Limestone combustor

Bed-to-Surface Heat Transfer. Bed-to-surface heat-transfer coefficients in fluidized beds are high. In a fast-fluidized bed combustor containing mostly Group B limestone particles, the dense bed-to-boiling water heat-transfer coefficient is on the order of 250 W/(m -K). For an FCC catalyst cooler (Group A particles), this heat-transfer coefficient is around 600 W/(600 -K). 

A fluidized bed is an excellent medium for contacting gases with sohds, and this can be exploited in a combustor because sulfur dioxide emissions can be reduced by adding limestone, CaCO, or dolomite, CaCO MgCO, to the bed. 

FluidiZed-Bed Combustion. Fluidized-bed combustors are able to bum coal particles effectively in the range of 1.5 mm to 6 mm in size, which are floating in place in an expanded bed (40). Coal and limestone for SO2 capture can be fed to the combustion zone, and ash can be removed from it, by pneumatic transfer. Very Htfle precombustion processing is needed to prepare either the coal or the sorbent for entry into the furnace (41). 

SNR s fluidized-bed cogeneiation system is an early example of the commercial development of AFBC technology. Foster Wheeler designed, fabricated, and erected the coal-fired AFBC/boHer, which generates 6.6 MWe and 37 MW thermal (also denoted as MWt) of heat energy. The thermal energy is transferred via medium-pressure hot water to satisfy the heat demand of the tank farm. The unit bums 6.4 t/h of coal and uses a calcium to sulfur mole ratio of 3 to set the limestone feed rate. The spent bed material may be reiajected iato the bed as needed to maintain or build bed iaventory. The fly ash, collected ia two multicyclone mechanical collectors, may also be transferred pneumatically back to the combustor to iacrease the carbon bumup efficiency from 93%, without fly ash reiajection, to 98%. 

PressurizedFIuidized-Bed Combustors. By 1983 the pressurized fluidized-bed combustor (PFBC) had been demonstrated to have capacities up to 80 MWt (49). PFBCs operate at pressures of up to 1500 kPa (220 psi) and fluidization velocities of 1—2 m/s. Compared to an AFBC of the same capacity, a PFBC is smaller, exhibits higher combustion efficiencies with less elutfiation of fine particles, and utilizes dolomite, CaCO MgCO, rather than limestone to capture SO2. 

Desulfurization with Dry Lime Limestone or lime or dolomite (CaCO,3-MgCO,3) in a fluidized bed coal combustor reacts with SO9 in... 

Other applications of microparticles include spray drying, stack gas scrubbing, particle and droplet combustion, catalytic conversion of gases, fog formation, and nucleation. The removal of SO2 formed in the combustion of high-sulfur coal can be accomplished by adding limestone to coal in a fluidized bed combustor. The formation of CaO leads to the reaction... 

The ratio of limestone to fuel required for effective desulphurisation depends upon both the mineral and sulphur content of the coal. It may be possible to burn low sulphur (<0.8%) western coal directly with 90% sulphur retention thus avoiding the necessity for flue gas desulphurisation and still meeting environmental SO2 emission regulations (18). Fluidyne (19) have indicated that a 3.65% Illinois coal with 0.3 lbs. of dolomite added per pound of coal in a fluidised bed combustor can reduce SO2 emission to less than 1.2 lbs. of SO2 per million BTU thermal output thus meeting U.S. EPA limits for large plants. 

Pulverized coal combustion systems are most commonly used in power plants. In pulverized coal combustion, temperatures typically reach around 1480 °C at atmospheric pressure. In the past couple of decades, fluidized bed combustion (FBC) technologies have been commercialized. These combustors often use limestone bed materials to capture sulfur gases. They operate at about 880 °C and usually at atmospheric pressure (Smoot and Smith, 1985), 38. 

The combustion of sulfur-rich char is accompanied by the production of an undesirable reaction product, viz., sulfur dioxide. However, most of the sulfur dioxide should be removed from the combustion gases before they leave the combustor. This may be accomplished by the introduction into the combustor of suitable additives which can absorb sulfur dioxide. Limestone is such an additive. The limestone reacts with sulfur dioxide in the presence of oxygen to form calcium sulfate, which is a solid product and can be easily removed from the reactor. In this work, a model is proposed for the prediction of sulfur dioxide removal from the combustion gases, based on knowledge of gas-solid reactions taking place on a single pellet. 

The mathematical model for char combustion described in the previous two sections is applicable to a bed of constant volume, i.e., to a fluidized bed of fixed height, Hq, and having a constant cross-sectional area, Aq. The constant bed height is maintained by an overflow pipe. For this type of combustor operating for a given feed rate of char and limestone particles of known size distributions, the model presented here can predict the following ... 

The above calculation is quite tedious and gets complicated by the fact that the properties which ultimately control the magnitude of these fourteen unknown quantities further depend on the physical and chemical parameters of the system such as reaction rate constants, initial size distribution of the feed, bed temperature, elutriation constants, heat and mass transfer coefficients, particle growth factors for char and limestone particles, flow rates of solid and gaseous reactants. In a complete analysis of a fluidized bed combustor with sulfur absorption by limestone, the influence of all the above parameters must be evaluated to enable us to optimize the system. In the present report we have limited the scope of our calculations by considering only the initial size of the limestone particles and the reaction rate constant for the sulfation reaction. 

Modelling of the Limestone-Sulfur Dioxide Reaction in a Fluidized-Bed Combustor" Fuel 1973, 121-127. 

Saraiva et al [121] presented an extended model for a circulating atmospheric fluidized bed combustor (CAFBC) which included hydrodynamics for the fast section at the top of the bed as well a bubbling bed section at the bottom of the CAFBC. For the fast section of the bed, one dimensional momentum and energy balances were used to predict the temperature and velocity profiles for gas and particles throughout the reactor. The model contain species mass balances for five gas species including SO2, as well as a model of SO2 retention by limestone particles. A bubbling bed model was considered to simulate the chemical process at the bottom of the combustor. 

The reactivity of limestones with respect to the reaction with sulfur dioxide varies markedly. For example, for a given fluidised bed combustor, the Ca S stoichiometric ratio required to achieve a 90 % reduction in sulfur emission at atmospheric pressure, varies from 2 to 5. The reasons for such a variation are not understood, but are likely to include decrepitation, catalytic effects of minor components such as iron, and the structure of the limestone and lime [12.12]. Laboratory test methods have been developed for predicting the performance of sorbents [12.13,12.14]. 

D.C. Fee et al., Sulfur control in Fluidised Bed Combustors Methodology for Predicting the Performance of limestone and Dolomite Sorbents , Argonne National Laboratory, Illinois, 1980. 

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