Crystallization of Phosphorus-Incorporated Solids from Liquid Phase of Hydrothermal Carbonization of Cow Manure

Saeed V Qaramaleki, John A Villamil, Angel F Mohedano, Charles J Coronella

Excessive amount of manure produced in concentrated animal feeding operations (CAFOs) pose serious environmental and economic challenges. At the same time manure as biomass feedstock offers a huge potential as a resource of nutrients and energy. Therefore, developing processes to utilize manure as a resource can have substantial benefits. Hydrothermal carbonization (HTC) has been investigated to produce solid biofuels from various forms of biomass in recent years. As a thermochemical process, HTC can convert low energy value manure into a carbonaceous solid fuel primarily through various deoxygenation reactions. HTC can achieve this through a network of different reactions taking place in an aqueous environment which gives this process a big advantage over other techniques since it eliminates the need to dry the feedstock[1]. Operating at elevated but subcritical temperatures and pressures results in a dramatic change in physical properties of water. Subcritical water as reaction medium has a positive affect on the progression of hydrolysis, dehydration and decarboxylation reactions[2]. However, hydrochar is not the only product obtained from HTC, other byproducts including process water is produced as well. The focus of current study is on the liquid byproduct from HTC process. We have previously investigated the influence of various HTC conditions on solubilizing nutrients allowing nutrient recovery as a byproduct separate from the fuel value. In present work, we studied crystallization and precipitation of phosphorus-containing solids from HTC aqueous product, therefore taking one more step forward towards closing the phosphorus loop.

HTC of cow manure was carried out at 170°C in different acidic conditions, the concentration of acids was constant at 0.3 molar. The reaction time was 10 minutes for each run. The liquid product obtained by applying these conditions was then used as supernatant for crystallization. Due to presence of numerous kinds of inorganic and organic species, thermodynamic equilibrium of the solution is complex. Any change in the condition of the solution such as pH, chemical activity, or temperature, can move the solution into supersaturation. In these experiments, the solution pH of the hydrothermal aqueous product was increased by adding NaOH . Moreover, extra ammonia and magnesium was supplied in order to adjust the ratio of the N/P and Mg/P. All the precipitation experiments were conducted at room temperature and the pH of the solutions was fixed at 9.5. Nitrogen to phosphorus molar ratio was set to 2 by adding extra ammonium chloride. Three different Mg/P molar ratios (1, 1.5 and 2) were studied in order to determine the favorable condition for higher phosphorus recovery.

Our results indicate that precipitation of phosphorus-containing solids from HTC liquid is heavily dependent on the type of the acid used during HTC. As can be seen from the graph, adding either formic or sulfuric acid allowed for highest P recovery in the precipitate, relative to addition of other acids. The recovery of P is relatively insensitive to Mg:P for these two acids. The reason for a better recovery efficiency of these acids might be attributed to the smaller molecular weight of the acids used. For citric acid the recovery efficiency is almost constant with regards to the change in dosage of Mg (18.1-20.6%). Conversely, addition of oxalic acid indicates noticeable variability in terms of P recovery efficiency.

[1] Kruse A.,Funke A.,and Titirici M.-M., Hydrothermal conversion of biomass to fuels and energetic materials, Curr. Opin. Chem. Biol., 2013, 17, 3, 515–521.

[2] Funke A. and Ziegler F., Hydrothermal carbonization of biomass: A summary and discussion of chemical mechanisms for process engineering, Biofuels, Bioprod. Biorefining, 2010, 4, 2, 160–177.