Combustion system


The main components of a typical biomass combustion system by VAS are indicated in the scheme below: 





  • I. Biomass Feeding


    The hydraulic moving floor (01.) carries the raw material into the fuel bunker wherefrom a chain conveyor as well as a twin hydraulic push-in system with a certified back-fire protection (02.) push the biomass fuel to the combustion chamber.


    The feeding rate is regulated automatically in order to control critical combustion parameters and ultimately sustain a steady thermal output.


  •  II. Combustion 


    The combustion system (03.) is designed as an inclined grate furnace with water-cooled feed cone and grate side cooling.


    Biomass is carried inside the furnace by a moving grate, where combustion is controlled by the the fuel feed rate and the supply of combustion air by three fans.  The primary and secondary fans (08. & 09.) of the recirculation system placed before the chimney, recirculate flue gas in order to maintain optimal combustion parameters inside the furnace.


    Precise combustion control is essential particularly when dealing with agricultural fuels which are typically characterized by aggressive chemical composition and low ash melting points. Even when the temperature inside the furnace needs to be maintained below 1000° in order to avoid ash melting and system damage, VAS moving grate system combined with precise combustion control and longer retention time of the combustion gas in the secondary and afterburning area of the furnace, deliver complete fuel combustion, high efficiency and low emissions    


    Several safety features are integrated in VAS' combustion system. In addition to a vertically arranged refractory-lined hot gas shaft right after the furnace, an emergency chimney (04.) placed on top of the combustion chamber will automatically release the heat from the furnace into the atmosphere if a failure in the thermal oil system is detected. The emergency chimney includes a hot gas resistant flap and opens under its own weight to safeguard against an event of loss of power.  


  •  III. Heat Exchange with Thermal Oil 


    The heat transfer oil system mainly consists of the thermal oil heat exchanger (05.) and the downstream economizers (06.)  


    Exhaust gases from the furnace are channeled into the thermal oil heat exchanger  to heat the thermal oil to the temperature required by the ORC unit. Exiting the thermal oil heat exchanger, exhaust gases enter the economizer tower to recover additional heat and preheat the combustion air and the thermal oil. The economizer consists of a series of different size heat exchangers including automatic heating surface blowdown as well as a heat exchanger for combustion air preheating.


    In addition to the thermal oil heat exchanger and the economiser, the thermohydraulic heat transfer oil system also includes the system for pressure maintenance, an emergency cooling facility and the synthetic heat transfer oil. 


    The thermal oil boiler pipes are cleaned during normal operation by fixed pneumatic nozzles on the bottom and lid area by 8 to approx. 12 bar compressed air (14.). Similarly all heat exchangers within the economiser tower are blown down fully automatically with 8 to approx. 12 bar compressed air by way of approx. 80 blowing lances with nozzles.


  • IV. Flue gas treatment 


    Exiting the economizer the exhausts gases are channelled to the Electrostatic Precipitator (07.) for the removal of Particulate Matter (PM).  The ESP typically reduces dust content from about 2.500 mg/Nm³ down to under 20 mg/Nm³ by two electrical fields powered by two different high voltage units within one housing.


    Depending on the characteristics of the the fuel and emission limits additional exhaust gas treatment systems such as bag filters, SNCR and additives feed in systems may be required before exhaust gases are sufficiently cleaned  to be vent into the atmosphere through the chimney (10.).


  • V. Ash removal 


    Bottom ash from the furnace, and fly ash from the economizer and the ESP are removed automatically by a hydraulic lock an ash conveying system (11. & 12.).


    Ashes are subsequently deposited in ash containers (13.


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