The fluidized-bed combustor with a cone-shaped bed (referred to as 'conical FBC') is used in this study, as providing the above-mentioned advantages over the combustors/furnace with a prismatic shape.
Figure 2 depicts the schematic diagram of the experimental set up with the conical FBC for the experimental tests using fuel-staged and reburning combustion methods. The combustor is designed to operate at the 300 kWth (maximum) heat input when firing various types of biomass. The combustor can also be operated for conventional combustion of individual fuels and fuel mixtures. This combustor is available at Sirindhorn International Institue of Technology (SIIT), Thammasat University, Rungsit Campus.
As seen in Figure 2, the combustor consists of two sections assembled coaxially: (1) the lower (conical) and (2) upper (cylindrical) sections of the combustor constructed of steel sheets of 4.5-mm thickness, while the reactor internal walls are lined with 50-mm-thick refractory-cement insulation to reduce heat losses across the walls. The conical section is 0.9 m high with 0.25 m inner diameter at the bottom plane and 40º cone angle, whereas the cylindrical section consists of five modules, each of 0.5 m height and 0.9 m inner diameter. This experimental set up allows operating using all the above combustion methods (see Section 5).
The primary and secondary/reburn fuels are delivered into the combustor via separate routes by two screw-type feeders. It can be seen in Figure 2 that primary fuel is injected into the conical section at a 0.65 m level above the air distributor, whereas secondary/reburn fuel is introduced into the lower region of the cylindrical section at a 1.15 m level above the air distributor. To ensure the desired fuel feed rates of the primary and secondary/reburn fuels, the rotational speed of both feeders is controlled using three-phase inverters.
Primary (or fluidizing) air is supplied to the conical FBC by a 25-hp air blower through the air pipe of 0.1 m inner diameter. In the tests for reburning, secondary air is tangentially supplied through a 0.04-m inner diameter pipe at a level of 1.6 m above the air distributor by the additional (5-hp) blower to ensure maximum possible burnout of remaining fuel fragments in the reburn zone. However, in the case of inappropriate incomplete combustion, tertiary may be introduced at a level of 2.1 m above the air distributor to ensure maximal combustion efficiency.
During the combustor start up, the bed material (fluidized by primary air) is preheated using a diesel oil-fired burner from the Riello Burners Co. (model “Press G24”), fixed at a 0.5 m level above the air distributor, until the bed temperature rises to a level of about 700 C. At this instant, the start-up burner is turned off, and the combustor starts its normal operation using (primary) fuel. In tests for fuel staging and reburning, secondary/reburning fuel is injected at the specified rate.
During the tests, temperature is recorded at different points along the reactor centerline and at stack, using eight stationary Chromel-Alumel thermocouples. In each trial at fixed operating conditions, O2, CO, CxHy as CH4, and NO are measured along the combustor height and at stack using a new model “Testo-350” gas analyzer. The heat flux along the combustor height is measured in all the test runs (provided that the heat flux meter is purchased).
A cyclone-type ash collector located downstream from the combustor is used to arrest predominant part of particulate matter (PM) originated from biomass (co-)combustion, which is then analyzed for the unburned carbon content (required for assessing the heat loss due to unburned carbon)