(158f) Upgrading of Low Rank Coal By The Integration of CCS/Coal-Drying

Coal is one of the
most important energy sources in the world for electricity and other
applications. Coals are generally classified as high and low rank coals depending
on their properties, especially heating value, moisture content, impurities
etc. Above all, lignite or sub-bituminous coal is a type of low-rank coal (LRC),
and it has high moisture content (20% – 50%), lower heating value (3,000 –
4,500kcal/kg on an as-received basis), and high volatile content. These fuels
hinder the operation of power plants because the high moisture content reduces
the power plant's efficiency and increases flue gas emissions. Also, there are
some disadvantages to using LRC such as costly transportation, potential safety
hazards during transportation and storage, and operational problems. However, LRC
is a competitive primary energy source for power generation. If these fuels are
upgraded, those can be used to replace relatively expensive bituminous coals or
be blended components with high-rank coal in existing boilers or in new
designed boilers. Hence, an appropriate drying process is necessary to develop
an energy-efficient power generation systems with lower environmental impacts. Among
the other drying systems, the fluidized bed drying system has the advantage of
temperature control due to uniformity of bed temperature and high drying rate since
the fluidized bed drying involves high heat transfer and mass transfer rates
through excellent mixing and gas-solid interactions. It offers a way of drying
coal in more economical and environmentally acceptable means.

Recently, many
power plants plan to build a CO₂ capture and storage (CCS) process
because regulations for greenhouse gas emissions have been reinforced. The CO₂
concentration in earth atmosphere is increased by combusting fossil fuels to
generate electricity. Capture of CO₂ in flue-gas streams is
essential for the control of CO₂ emissions. Especially, one of the
advanced concepts for capturing CO₂ is an absorption process with
dry re-generable sorbents. This CO₂ capture process discharges CO₂/steam
mixture gas around 150^oC of temperature after regeneration reactor. Therefore,
we are to develop the fluidized bed dryer using the outlet gas out of the CCS
process.

In this study,
the purpose of our work is to integrate a CCS and a fluidized bed coal drying
system. We produced the upgraded Indonesian coal which has a high moisture
content of 35 wt.% using a batch type laboratory-scale fluidized bed dryer.

Figure 1. Schematic diagram of integrated CCS/coal-drying
system.

Figure 1 shows the concept of integrated
CCS/coal-drying system. As shown in the process flow diagram, the CO₂
in the flue gas from fossil fuel-fired power plants is captured through CO₂
capture process. Next, the CO₂ and steam after a CCS process is used
as heat sources for drying LRC. Furthermore, CO₂ is used as
fluidizing gas and steam is used for indirect coal drying through the heat
exchanger. The experiment was performed using CO₂ as a fluidization
gas in a fluidized bed dryer (a 500 mm tall pipe with an inner diameter of 80
mm) at a gas velocity ranging from 4 – 6 U_mf using a compressed
carbon dioxide gas cylinder. The inlet gas temperature was varied from 100 to
200^oC using the preheater. The drying rate of the coal was
investigated in terms of the inlet gas temperature and the gas velocity in
order to determine the optimum operating conditions. Changes in the moisture
content of the coal, before and after the experiments, were characterized by a
proximate analysis, an ultimate analysis, the higher heating value (HHV), the
lower heating value (LHV), a particle size analysis, a thermo gravimetric
analysis (TGA). In summary, we are to implement the integrated CCS/coal-drying
system using CO₂ and steam as heat sources for LRC drying.