Design Optimization of an Ammonia-Based Distributed Sustainable Agricultural Energy System


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Small-scale, distributed production of ammonia better enables the use of renewable energy for its synthesis than the current paradigm of large-scale, centralized production. Pursuant to this idea, a small-scale Haber-Bosch process has been installed at the West Central Research and Outreach Center (WCROC) in Morris, MN [1] and there is ongoing work on an absorbent-enhanced process at the University of Minnesota [2], [3]. Using renewables to make ammonia would greatly improve the sustainability of fertilizer production, which currently accounts for 1% of total global energy consumption [4]. The promise of renewable-powered, distributed ammonia production for sustainability is in fact not limited to fertilizer, because ammonia also has potential as an energy-dense, carbon-neutral fuel. For example, using ammonia produced from renewable energy for nitrogen fertilizer, grain drying fuel and tractor fuel at the WCROC farm would reduce more than 90% of the fossil energy footprint associated with corn production [5].

In this light, we envision a distributed sustainable agricultural (farm) energy system (DSAE) fundamentally based on the idea of ammonia as not only a fertilizer, but also a fuel and a method of energy storage. Specifically, this system will use only renewable energy to produce ammonia for use as fertilizer and agricultural fuel (for cropping equipment and grain drying) at the scale of a single farm or an agricultural cooperative. It will also use renewables to meet local power and heat demands in a manner synergistic to distributed ammonia production; the difference in power and heat (hourly) and ammonia (monthly or biannually) demand time scales gives rise to opportunities for temporally flexible ammonia production and locally controllable power generation using ammonia. Heat integration will also be possible due to the exothermic nature of ammonia synthesis.

Based on the above, we consider a DSAE superstructure that consists of PV arrays, wind turbines, a battery bank, electrolysis (for hydrogen production), pressure-swing adsorption (for nitrogen production) and modular ammonia synthesis units, hydrogen, nitrogen and ammonia storage vessels, hydrogen- or ammonia-fed fuel cells, ammonia-fed microturbines and electric boilers. Additionally, excess power can be sold to the utility to generate revenue, but this sale is constrained to prevent sharp inter-hour changes which can adversely affect grid operation.

For this superstructure, we develop a framework for design optimization. The 20 year net present cost (NPC) of this system is minimized by determining the optimal selection and size of units. The design of this system is inextricably coupled with its scheduling due to the time-varying nature of renewable generation as well as power and heat demands. Therefore, we consider one year of operation during which set points of installed units are allowed to vary hourly. For the sake of computational tractability, 12 representative weeks (2016 hours), one for each month, are used in the place of a full year-long scheduling horizon (8760 hours). The operating cost over 20 years is determined by first assuming identical operation in each month from each representative week and then assuming identical operation in each year from the scheduling year (scaled using a discount rate). This combined design-scheduling problem is formulated as a mixed-integer linear program (MILP) through the use of piecewise linear capital cost correlations and linear production-consumption relationships for unit operation.

We apply this framework to a case study inspired by the WCROC farm and the nearby UMM Morris Campus. The farm consists of 290 acres of corn and 116 acres of soy. The UMM Campus has hourly average power and heat loads of 985 kWh and 3480 kWh, respectively. UMM strives to be a net-zero energy campus by 2020 and to this end, two 1.65 MW wind turbines, a 20 kW solar PV system and biomass gasification district heating and cooling system with a 300 kW back pressure steam turbine have been installed so far. For this case study, we assume that the DSAE system can utilize the already-installed wind turbines and PV array while also having the ability to install additional renewable generation capacity. We further assume that renewable ammonia is required as fertilizer and as fuel for cropping operations and grain drying for all WCROC crops and that the DSAE system is responsible for serving the power and heat loads not met by biomass gasification and the associated back pressure steam turbine. Hourly power and heat demands as well as the ammonia demand profile are based on historical data from the Morris-WCROC site. Renewable generation profiles are synthesized using relevant weather data (e.g. insolation, ambient temperature, wind speed) for a typical meteorological year in Morris, MN. Based on the results of this case study, we perform sensitivity analysis to determine the unit performance improvements or capital cost reductions that will be most effective in reducing the NPC of the DSAE system.

[1] M. Reese, C. Marquart, M. Malmali, K. Wagner, E. Buchanan, A. McCormick and E. L. Cussler, “Performance of a Small-Scale Haber Process,” Industrial & Engineering Chemistry Research, no. 55, pp. 3742-3750, 2016.
[2] M. Malmali, Y. Wei, A. McCormick and E. L. Cussler, “Ammonia Synthesis at Reduced Pressure via Reactive Separation,” Industrial & Engineering Chemistry Research, vol. 55, no. 33, pp. 8922-8932, 2016.
[3] K. Wagner, M. Malmali, C. Smith, A. McCormick, E. Cussler, M. Zhu and N. C. Seaton, “Column Absorption for Reproducible Cyclic Separation in Small Scale Ammonia Synthesis,” AIChE Journal, vol. 63, no. 7, pp. 3058-3068, 2017.
[4] B. Swaminathan and K. Sukalac, “Technology transfer and mitigation of climate change: The fertilizer industry perspective,” in InIPCC Expert Meeting on Industrial Technology Development, Transfer and Diffusion, Tokyo, 2004.
[5] J. Tallaksen, Life Cycle Assesment of Corn Grain at WCROC, Morris, MN: Morris SunTribune, 2016.