This work presents a dynamic model of the reactive side of large-scale fluidized bed (FB) boilers that describes the infurnace transient operation of both bubbling and circulating FB boilers (BFB and CFB, respectively). The model solves the dynamic mass and energy balances accounting for the bulk solids, several gas species, and the fuel phase. The model uses semiempirical expressions to describe the fluid dynamics, fuel conversion, and heat transfer to the furnace walls, as derived from units other than the studied ones. The model is validated against operational data from two different industrial units: an 80 MW CFB and a 130 MW BFB, both at steady-state and transient conditions. The validated model is used to analyze: (i) the performance of the reactive side of two FB boilers under offdesign, steady-state conditions of operation; and (ii) the open-loop transient response when varying load or fuel moisture. The results underline the key role of heat capacity on the stabilization time. Within a given unit, the differences in heat capacity between the top and bottom of the furnace affect also the stabilization times, with the furnace top (lower heat capacity) being 1−3 times faster in the CFB unit and up to 10 times faster in the BFB unit. Due to the differences in gas velocity, the investigated boilers are found to stabilize more rapidly to input changes when running at full load than at partial load. Lastly, a variable ramping rate analysis shows that the inherent transient responses of the reactive side disappear when disturbances are introduced at (slower) rates, typical of industrial operation. Thus, the reactive side could be modeled as pseudo-static.
The cyclic carbonation-calcination of CaCO3 in fluidized bed reactors not only offers a possibility for CO2 capture but can at the same time be implemented for thermochemical energy storage (TCES), a feature which will play an important role in a future that has an increasing share of non-dispatchable variable electricity generation (e.g., from wind and solar power). This paper provides a techno-economic assessment of an industrial-scale calcium looping (CaL) process with simultaneous TCES and CO2 capture. The process is assumed to make profit by selling dispatchable electricity and by providing CO2 capture services to a certain nearby emitter (i.e., transport and storage of CO2 are not accounted). Thus, the process is connected to two other facilities located nearby: a renewable non-dispatchable energy source that charges the storage and a plant from which the CO2 in its flue gas flow is captured while discharging the storage and producing dispatchable electricity. The process, which offers the possibility of long-term storage at ambient temperature without any significant energy loss, is herein sized for a given daily energy input under certain boundary conditions, which mandate that the charging section runs steadily for one 12-h period per day and that the discharging section can provide a steady output during 24 h per day. Intercoupled mass and energy balances of the process are computed for the different process elements, followed by the sizing of the main process equipment, after which the economics of the process are computed through cost functions widely used and validated in literature. The economic viability of the process is assessed through the breakeven electricity price (BESP), payback period (PBP), and as cost per ton of CO2 captured. The cost of the renewable energy is excluded from the study, although its potential impact on the process costs if included in the system is assessed. The sensitivities of the computed costs to the main process and economic parameters are also assessed. The results show that for the most realistic economic projections, the BESP ranges from 141 to −20 $/MWh for different plant sizes and a lifetime of 20 years. When the same process is assessed as a carbon capture facility, it yields a cost that ranges from 45 to −27 $/tCO2-captured. The cost of investment in the fluidized bed reactors accounts for most of the computed capital expenses, while an increase in the degree of conversion in the carbonator is identified as a technical goal of major importance for reducing the global cost.
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