Distinctive features of burning of a solid biofuel in a cyclone-bed furnace

Abstract

A study has been made of the distinctive features of burning of a solid biofuel (wood, peat) in a cyclone-bed furnace. The limiting values of the moisture of wood chips as received and of the mass content of milled peat in the chip-milled peat mixture, at which the regime of its combustion is unstable, have been found. The tem- perature, velocity, and pressure distribution in the indicated furnace have been established. The dependences of the concentration of carbon and nitrogen monoxides on the excess air in exhaust gases in burning lump wood and crushed peat briquettes have been established. tangential blowing, excess air, carbon monoxide, nitrogen monoxide, temperature in the furnace, moisture of the fuel as received. Introduction. It is critical at present to improve the efficiency of furnace facilities designed for burning of fuels of biological origin such as wood and peat. These fuels are low-grade and generally have a high moisture con- tent and a comparatively low heat of combustion. The polyfractional composition varying in the process of operation and qualitative characteristics of this substandard fuel require special furnace facilities. For efficient burning of the in- dicated fuel, it is promising to use the so-called cyclone-bed furnaces, in which bed burning of a solid fuel is com- bined with the swirling-type afterburning of fines and gaseous fuel products in the superbed space (1, 2). This makes it possible to realize the furnace process with a higher thermal stress of the furnace volume, to improve the quality of burnout, and to raise the unit's efficiency by reducing excess air. Experimental Setup. The experimental setup for studying the burning of a solid biofuel represents an air- heater boiler equipped with a cyclone-bed furnace. The double furnace (Fig. 1) consists a cyclone-bed combustion chamber and an afterburner section separated by a constriction. Primary air is injected under a gas-distributing grate (bottom (undergrate) blowing); secondary air is injected tangentially (tangential blowing) via two rows of diametrically opposed holes on the cylindrical wall of the combustion chamber (12 holes of diameter d in = 7 mm). Combustion products from the afterburner section are removed to a gas-tube heat exchanger. The heating surfaces are cooled with air. To intensify heat transfer through the wall of the afterburner section there is "active heat insulation" around the latter; one basic element of the active heat insulation is a granular-material bed (3). As the fuel we used lump wood of length about 30 mm, 5 × 15 × 25 mm chips, milled peat of length 7-10 mm, and crushed peat briquettes of 15-25 mm in length; the moisture of the fuel as received did not exceed 60%. The fuel was charged to the gas-distributing grate in batches at intervals of 20 to 120 s. We carried out the experi- ments at different constriction diameters: d out ⁄ D = 0.3-0.5. Regimes of blowing-air feed that ensured the shares of bottom blowing ϕ = 0.3 and 0.5 were used. Heat release in the furnace was determined from the rate of flow of the fuel whose low heat value was Q low = 2467 kcal/kg for the wood fuel (lump wood, chips with W r = 40%), Q low = 1925 kcal/kg for the milled peat with W r = 45%, and Q low = 3392 kcal/kg for the crushed peat briquettes with W r = 11.3%. Fuels and mixtures of another (higher) moisture content were prepared from the indicated wood fuel and milled peat by moisturizing them to required values. The low heat value of the fuel was recalculated with a correction for its actual moisture content in accordance with (4, 5); the flow rate of the fuel corresponding to its moisture as re-