(60b) Recovering Hydrogen Biofuel from Wet-Biowaste

Most fossil-based hydrogen is produced by steam methane reforming (SMR) and renewable hydrogen from the electrolysis of water. Without considering plant efficiency, capital or operating costs, hydrogen from SMR has a minimum value of 1.45 times the price of methane, a valuable and resource-limited commodity. In addition, for every ton of hydrogen produced, 10 ton of greenhouse-gas carbon dioxide is co-produced and released from the feedstock and in processing. The electrolysis of water requires a clean, pure feedstock and requires considerably more energy than what hydrogen will produce when reacted with oxygen (air).

In contrast to inorganic hydrogen production as described above, organic biofuels are produced by anaerobic digestion and fermentation, which are proven and well-established technologies. Anaerobic digestion uses decaying organic material to produce a syngas where there is waste that needs disposal, or significant subsidies for using energy crops. Fermentation has been producing alcohol beverages for thousands of years from cultivated crops, now it’s used to produce subsidized ethanol from corn and sugar, impacting feed and food costs. To quiet the “food vs. fuel” debate, effort is now directed at cultivating algae and grass feedstock. Cultivation requires fossil-fuel for planting, fertilizing, harvesting, transporting and distillation of product, which without subsidies would be highly unprofitable.

Anaerobic digestion and fermentation both use biological microorganisms (‘bugs’), which are temperature dependent, require large volumes with slow rates & low-yields, and are prone to contamination (sulfur) or conditions (temperature) that kills the ‘bugs’ employed to do the work. In addition, conventional biofuel producers must pay for crop feedstocks whereas biowaste producers pay for biowaste disposal; yet, biowaste has higher energy content than crop feedstocks.

An alternative, recovering hydrogen biofuel from wet-biowaste:

In the U.S., waste-water treatment plants (WWTP), municipal solid waste, confined area feeding operations, and agriculture produce over 1 billion tons of organic biowaste annually that can cost from $40-$200 per ton to treat before use or disposal. In addition, the amount and cost is increasing.

Fuel and electricity represent a substantial cost to WWTPs; one is required for nearly all stages in the treatment process, from the collection of raw sewage to the discharge of treated effluent. An estimated 3-4% of total U.S. electricity consumption is used for the transfer and treatment of wastewater, corresponding to more than 45 million tons of greenhouse gas (GHG) emissions annually.

WWTP in the U.S. produced 7.9 million tons of sewage biowaste in 2014. The latent energy in this biowaste could meet 12% of the nation’s electricity demand. A 10 million gallon/day WWTP producing 7 tons/day of biowaste will require 11 to 17 MWhe of electricity daily. In addition, due to water shortages, higher energy & capital costs and a changing climate, water-energy issues are of growing importance in biowaste management at WWTP.

In contrast to being a public energy, economic and environmental burden, Chemergy’s patented HyBrTec technology allows WWTPs to be energy independent and profitable producers of thermal energy and renewable fuel. All organic biowaste contains an unused remnant of stored solar energy that HyBrTec recovers as renewable hydrogen, non-anthropogenic carbon dioxide, thermal energy and inorganic residuals suitable as a micro-nutrient fertilizer.

Processing wet-biowaste, HyBrTec exploits two well-established steps that are scalable from pounds to tons per minute with commercially available components. First, the wet-biowaste (C₆H₁₀O₅ & H₂O) is ‘burned’ or oxidized with bromine (Br₂) to produce hydrogen bromide (HBr), carbon dioxide (CO₂) and heat (C₆H₁₀O₅ + 7H₂O + 12Br2 → 24HBr + 6CO₂). The HBr reacts with unreacted water forming concentrated hydrobromic acid (HBr_aq). Bromination is followed by electrolysis of the hydrobromic acid into hydrogen (H₂) and reagent Br₂ and dilute acid that is recycled back into the process.

Hydrogen bromide electrochemistry is reversible (2HBr ↔ H₂ + Br₂) allowing the charging (storing) and discharging (production) of electrical energy. This enables a HyBrTec reversible electrolysis system to co-provide efficient electrical energy storage. The hydrogen-bromine energy storage system operates as a flow battery where the reactants are stored outside the electrochemical PEM stack, which simplifies capacity growth. Energy storage increases the value of off-peak and intermittent solar & wind energy and promotes development of micro- and smart-grids that can exploit local sources of energy, improve efficiency, reduce needed capacity and mitigate disruptions.

Feedstocks include: sewage, manure, agricultural residuals, and municipal solid waste including plastics. Also, wastewater is a reagent in the process producing additional hydrogen. Unique to HyBrTec is that 175-200 C heat (26.3 kWh_th/kgH₂) is released, which can be used to reduce the feedstock water content to a desired 50%. Also, electroosmotic water transfer from the bromine anode to the hydrogen cathode increases the acid concentration at the anode, which lowers cell voltage and results in 4-6 gallons of potable water per kgH₂.

If the residual energy content of the biowaste is omitted, HyBrTec offers a theoretical biowaste-to-hydrogen-to-energy efficiency greater than 100%. This is simply because the theoretical voltage to dissociate hydrogen carrier hydrogen bromide into hydrogen and bromine (0.58 Volt) is less than the voltage produced in the hydrogen reaction with oxygen (1.23 Volt) forming water, providing a theoretical biowaste-to-energy efficiency of 212% (1.23V/0.58V). However, if the electrolyzer operates at an actual 0.75-0.8 Volts (16-21.23 kWh/kgH₂ depending on electrolyte temperature) the efficiency is 154%, and if the hydrogen is used in a 65% efficient fuel cell with air the electric-to-electric efficiency is reduced further to 100%. The efficiencies above disregard the latent energy content of the negative-valued feedstock; however, if included the biowaste-to-electricity efficiency is a healthy 67%.

Hydrogen can enrich natural gas to increase its Btu content and reduce NOx emissions, used directly as a clean fuel or sold as a commodity. Furthermore, byproduct carbon dioxide is organic in origin and not considered a GHG, can be vented, or used with recovered hydrogen to synthesize methanol, ethanol or methane using Fischer-Tropsch or the Audi e-gas process. Also, recovered hydrogen can be combined with nitrogen from on-site membrane air-separation to produce ammonia, which is a higher-value commodity and easily transportable. Other opportunities to monetize the carbon dioxide byproduct include reacting with ammonia to produce even higher value urea.

Beginning in 2010 the U.S. Department of Energy and the Florida Hydrogen Initiative funded laboratory bench-top R&D of HyBrTec. Based on the success of this research and at the recommendation of Lawrence Livermore National Laboratory (LLNL), the California Energy Commission (CEC) in 2013 funded Chemergy in a cost-shared program to determine suitability of biosolids from the Delta Diablo WWTP, prepare a detailed system design & cost estimate of a demonstration system and an economic analysis of a HyBrTec commercial system.

The results of the CEC program verified that 21.44 kg of hydrogen could be produced per wet-ton of sewage sludge and providing a 95% reduction of the wet-mass. An economic analysis for a small system processing 11 wet-tons of sewage sludge per day into 236 kg of hydrogen predicted a hydrogen production cost of $2/kg and a 14% Internal Rate of Return. Economic benefits from an increased scale, energy storage, state & federal renewable energy tax incentives, loan guarantees, cap & trade credits and subsidies were not included in the analysis, which if taken would increase the IRR.

Summary:

HyBrTec offers advantages over conventional hydrogen and biofuel production:

• Wet-biowaste is abundant and an environmental and economic burden.
• The technology exploits two advantages that reduce capital and energy:
• At moderate temperature & pressure, processing is fast and yields are high, which minimizes the size, footprint and cost of equipment.
• The chemical bonds to release hydrogen are weak, requiring less than half the energy (40%) that hydrogen will produce when burned with oxygen (air).
• HyBrTec is a highly scalable technology able to process lb/day or tons/minute with commercially available components anywhere biowaste is produced.
• With electricity from renewable solar & wind resources, HyBrTec is GHG neutral.
• Hydrogen and by-product carbon dioxide can produce conventional fuels including ethanol, methane, green diesel or higher-valued chemical commodities.
• HyBrTec offers a profitable triad: 1) eliminates biowaste, 2) low-cost ($2/kg) renewable hydrogen and 3) co-provides efficient energy storage.
• $2/kg hydrogen fueling a 50% efficient fuel cell vehicle is equivalent to gasoline at $1/gallon without the harmful ‘well-to-wheels’ environmental and health effects.
• Accrues benefits from state and federal renewable-energy tax incentives, loan guarantees, GHG cap & trade programs and energy storage subsidies.

In closing, Chemergy’s HyBrTec technology promotes a highly advantageous paradigm shift departing from: 1) costly non-renewable resources to negative-valued renewable resources, 2) large, expensive centralized plants to low-cost, well-distributed systems, 3) collection, transporting & disposal, to on-site utilization.