as natural gas and oil . Combustion of liquid fuel occurs when the fuel flows upwards into the air which is achieved by the force of a fuel pump. This pump will atomize the liquid fuel from a burner in the form of mist or spray . When the atomized liquid fuel comes in contact with the heat in the furnace, it will ignite as a flame in the air or become burned in the air.

A distinct example in applying a suspension - fired combustion system for use with solid fuel can be found in a pulverized coal burner, which is one of the forms of a suspension-fired system . But in general, this type of combustion has a specific name in accordance with the type of furnace used for combustion . In the example, the furnace is called a pulverized coal boiler . Its operation principle is based on pulverization of coal . Transport air is used as a means to transport pulverized coal and at the same time is utilized as combustion air . Excess air will be added to ensure a more complete combustion. Pulverized coal which is injected from the burner is thus burned during it suspends in the air.

The suspension-fired combustion burners used in the power plant of A.T.

Biopower Company Limited is also based on the same principle. That is to say, rice husks are ground by a grinding machine so that the finely ground rice husks in small fragments will have increased surface areas for combustion, resulting in rapid combustion and at the same time have lighter weight suitable for being transported by the transport air. Therefore, when these ground rice husks are injected through the burner, major quantities of rice husks will be burned during subject to suspension - fired combustion in the furnace while light-weight fly ash which occurs in the combustion will be blown away by air pressure.

However, as the combustion rate of rice husks is much slower than that of coal, some portions of rice husks with larger particles which have not yet burned completely will fall onto a grating plate at the bottom of the furnace and are subject to further combustion to become bottom ash wherein the grating plate serves to control the amount of un-burnt carbon in the bottom ash by adjusting the time in tilting these grating plates to extend or shorten combustion time . When the grating plate is tilted, bottom ash will fall down and is swept out of the suspension - fired combustion system by a screw conveyor . In respect of combustion of rice husk fuel in a suspension - fired furnace, there is no preheating of fuel while hot air will serve as transport air to transfer heat to fuel in a short period of time before it is sent to the furnace . Another reason why there is no preheating of fuel is due to the special properties of rice husks . Even though they are piled outside a facility and directly subject to the influence of heavy rain, rain water can only penetrate up to 2 centimeters while the moisture content of is only averaged around 9-10%.

Suspension-fired combustion furnaces employing agricultural residues have never been used in Thailand before . Furnaces which are widely used in Thailand include stroker combustion or fluidized bed combustion systems. For example, in general sugar factories which use bagasse as fuel, stoker furnaces are mainly employed or in rice mills which use rice husks to generate power in their factories, both stoker or fluidized bed furnaces are frequently employed. Suspension - fired combustion by employing rice husks as fuel is not much used because it is based on expensive technology when compared with other types of combustion. However, this type of combustion has been used more than 20 years, particularly in North America . For example, Agrilectric Power plant has used this type of furnace under the US environmental laws and at present there are several power plants being constructed based on this technology in South America and Australia as details shown in Table