THERMOCHEMICAL CONVERSION: DIRECT COMBUSTION - Biomass as a Sustainable Energy Source for the Future - page 286

the entire solid flow in the furnace is entrained and circulated. The CFB takes the total
space of the furnace. In both the BFB and the CFB, the residence time of the solids in
the furnace is markedly longer than that of the gas. Figure 9.7 presents an example of a
CFB combustor integrated in a boiler system.
9.4.3 Pulverized Fuel Firing/Entrained Flow Combustion
In pulverized fuel firing systems, the particles are carried along with the air and com-
bustion gas flow. Pulverized fuel and combustion air are injected into the furnace and
mixed. As the solid fuel has been milled up to very small size and the flow velocities of
the combustion gases are high, there is no slip velocity, and the residence times of the
fuel particles and the gas are almost equal. The combustion of pulverized fuel is a
rapid process, which is distributed over the entire furnace, so higher capacities are
possible than in grate or fluidized bed firing systems.
9.4.4 Comparison of Different Firing Systems
Table 9.1 presents the advantages and disadvantages of the different combustion sys-
tems. The choice of the combustion system depends on the properties of the fuel and
on the capacity for generating steam. Capacities for different solid fuel combustion
systems offered on the market are shown in Table 9.2.
9.4.5 Power Generation
The heat produced by burning biomass is often transferred to a working medium,
which is used subsequently in a heat engine to create mechanical energy for power
generation. Steam turbines are the most widely spread technology for electricity gen-
eration in thermal power plants with capacities from 1 to several 100 MW. Other pro-
cesses to generate power are based on the organic Rankine cycle, the externally fired
combined cycle, or the Stirling engines. Biomass-fueled combustion plants normally
have net electrical efficiencies (based on the lower heating value) between 25 and
30%. The main reason for this low efficiency is that the work produced by expansion
of the gases, as they are combusted and heated up, is not utilized in standard combus-
tion equipment such as boilers and furnaces.
9.4.6 Oxyfuel Combustion and Carbon Capture
Oxyfuel combustion is being developed for CFBs and also for pulverized coal plants.
The main products of oxyfuel combustion are the usual carbon dioxide and water.
Pulverized coal oxyfuel combustion burns coal in a mixture of recirculated flue
gas and pure oxygen, rather than in air. The flue gas that is not recirculated is rich
in carbon dioxide and water vapor, which makes it a good candidate for treatment
by condensation of the water vapor and subsequent storage of the CO 2 .
A driver for using optimized oxyfuel combustion power plants is the fact that they
produce ultralow emissions. In addition, oxyfuel power cycles have the flexibility to
use coal- or biomass-derived syngas containing both CO and H 2 for combustion in a