Estimating the fuel moisture content to control the reciprocating grate furnace firing wet woody biomass

• Published in: Energy conversion and management 2017-10, Vol.149, p.937-949
• Main Authors: Striūgas, N., Vorotinskienė, L., Paulauskas, R., Navakas, R., Džiugys, A., Narbutas, L.
• Format: Article
• Language: English
• Subjects: Automatic control; Biomass; Boiler furnaces; Boilers; Combustion; Condensing economiser; Data smoothing; District heating; Flue gas; Fuel combustion; Furnaces; Heat balance; Mathematical analysis; Moisture content; Oscillations; Parameters; Studies; Thermal capacity; Water content; Wood chips
• ISSN: 0196-8904; 1879-2227
• DOI: 10.1016/j.enconman.2017.04.014

•Combustion of biomass with varying moisture content might lead to unstable operation of a furnace.•Method for automatic control of a furnace fired with wet biomass was developed.•Fuel moisture is estimated by cost-effective indirect method for predictive control.•Fuel moisture estimation methods and furnace control algorithm were validated in an industrial boiler. In small countries like Lithuania with a widespread district heating system, 5–10MW moving grate biomass furnaces equipped with water boilers and condensing economisers are widely used. Such systems are designed for firing biomass fuels; however, varying fuel moisture, mostly in the range from 30% to 60%, complicates the automated operation. Without manual adjustment of the grate motion mode and other parameters, unstable operation or even extinction of the furnace is possible. To ensure stable furnace operation with moist fuel, the indirect method to estimate the fuel moisture content was developed based on the heat balance of the flue gas condensing economiser. The developed method was implemented into the automatic control unit of the furnace to estimate the moisture content in the feedstock and predictively adjust the furnace parameters for optimal fuel combustion. The indirect method based on the economiser heat balance was experimentally validated in a 6MW grate-fired furnace fuelled by biomass with moisture contents of 37, 46, 50, 54 and 60%. The analysis shows that the estimated and manually measured values of the fuel moisture content do not differ by more than 3%. This deviation indicates that the indirect fuel moisture calculation method is sufficiently precise and the calculated moisture content varies proportionally to changes in the thermal capacity of the economiser. By smoothing the data using sliding weighted averaging, the oscillations of the fuel moisture content were identified.