Title:
Integration of a Torrefaction Process in a Large-Scale Pulverized Coal-Fired Power Plant for the Reduction of Energy Requirement for Biomass Milling at Higher Co-Firing Rates
Author(s):
Distler, T., Everts, B., Kather, A.
Document(s):
Paper
Abstract:
When increasing the cofiring rate of wood chips with coal in largescale pulverized coalfired power plants to rates up to 50 % rated thermal input, the effort to grind the biomass in terms of internal power consumption increases tremendously and tends to become the largest contributor to overall internal power demand. The pretreatment of wood chips using a torrefaction process can be a suitable measure to reduce this effort to a magnitude comparable to milling coal. In this paper, the dependency of the demand of electrical energy for milling and the demand of thermal energy used for an integrated torrefaction process and the impact of both on the overall process parameters efficiency and power output are analysed for an existing hard coal fired power plant with a net efficiency of 39,6 %net,LHV. The heat demand of the torrefaction reactor to heat up, dry and torrefy the biomass is supplied by steam extracted from the power plant process. The basis of this work is a detailed process model of an existing German hard coal fired power plant using the power plant simulation software Ebsilon® professional. The model was adjusted and supplemented with models for biomass milling and drying obtained in literature as well as with a torrefaction model based on the work of di Blasi/Lanzetta and Bates. The extraction of steam from the power plant to reach the targeted 250 – 300 °C for torrefaction operation is possible at two different positions. One position is the pipe from the HP turbine to the reheater, allowing a maximal torrefaction temperature of 250 °C. This integration variant appears to be not feasible because of very long residence times for torrefation caused by low reaction rates at 250 °C. The second possible position for steam extraction is the live steam pipe with about 190 bar steam pressure. For this integration variant and torrefaction at the maximum torrefaction temperature of 300 °C, an optimal operation point with maximal net power production and good overall efficiency was found with a degree of torrefaction/anhydrous weight loss (AWL) of 16 %. Compared to direct cofiring of wood chips with 20 % humidity, the analysed process with integrated torrefaction at 300 °C offers advantages of approximately + 0.14 %pts. in net efficiency per 10 % cofiring rate. The derating of net power output with direct cofiring is reduced by 15.8 MW per 10 % cofiring rate with indirect cofiring. The hammer mill consumes 21.4 MW per 10 % cofiring rate for fresh wood milling and 355 kW per 10 % cofiring rate for milling of torrefied material with AWL 16 %.
Keywords:
coal, efficiency, torrefaction, wood chips, cofiring, integration
Topic:
Biomass Conversion Technologies for Heating, Cooling and Electricity
Subtopic:
Solid biofuels combustion in large utilities
Event:
22nd European Biomass Conference and Exhibition
Session:
2AO.7.4
Pages:
418 - 425
ISBN:
978-88-89407-52-3
Paper DOI:
10.5071/22ndEUBCE2014-2AO.7.4
Price:
FREE
