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State-of-the-Art Renewable Energy Technology
Published in Ramendra Sundar Dey, Taniya Purkait, Navpreet Kamboj, Manisha Das, Carbonaceous Materials and Future Energy, 2019
Ramendra Sundar Dey, Taniya Purkait, Navpreet Kamboj, Manisha Das
Biomass burning has been one of the oldest practicing renewable technologies for heat and power production. Particularly in forest and sugar industries, solid biowastes – mainly wood chips and pellets – are used to provide process heat on site, with surplus electricity sold off-site to generate revenues [7]. The combustion of wet biomass wastes, such as sewage sludge and agricultural slurries in grate-firing boilers, often produces biogas (e.g. cow dung to methane) or other fuels to be used for heat generation and various other solid by-products that find use in various industries [4].
End-of-Pipe Treatment Techniques
Published in Guttila Yugantha Jayasinghe, Shehani Sharadha Maheepala, Prabuddhi Chathurika Wijekoon, Green Productivity and Cleaner Production, 2020
Guttila Yugantha Jayasinghe, Shehani Sharadha Maheepala, Prabuddhi Chathurika Wijekoon
The incineration of sewage sludge mostly takes place in rotary kilns, multiple hearths, or fluidized bed incinerators. Co-combustion in grate-firing systems, coal combustion plants, and industrial processes is also applied. Sewage sludge often has a high water content and therefore usually requires drying, or the addition of supplementary fuels to ensure stable and efficient combustion.
A hybrid EDC/Flamelet approach for modelling biomass combustion of grate-firing furnace
Published in Combustion Theory and Modelling, 2019
Mohammadreza Farokhi, Madjid Birouk
CFD simulation of grate-firing biomass combustion usually consists of two sub-stages, known as biomass conversion in the bed and gas-phase combustion in the zone located above the bed (i.e. freeboard). Bed modelling accounts for thermal decomposition of solid fuel and surface reactions of char particles on the grate of the furnace (i.e. fuel-bed). This process describes the conversion of biomass particles located in the fuel-bed area into equivalent gas-species with similar energy and elemental mass fraction as those of biomass particles [6]. Freeboard modelling describes the interaction between volatile gases released from the fuel-bed and fresh-air in the freeboard [7]. The accuracy of the predictions of CFD simulation depends strongly on the adopted sub-models (e.g. bed, turbulence, and combustion models as well as chemical mechanism) [7,8]. Gas-phase combustion model plays a key role in the predictions of temperature and gases emissions [9,10]. The gas-phase combustion of the volatile gases in the freeboard commonly features a predominantly turbulent non-premixed flame. The decoupled mixing/chemistry combustion process, known as steady flamelet model (SFM) [11], has been extensively used in modelling non-premixed combustion, owing to its ability to incorporate detailed chemistry at low computational cost. However, since the gases released from a solid biomass fuel cannot be described by a single mixture fraction quantity, additional variables are needed to account for tabulated flamelet data [12]. Thus, the application of standard SFM in modelling grate-firing biomass furnaces is not suitable [7] and consequently species transport combustion schemes (e.g. Eddy Dissipation Concept (EDC) [13]) are commonly used. EDC benefits from real-time interaction between chemical and flow mixing processes, as well as the possibility to incorporate kinetic rates of chemical mechanisms in modelling species reaction rates. However, the application of this model poses a challenge when modelling weakly turbulent reacting flow conditions, as well as reacting flow with comparable chemical and flow time scales [10]. Such conditions may prevail close to the fuel-bed of grate-firing biomass furnaces, where high temperature volatile gases leave the solid fuel with relatively low velocity [14,9]. Moreover, since the model has to integrate chemical kinetics of species in each computational cell, using detailed chemistry leads to a high computational cost, and consequently, the model can only be used with a reduced chemistry for simulating complex and industrial geometries [7,15].