(192h) Assessment of Oxy-Fuel Combustion System with Coal and Biomass Co-Firing Based on Circulating Fluidized Bed

Oxy-fuel combustion
with circulating fluidized bed (CFB) is one of the most competitive technology
for carbon capture and sequestration. Circulating fluidized bed (CFB)
combustion as a typically technical direction in oxy-fuel combustion, has
several advantages against pulverized boiler, e.g., fuel flexibility, low NOx emission,
reduction of recirculated flue gas and higher oxygen concentration. So far, few
of the research related to oxy-fuel combustion refers to the system performance
with the integration of CFB power plant, air separation unit (ASU) and carbon
dioxide compression and purification unit (CPU), especially the system
efficiency influenced by different operational parameters. In addition, biomass
as an important clean energy source, the
system performance of coal and biomass co-firing system combined with oxy-fuel
combustion is still unknown.

In this work, a
supercritical oxy-fuel combustion system based on CFB boiler firing coal and
the mixture of biomass and coal is established according to the conservation of
material and energy as shown in Figure 1. Three
primary units are arranged in the system including an ASU, a CFB power plant
and a CPU. Coal and the blends of coal and biomass burn in the boiler with the
oxidants consisting of the oxygen from ASU and the recirculated flue gas. And
the combustion process is simplified to pyrolysis, volatile combustion and char
combustion. After the validation of adiabatic combustion temperature, the
influence of wet/dry cycle, exhaust flue gas temperature and combustion
pressure on the net electrical efficiency are evaluated. The excess oxygen
ratio and oxygen concentration in the system are determined as 1.06 and 26%, respectively
based on the previous research which is satisfied to achieve better performance.
The results showed that under the same exhaust flue gas temperature, the flue
gas loss of dry cycle is higher than wet cycle mainly because the energy loss
of water condensation in dry cycle. Although the co-firing cases have less flue
gas loss than coal combustion in wet cycle, the supplement of biomass will
decrease the net electrical efficiency because of the pre-treatment of biomass as
shown in Figure 2. In addition, a sharp
increase occurs when the exhaust flue gas reaches below 60 centigrade because
the steam vapor in the flue gas begin to condense. Elevating the combustion
pressure is an effective way to improve the heat recovery. The maximum heat recovery at 1MPa is 30MW higher
than the atmospheric condition. However, the selection of combustion pressure
should also consider the auxiliary energy consumptions of ASU and CPU. In
addition, air ingression will significantly influence the net efficiency that 10%
of the theoretical air will cause 1% reduction of net electrical efficiency.

Figure 1. Flow
diagram of oxy-fuel combustion power plant based on CFB boiler applied in this
work.

Figure 2. The flue gas loss and
net electrical efficiency under varied exhaust flue gas temperature.