Scale‐up in combustion has been a fundamental driver in the past to utilize the economy of scale effect in industrial combustion plants. Several scaling methods have been used successfully in engineering applications to transfer the properties of smaller equipment onto larger equipment. Today, fluid dynamic scaling based on scaling criteria derived from fundamental fluid dynamic equations, describing the movement of the two‐phase medium in a fluidized bed reactor, can be performed with sufficient accuracy, using cold flow models. However, combustion scaling using laboratory data obtained from a small‐scale boiler inherits the difference that complete fluid dynamic similarity can never be obtained. In complete boiler design the main problem is the scale‐up of known features to very large sizes; scaling can then use elements already developed in similar size and employ them in multiple numbers to reach the required throughput. In bubbling fluidized bed technology the upscaling has reached a limit, while in circulating fluidized bed boilers the strong upscaling tendency shows a slower movement. The main challenges today are the substitution of an existing fuel with a more challenging counterpart, or the use of low‐grade fuel. This can cause severe problems in industrial‐sized plants, mainly in terms of fuel feeding, ash discharge, and the erosion and corrosion behaviors of the flue gases. Further challenges of today exists in the increase in pressure on the return of investment of R&D projects, the high requirements on plant availability, even on prototypes, and the constant reduction in R&D budgets.
Biomass, a renewable fuel source for generating energy, is available in large quantities in the USA. Typical biomass consists of wood chips, construction and demolition wood, bark, residual logging debris, saw dust, paper rejects, and paper and sewage sludge. Composition and moisture content of biomass vary greatly and affect its heating value. There are several combustion technologies available to generate power from biomass. Fluidized bed boilers are preferred, because of their ability to burn a wide variety of biomass fuels while achieving high combustion efficiency and low emissions. This paper discusses basic design and operation features of bubbling (BFB) and circulating fluidized bed (CFB) boilers, both offering high fuel flexibility. In fluidized bed combustion, reactive biomass fuels are almost completely burned out because of close contact between the hot bed material and the fuel. In advanced BFB and CFB boilers, an open bottom design is used for ash and coarse material removal through the fluidizing air distribution system. This allows combustion of fuels containing large inert particles, such as rocks and metal pieces. If limestone is added to the bed, SO2 emissions are reduced. By using ammonia or urea in high temperature areas, NOx emissions are reduced. In order to achieve very low emissions, back-end flue gas treatment for SO2, NOx, HCl, HF, and Hg is required. To treat flue gases, several technologies can be used — such as activated carbon and sodium bicarbonate or Trona injection, Turbosorp® circulating dry scrubber, and SCR. Normally the preferred particulate matter cleaning device is a baghouse since the filter cake allows further reactions between pollutants and sorbents. Different fluidized bed designs are shown and recommended for various biomass fuels. This paper describes design, fuels, and emissions for an advanced BFB boiler producing steam at a rate of 230,000 lb/hr/930 psig/860°F (29.0 kg/s/64 barg/460°C).
The sections in this article are Introduction General Aspects of Combustion Excess Air Formation of Nitrogen Oxides Staged Combustion and Flue‐Gas Recirculation in Gas‐ and Oil‐Fired Applications Low Excess Air Staged Combustion Flue‐Gas Recirculation Reburning Stage Combustion in Modern Grate‐Fired Applications Advanced Secondary Air Supply Stage Combustion and Flue‐Gas Recirculation in Modern Bubbling Fluidized‐Bed Boiler Applications Staged Combustion Bed Fluidization Bed Temperature Outlook Summary
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