(670h) Coal and Biomass to Liquids and Power Generation Via Fischer-Tropsch Synthesis and Fuel Cells

Introduction

Security
of energy supply, economic sustainability, and concerns over global climate
change are strategic energy objectives of oil-importing countries [1,2]. Thus, considerable interest is focused on the so-called
coal to liquid (CTL) processes, based on Fischer-Tropsch (F-T) synthesis. The
perspective of coal as energy source depends on the success of emerging ?clean
coal technologies? (CCT) [2]. However, synthetic fuels made from coal, even
with CO₂ capture and storage (CCS), have greenhouse gas (GHG)
emission rates equal or higher than the petroleum products they replace. One
approach to address this issue is to identify options that will offset the GHG
emissions of coal processing. Co-feeding of biomass in CTL processes (called
coal and biomass to liquids - CBTL) is one of the options considered. Biomass
is among the most promising renewable energy sources, due to its neutral CO₂
life cycle. The option of biomass as a stand-alone source of fuel production is
not yet feasible, because sustainably-recovered biomass is expensive. However,
co-feeding of biomass in coal-based plants provide a viable solution for controlling
coal GHG emissions. Moreover, coal power plants are based on combined cycle
(CC) technologies. Of scientific interest is the replacement of the CC by fuel
cells (FC), to improve process efficiency and reduce carbon footprint. The
objective of this work is to evaluate the potential of emerging and efficient
technologies for the production of clean energy (liquid fuels and electricity)
based on coal and biomass. The technology options, evaluated include: (1) co-feeding of biomass in CTL plants; and (2)
partial or complete replacement of CCs with FCs.

Simulation Design Strategy

Coal
and biomass to liquids through F-T synthesis is a multi-step and energy
intensive process. Briefly, carbonaceous material (coal and/or biomass) is
gasified and the gas is processed to make purified synthesis gas. The F-T process
polymerizes syngas into valuable gasoline and diesel range hydrocarbons. In
this work, the feasibility of biomass addition to coal-based F-T synthesis was analyzed
using Aspen Plus. Southern pine wood and Illinois coal #6 were used as
feedstocks in this study [3]. As shown by the respective ultimate analysis (dry
basis) of Figure 1(b), Illinois #6 coal has approximately 20wt% more carbon and
30wt% less oxygen than Southern Pine, corresponding to a more valuable feedstock
(in terms of heating value), but also higher CO₂ emissions. The high
oxygen content in biomass diminishes its heating value. On the other hand, coal
has 12wt% ash and high sulfur content, which need to be removed during the
process.

The
process flowsheet developed is briefly shown in
Figure 2 and consists of the following subsystems: (1) preparation unit where
coal and/or biomass are milled, dried and grinded; (2) air separation unit
(ASU) where high purity oxygen (95vol%) is produced for the gasification using
conventional cryogenic technology; (3) entrained-flow oxygen-blown gasification
unit, operating at ca. 2500°F/600psia,
where coal and/or biomass are gasified to syngas; (4) water gas shift (WGS) unit
for the adjustment of H₂:CO ratio at 2.0 (as required for Co-based
F-T synthesis); (5) acid gas removal unit based on the Rectisol
process for the recovery of CO₂ and H₂S; (6) Claus unit
for the conversion of H₂S to elemental sulfur; (7) low temperature
Co-based F-T synthesis unit (operating at 480°F/450psia), where the clean
syngas is converted to valuable liquid fuels; and (8) power production unit. For
the power island two cases were considered: a) gas and steam CC, and b) solid
oxide fuel cell (SOFC). In the CC, the gas turbine utilizes tail gas from the
F-T synthesis and steam turbine utilizes steam generated in the Heat Recovery Unit
(HRU). In the case of the SOFC system the tail gas from the F-T unit first
enters a pre-reforming step, where all the light hydrocarbons are reformed and
then enters the FC Anode, where any remaining CH₄ is reformed, CO is
shifted and H₂ is oxidized with oxygen from the Cathode. The SOFC
operates at 1650°F and 1atm, at 85% fuel utilization factor. Part of the
unconverted fuel is recycled to the pre-reformer, and the remaining is
combusted. Aggressive CO₂ capture (after F-T synthesis) was
considered, but was not included in this study due to its high energy penalty.

Results and Discussion

Five
scenarios of biomass to coal ratio were evaluated (0, 15, 50, 75 and 100 wt%) with respect to their overall thermodynamic efficiency
(to liquids and power). The Co-based F-T synthesis was simulated on the basis
of a semi-empirical mathematical model [4]. In Figure 3 the effect of biomass
addition on the major plant inputs is presented for
a base case of processing 1.50E+06 lb/hr of coal and/or biomass. The oxygen
demand for the gasification process is gradually reduced as the addition of
biomass in the CBTL process is increased. The oxygen content of biomass can
serve as an oxidant source, reducing the requirements in oxygen supply.
Consequently, the power demands in the ASU unit (the most energy intensive unit
in the plant) are significantly reduced as more biomass is added in the plant.
Moreover, the steam requirements in the WGS unit are significantly reduced by
increasing biomass addition. Biomass has a higher H:C
ratio than coal, resulting in higher H₂:CO ratios after gasification;
hence, smaller steam requirements. Note that the shift steam is taken from an
intermediate extraction point in the HRU. Thus, any steam extracted for the
shift is not expanded in the steam turbine to produce power; therefore the WGS
step is typically viewed as a burden on the steam cycle.

Finally,
the methanol required in the Rectisol unit is lower
with increasing biomass addition. The reason for this is the very small sulfur
content of biomass and the lower CO₂ of the shifted syngas. It
should also be noted that the CO₂ formed by biomass gasification is
not considered as emission (assuming a close-to-zero carbon life cycle for
biomass). Thus, the energy demand is significantly reduced by co-feeding
biomass to the plant. However, less energy is produced (both liquids and
electricity), due to its lower energy density. In Figure 4(a) the energy input
(based on HHV of feed), the energy production (based on HHV of liquids and
electricity via the CC) and the plant energy consumption are plotted as a function
of biomass addition. In Figure 4(b) the efficiency of the CBTL plant to liquids
and CC electricity, and the overall efficiency are plotted as a function of the
biomass addition. It is obvious that, albeit the lower power production, the
overall efficiency of the CBTL plant increases by adding biomass, due to the
lower plant energy requirements.

To increase the
overall electricity production, the replacement of the CC with SOFCs was
studied. A tubular SOFC system, developed by Siemens Power Generation Inc. (SPGI)
[5], was considered. The current density was set to 2000A/m². The
results are presented in Table 1. The SOFC system significantly improves the
electricity production compared to the CC.

Table 1: Net electricity production based on CC
and SOFC as a function of biomass addition

Conclusions

Biomass
co-feeding in existing coal-fired power plants was considered as an alternative
to control GHG emissions in F-T liquid fuels synthesis. The results of this
study show that as the biomass addition is increased liquid production and
power generation are reduced. However, the overall efficiency of the plant is
increased, due to the lower power consumption. The replacement of the
conventional CC for power generation by SOFCs was also studied showing
enhancements in the efficiency of the power generation island.

Acknowledgment

This work was supported
in part by the Connecticut Center for Advanced Technologies and the Faculty
Large Grant of the University of Connecticut.

References

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