Grate biomass

Grate-fired combustors are in use for old biomass-fired plant, while fluid bed combustors are rapidly becoming the preferred technology for biomass combustion because of their low NOx emissions. 

Thermal Biomass Conversion and NOx Emissions in Grate Furnaces... 

A popular small-scale thermal biomass conversion method is combustion in grate furnaces. To meet the emission regulations for such a furnace, the operating conditions and design of the furnace have to be chosen carefully. Numerical models, known as computational fluid dynamics (CFD), can support the making of these choices, provided that accurate sub-models for the phenomena occurring in the oven are available. 

Fig. 8.10 2D predicted profiles of (a) mean mixture fraction, (b) mean temperature and (c) mean NO mass fraction for the biomass grate furnace. 

The present research has treated important parts of the modeling of combustion and NOx formation in a biomass grate furnace. All parts resulted in useful approaches. For all these approaches successful first steps were taken. Currently, more research is underway to obtain improved results NH3 production is measured in the grid reactor with the tunable diode laser, detailed kinetics will be attached to the front propagation model, including the measured NH3 release functionalities, and for the turbulent combustion model heat losses are taken into account. In addition, the fuel layer model has to be coupled to the turbulent combustion model in the furnace. 

Albrecht, B.A., Bastiaans, R.J.M., van Oijen, J.A., de Goey, L.P.H. NOx emissions modelling in biomass combustion grate furnaces, in Proceeding of the 7th European Conference on Industial Furnaces and Boilers (INFUB ), Reis, A., Ward, J., Leuckel, W., (Eds.), Porto, Portugal, ISBN 972-99309-1-0, (2006). 

Scharler, R., Fleckl, T., Obernberger, I. Modification of a Magnussen constant of the Eddy Dissipation Model for biomass grate furnaces by means of hot gas in-situ FT-IR absorption spectroscopy,... 

Keywords Packed-bed combustion, thermochemical conversion of biomass, solid-fuel combustion, fuel-bed combustion, grate combustion, biomass combustion, gasification, pyrolysis, drying. 

In grate-fired boilers, the biomass is fed in a thin layer, so it is evenly distributed over a sloped stationary, traveling, or vibrating grate. Improved control over the combustion process can be achieved with better carbon conversion. Fluidized-bed combustors are more complex systems, but offer much better control of combustion temperature, improved carbon conversion, and fuel flexibility. Using a boiler to produce both heat and electricity (co-generation) can improve the overall system efficiency to as much as 85 percent. Boiler efficiencies are affected by fuel moisture content, air-fuel ratio, excess air, combustion temperature, and biomass ash content. 

TABLE 6.2 Typical Moisture Contents and Heating Values of Waste Biomass for Combustion in Fluid-Bed, Grate, and Suspension Firing Units... 

Stand-alone Grate firing Municipal waste RDF Industrial waste Biomass... 

We gratefully acknowledge the assistance of Dan Fuleki of Magellan, Orenda Aerospace Division, in providing performance data on gas turbines, and insight into the operating requirements of gas turbines with biomass derived fuels. 

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