281554 Coal and Biomass to Liquids and Power Generation Via Fischer-Tropsch Synthesis and Fuel Cells

Thursday, November 1, 2012: 2:50 PM
322 (Convention Center )
Julia Valla1, Shoucheng Du1 and George M. Bollas2, (1)Department of Chemical & Biomolecular Engineering, University of Connecticut, Storrs, CT, (2)Department of Chemical, Materials & Biomolecular Engineering, University of Connecticut, Storrs, CT

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.

Figure 1: Illinois #6 coal and Southern Pine wood proximate (a) and ultimate (b) analysis

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.

Figure 2: Flowsheet of CBTL-CC and CBTL-FC

 

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.

Figure 3: Effect of biomass addition on the plant demands

 

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.

Figure 4: a) Energy input, production and consumption, and b) Energy efficiencies as a function of biomass addition in the CBTL plant with parallel CC

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

Biomass addition %/Net electricity

0

50

100

CC, MW

907

820

770

SOFC, MW

2010

1935

1880

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

[1] T. J. Tarka, “Affordable, Low-Carbon Diesel Fuel from Domestic Coal and Biomass,” DOE/NETL-2009/1349

[2] T. G. Kreutz, E. D. Larson, G. Liu, R. H. Williams, “Fischer-Tropsch Fuels from Coal and Biomass,” 25th Annual International Pittsburg Coal Conference, 29 September – 2 October 2008, Pittsburg, Pennsylvania, USA

[3] G. Vaughan, S. Luz-Acosta, C. Roberts and M. R. Eden “Systems Analysis of Coal and Biomass Based Fuel Production Strategies” Consortium for Fossil Fuel Science Meeting, Pittsburg, PA, August 3-4, 2010

[4] C. N. Hamelinck, A. P.C. Faaij, H. den Uil, H. Boerrigter, “Production of FT transportation fuels from biomass: technical options, process analysis and optimization, and development potential,” Energy 29 (2004) 1743-1771

[5] W. Doherty, A. Reynolds, D. Kennedy, “Computer simulation of a biomass gasification solid oxide fuel cell power system using Aspen Plus,” Energy 35 (2010) 4545-4555


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