Integration of hydrothermal liquefaction of Cyanophyta and supercritical water oxidation of its aqueous phase products: Biocrude production and nutrient removal.

Cyanophyta has the potential to produce biocrude via hydrothermal liquefaction (HTL). However, aqueous phase products (APs), as by-products of HTL, pose a risk of eutrophication for the high levels of carbon, nitrogen, and phosphorus. Supercritical water oxidation (SCWO) can efficiently convert organics into small molecules, offering a technique for the harmless treatment of APs. Effects of holding time, pressure, and moisture content on the biocrude yields from isothermal HTL (300 °C) and fast HTL (salt bath temperature of 500 °C) were comprehensively investigated. Biocrude properties were characterized by elemental analysis, FT-IR and GC-MS. Subsequently, the APs obtained under the conditions producing the highest biocrude yield were subjected to SCWO at 550 °C with different oxidation coefficients (n) from 0 to 2. Removal rates of chemical oxygen demand (COD), ammonia nitrogen (NH3-N), and total phosphorus (TP) were further explored. The results show that the highest biocrude yields from isothermal HTL and fast HTL were 24.2 wt% (300 °C, 1800 s, 25 MPa, and 80 wt% moisture content) and 21.9 wt% (500 °C, 40 s, 25 MPa, and 80 wt% moisture content), respectively. The biocrude primarily consisted of N-containing heterocyclic compounds, amides, and acids. SCWO effectively degraded the COD and TP in APs, while the NH3-N required further degradation. At n = 2, the highest removal rates of COD, NH3-N and TP were 98.5 %, 22.6 % and 89.1 %, respectively.

Supercritical water oxidation of quinazoline: Reaction kinetics and modeling.

This paper presents a first quantitative kinetic model for supercritical water oxidation (SCWO) of quinazoline that describes the formation and interconversion of intermediates and final products at 673-873 K. The set of 11 reaction pathways for phenol, pyrimidine, naphthalene, NH3, etc, involved in the simplified reaction network proved sufficient for fitting the experimental results satisfactorily. We validated the model prediction ability on CO2 yields at initial quinazoline loading not used in the parameter estimation. Reaction rate analysis and sensitivity analysis indicate that nearly all reactions reach their thermodynamic equilibrium within 300 s. The pyrimidine yielding from quinazoline is the dominant ring-opening pathway and provides a significant contribution to CO2 formation. Low sensitivity of NH3 decomposition rate to concentration confirms its refractory nature in SCWO. Nitrogen content in liquid products decreases whereas that in gaseous phase increases as reaction time prolonged. The nitrogen predicted by the model in gaseous phase combined with the experimental nitrogen in liquid products gives an accurate nitrogen balance of conversion process.

Supercritical water oxidation of Quinazoline: Effects of conversion parameters and reaction mechanism.

The supercritical water oxidation reaction of quinazoline and a set of related reaction products were investigated in batch reactors by varying the temperature (T, 400-600 °C), time (t, 0-400 s), water density (ρ, 70.79-166.28  kg m(-3)) and oxidation coefficient (OC, 0-4.0). The TOC removal efficiency (CRE) increased significantly as the OC increased, whereas this effect was very limited at high OC (>2.0). Lack of oxygen resulted in low CRE and TN removal efficiency (NRE), also cause coke-formation, and giving high yield of NH3 and nitrogenous organic intermediates. Prolonging reaction time did not provide an appreciable improvement on CRE but remarkably increased NRE at temperature higher than 500 °C. Pyrimidines and pyridines as the nitrogenous intermediates were largely found in GC-MS spectrum. Polymerization among benzene, phenyl radical and benzyl radical played important roles in the formation of PAHs, such as naphthalene, biphenyl, phenanthrene. These collective results showed how the yield of intermediate products responded to changes in the process variables, which permitted the development of a potential reaction network for supercritical water oxidation of quinazoline.

Partial oxidation of landfill leachate in supercritical water: Optimization by response surface methodology.

To achieve the maximum H2 yield (GYH2), TOC removal rate (TRE) and carbon recovery rate (CR), response surface methodology was applied to optimize the process parameters for supercritical water partial oxidation (SWPO) of landfill leachate in a batch reactor. Quadratic polynomial models for GYH2, CR and TRE were established with Box-Behnken design. GYH2, CR and TRE reached up to 14.32mmol·gTOC(-1), 82.54% and 94.56% under optimum conditions, respectively. TRE was invariably above 91.87%. In contrast, TC removal rate (TR) only changed from 8.76% to 32.98%. Furthermore, carbonate and bicarbonate were the most abundant carbonaceous substances in product, whereas CO2 and H2 were the most abundant gaseous products. As a product of nitrogen-containing organics, NH3 has an important effect on gas composition. The carbon balance cannot be reached duo to the formation of tar and char. CR increased with the increase of temperature and oxidation coefficient.

Supercritical water oxidation of polyvinyl alcohol and desizing wastewater: influence of NaOH on the organic decomposition.

Polyvinyl alcohol is a refractory compound widely used in industry. Here we report supercritical water oxidation of polyvinyl alcohol solution and desizing wastewater with and without sodium hydroxide addition. However, it is difficult to implement complete degradation of organics even though polyvinyl alcohol can readily crack under supercritical water treatment. Sodium hydroxide had a significant catalytic effect during the supercritical water oxidation of polyvinyl alcohol. It appears that the OH- ion participated in the C-C bond cleavage of polyvinyl alcohol molecules, the CO2-capture reaction and the neutralization of intermediate organic acids, promoting the overall reactions moving in the forward direction. Acetaldehyde was a typical intermediate product during reaction. For supercritical water oxidation of desizing wastewater, a high destruction rate (98.25%) based on total organic carbon was achieved. In addition, cases where initial wastewater was alkaline were favorable for supercritical water oxidation treatment, but salt precipitation and blockage issues arising during the process need to be taken into account seriously.

Supercritical water oxidation of landfill leachate.

In this paper, ammonia as an important ingredient in landfill leachate was mainly studied. Based on Peng-Robinson formulations and Gibbs free energy minimization method, the estimation of equilibrium composition and thermodynamic analysis for supercritical water oxidation of ammonia (SCWO) was made. As equilibrium is reached, ammonia could be totally oxidized in SCW. N(2) is the main product, and the formation of NO(2) and NO could be neglected. The investigation on SCWO of landfill leachate was conducted in a batch reactor at temperature of 380-500 °C, reaction time of 50-300s and pressure of 25 MPa. The effect of reaction parameters such as oxidant equivalent ratio, reaction time and temperature were investigated. The results showed that COD and NH(3) conversion improved as temperature, reaction time and oxygen excess increased. Compared to organics, NH(3) is a refractory compound in supercritical water. The conversion of COD and NH(3) were higher in the presence of MnO(2) than that without catalyst. The interaction between reaction temperature and time was analyzed by using response surface method (RSM) and the results showed that its influence on the NH(3) conversion was relatively insignificant in the case without catalyst. A global power-law rate expression was regressed from experimental data to estimate the reaction rate of NH(3). The activation energy with and without catalyst for NH(3) oxidation were 107.07 ± 8.57 kJ/mol and 83.22 ± 15.62 kJ/mol, respectively.

Partial oxidation of municipal sludge with activited carbon catalyst in supercritical water.

The partial oxidation (POX) characteristics of municipal sludge in supercritical water (SCW) were investigated by using batch reactor. Effects of reaction parameters such as oxidant equivalent ratio (OER), reaction time and temperature were investigated. Activated carbon (AC) could effectively improve the mole fraction of H(2) in gas product at low OER. However, high OER (greater than 0.3) not only led to the combustion reaction of CO and H(2), but also caused corrosion of reactor inner wall. Hydrogenation and polymerization of the intermediate products are possible reasons for the relative low COD removal rate in our tests. Metal oxide leached from the reactor inner wall and the main components of the granular sludge were deposited in the AC catalyst. Reaction time had more significant effect on BET surface area of AC than OER had. Long reaction time led to the methanation reaction following hydrolysis and oxidation reaction of AC in SCW in the presence of oxygen. Correspondingly, the possible reaction mechanisms were proposed.