Hydrothermal Carbonization

Overall Summary

Primary Application

• Dairy farms with over 3,000 cows or smaller dairies with significant supply of organic substrates to mix with manure
• HTC is designed to process scraped manure, manure slurries, bedded pack and AD digestate, ideally the total solids of the mixed feedstock should be over 15%

Economic/Return on Investment Considerations

• The capital cost of HTC systems is relatively high.
• The operating costs of these systems are low as they provide their own energy and require no additives if the feedstock is dry enough and has a high enough energy content.
• Depending on the system, HTC can produce a high quality hydrochar which is a salable product similar to biochar
• The economic viability of HTC for dairy manure needs to be worked out, the salability of the hydrochar as a renewable alternative for coal and how to utilize the process water are the key questions

Industry Uptake

• Commercial HTC systems to process cow manure are not commercially available

Technology Maturity

• Hydrothermal carbonization of cow manure is an emerging technology
• Commercial HTC systems are not commercially available to process dairy manure
• A limited number of HTC plants are processing municipal biomass wastes in Europe and Asia with one commercial system in the U.S.

Primary Benefits

• The primary advantage of HTC is it can process wet biomass feedstocks, so no pre-processing of untreated wet manure is required
• HTC removes up to 90% of phosphorus in manure in the form of calcium phosphate
• There are no toxic waste products
• Excess process water contains soluble fertilizer components that can be recovered
• HTC hydrochar can be mixed with fossil coals for co-combustion in a thermal electric power plant 
• The HTC process is exothermic and therefore produce more energy that it consumes providing a favorable energy balance and significant renewable thermal heat production if the feedstock contains enough solids (>15%)
• Total pathogen destruction due to the high HTC pressure and temperature

Secondary Benefits

• HTC takes only hours to process manure when compared to other technologies resulting a smaller plant size
• With emission controls, HTC significantly reduces odors as compared to typical liquid lagoon storage

• Greenhouse gas (GHG) mitigation is also high as compared to an anaerobic lagoon storage baseline, with potential benefits as a renewable replacement for coal as well

How it works?

• Raw or pre-treated wet manure is conveyed into a pressure vessel where it is heated under high pressure to convert manure into a coal-like product, called hydrochar
• The HTC process mimics the natural process of coal formation in just a few hours essentially “pressure cooking” manure at 180-250^â—¦C under 145-750 psi for a period of one to eight hours

• 80% to 90% of the phosphorus in the manure feedstock can be recovered in the form of calcium phosphate by subsequent acid treatment and filtration of the hydrochar
• About half of the nitrogen and most of the potassium are retained in the liquid fraction of in the HTC reactor and can be used as fertilizer products.

Pretreatment and Post-treatment Required

• HTC is a wet process and no manure solid separation, drying or other pretreatment is required if the solids content of the manure is >15%
• Additional technology is required for recovering hydrochar and extracting nutrients from the liquid fraction

Limitations

• HTC capital and O&M costs are not known but anticipated to be medium to high
• Does not work well with manure containing bedding sand
• Does not work with highly diluted manure or other organic substrates
• HTC technology requires dedicated, skilled labor to operate and maintain

Other Considerations

• HTC economics are a challenge. 
• Markets for hydrochar and calcium phosphate need to be developed to generate sufficient return on investment
• Operating the system requires a full-time commitment of trained, dedicated operators
• More research is needed to determine the potential of HTC and its products with respect to carbon sequestration in soil and GHG mitigations

Overall Summary

Hydrothermal Carbonization is a thermochemical technology, operating within the temperature ranges of 180-300^oC. It is occasionally referred to as “Wet Torrefaction”.  Recently, significant interest has developed around HTC of organic materials, including dairy manure, primarily due to its potential for serving a dual purpose: energy densification via production of the resulting solid, commonly called hydrochar, and partitioning of nutrients into the resulting liquid stream. Despite this interest, to date no commercial applications on dairy manure exist within the U.S.

Specific studies on dairy manure, show HTC treatments with a mass yield of char in the 50-60% range, producing a char with heating values equivalent to coal while maintaining the bulk of the phosphorus (~90%) and half of the nitrogen. The liquid fraction contains various organics, the other half of nitrogen, primarily as a mix of ammonia and organic-N, as well as most of the potassium. Concerns exist regarding the value-added use of the resulting liquid fraction, with some studies acidifying the manure during the HTC process to increase the transport of phosphorus and nitrogen to the liquid stream. Studies show clear pathogen destruction at the temperature ranges utilized, while volatile and odorous emissions require controls, as is the case with any thermal process. With such controls, significant savings could be incurred in controlling odor emissions as compared to typical liquid storage under anaerobic conditions, however data is lacking in regard to these criteria. Greenhouse gas (GHG) mitigation is also potentially high as compared to an anaerobic lagoon storage baseline-with savings incurred from a combination of methane reduction and sequestration of fixed carbon into the soil. More research though is needed to determine the potential of HTC and its products with respect to carbon sequestration in soil and GHG mitigations resulting from such treatment.