Donghai Xu

Treatment of municipal sewage sludge in supercritical water: A review.

With increasing construction of wastewater treatment plants and stricter policies, municipal sewage sludge (MSS) disposal has become a serious problem. Treatment of MSS in supercritical water (SCW) can avoid the pre-drying procedure and secondary pollution of conventional methods. SCW treatment methods can be divided into supercritical water gasification (SCWG), supercritical water partial oxidation (SCWPO) and supercritical water oxidation (SCWO) technologies with increasing amounts of oxidants. Hydrogen-rich gases can be generated from MSS by SCWG or SCWPO technology using oxidants less than stoichiometric ratio while organic compounds can be completely degraded by SCWO technology with using an oxidant excess. For SCWG and SCWPO technologies, this paper reviews the influences of different process variables (MSS properties, moisture content, temperature, oxidant amount and catalysts) on the production of gases. For SCWO technology, this paper reviews research regarding the removal of organics with or without hydrothermal flames and the changes in heavy metal speciation and risk. Finally, typical systems for handling MSS are summarized and research needs and challenges are proposed.

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.

Treatment of sewage sludge in supercritical water and evaluation of the combined process of supercritical water gasification and oxidation

Influences of temperature and oxidation coefficient (n) on sewage sludge treatment in supercritical water and its corresponding reaction mechanism were studied. Moreover, the combined process of supercritical water gasification (SCWG) and supercritical water oxidation (SCWO) was also investigated. The results show that ammonia nitrogen, phenols and pyridines are main refractory intermediates. The weight of solid products at 873K and n=4 is only 3.5wt.% of the initial weight, which is lower than that after combustion. Volatile organics in solid phase have almost released at 723K and n=0. Highest yield of combustible gases was obtained at n=0, and H2 yield can reach 11.81mol/kg at 873K. Furthermore, the combination of SCWG at 723K and SCWO at 873K with a total n=1 is feasible for its good effluent quality and low operation costs.

Supercritical water oxidation of acrylic acid production wastewater

Supercritical water oxidation (SCWO) of wastewater from an acrylic acid manufacturing plant has been studied on a continuous flow experimental system, whose reactor was made of Hastelloy C-276. Experimental conditions included a reaction temperature (T) ranging from 673 to 773 K, a residence time (t) ranging from 72.7 to 339 s, a constant pressure (P) of 25 MPa and a fixed oxidation coefficient (α) of 2.0. Experimental results indicated that reaction temperature and residence time had significant influences on the oxidation reaction, and increasing the two operation parameters could improve both degradation of chemical oxygen demand (COD) and ammonia nitrogen (NH3‒N). The COD removal efficiency could reach up to 98.73% at 25 MPa, 773 K and 180.1 s, whereas the destruction efficiency of NH3‒N was only 43.71%. We further carried out a kinetic analysis considering the induction period through free radical chain mechanism. It confirms that the power-law rate equation for COD removal was 345 exp(−52200/RT)[COD]1.98[O2]0.17 and for NH3‒N removal was 500 exp(−64492.19/RT)[NH3‒N]1.87[O2]0.03. Moreover, the induction time formulations for COD and NH3‒N were suspected to be exp(38250/RT)/173 and exp(55690/RT)/15231, respectively. Correspondingly, induction time changed from 2.22 to 5.38 s for COD and 0.38 to 1.38 s for NH3‒N. Owing to the catalysis of reactor inner wall surface, more than 97% COD removal was achieved in all samples.

Effect of temperature, water loading, and Ru/C catalyst on water-insoluble and water-soluble biocrude fractions from hydrothermal liquefaction of algae

Hydrothermal liquefaction (HTL) converts algal biomass into a crude bio-oil (biocrude) and aqueous-phase products. The effect of temperature, water loading, and added H2 and/or Ru/C catalyst on the properties of the biocrude that spontaneously separates from the aqueous phase post reaction and also the biocrude that is extractable from the aqueous phase by dichloromethane is explored herein. This report is the first to elucidate how the yields, compositions, heating values, and energy recoveries of the two biocrudes vary with the processing conditions above. Increasing temperature from 350 to 400°C increased the yield of water-insoluble biocrude (38.1-42.5wt%) and its hexane-soluble subfraction (63.7-85.6wt%) while decreasing the yield of extractable, water-soluble biocrude (6.6-2.5wt%). The Ru/C catalyst had the same effect. Reaction temperature and catalysts could be used to manipulate the proportions of water-soluble and water-insoluble biocrude from algae HTL and thereby manipulate biocrude quantity and quality.

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.

Ni-Ru/CeO2 Catalytic Hydrothermal Upgrading of Water-Insoluble Biocrude from Algae Hydrothermal Liquefaction

Hydrothermal liquefaction (HTL) of algae is a promising crude bio-oil (biocrude) production technology, which can convert wet algae into water-insoluble biocrude and other coproducts. In this work, algae HTL at 350°C and 20 min was conducted to obtain water-insoluble biocrude (B1), which was then hydrothermally upgraded at 450°C, 60 min, or with added H2 and/or homemade catalyst (i.e., Ni-Ru/CeO2 or Ni/CeO2) for the first time. The characteristics (e.g., yield, elemental component, energy recovery, and molecular and functional group compositions) of upgraded water-insoluble biocrude (B2) as well as light biocrude thereof were analyzed comprehensively. The results show that Ni-Ru/CeO2+H2 led to the highest yield and HHV (higher heating value), the best elemental compositions quality of B2, and the largest fraction and the best light of light biocrude in B2. Ni-Ru/CeO2+H2 had good catalytic desulfurization effect and could transform high-molecular-weight compounds into low-molecular-weight compounds in B1 upgrading. At the condition above, 46.2% of chemical energy in the initial algae could be recovered by B2, while average 54.9% of chemical energy in B2 was distributed in its light biocrude (hexane-soluble) portion. On the whole, Ni-Ru/CeO2+H2 can be considered as the optimal additive in all tested cases.

Evaluation of the Combined Process of Aammonia Distillation-Evaporation Concentration - Supercritical Water Gasification - Supercritical Water Oxidation for Coking Wastewater Treatment

A combined process of ammonia distillation - evaporation concentration - supercritical water gasification (SCWG) supercritical water oxidation (SCWO) was proposed for coking wastewater treatment. The NH3 in wastewater was first recovered in ammonia distillation section. The deammoniated wastewater was concentrated in the evaporation concentration section, in which the quantity of the wastewater was decreased accordingly. Then the concentrated wastewater entered into the SCWG section to produce the syngas such as H2, CO, CH4, and liquid effluent entered into SCWO section, where the pollutants were completely degraded. The operation parameters of the combined process were simulated by Aspen Plus software and the investment and the operation cost were also analyzed. For a wastewater of 5000kg/h, the investment of the combined process is 6000000 ¥. The profit from byproduct of ammonia, syngas and steam is 70¥/t. For the whole process, it can gain a profit of 5.1 ¥/t (wastewater).

Supercritical Water Oxidation of Lurgi Coal Gasification Wastewater and Evaluation of the Combined Process with Gasification

Degradation of Lurgi coal gasification wastewater in supercritical water was investigated in a batch reactor, and a combined process of supercritical water oxidation (SCWO) and gasification was put forward to utilize the material and energy in SCWO effluent. The technical feasibility and economy were evaluated. In the SCWO process, when operated at condition of 500 °C, 25 MPa, oxygen ratio of 3.5 and residence time of 15 min, the concentrations of COD, NH3-N and volatile phenol in effluent reduced to 27, 11 and 0.4 mg/L, respectively, where the removal efficiencies were 99.81%, 99.85% and 99.99%, accordingly. The effluent could reach national discharge standard I (GB 8978-1996), and can be used as the gasification agents of gasifier via a pressure reducer. For a single Lurgi gasifier, the combied process could save 23 t/h of steam and 1550 kg/h of O2, which is equal to 4996 yuan/h.

Corrosion Behavior of Candidate Materials for Supercritical Water Oxidation Reactor for Sewage Sludge Processing Plants

In this study, corrosion behavior of Incoloy 800 was studied in supercritical water containing sewage sludge and oxygen at various temperature from subcritical to supercritical environment. Surface and cross-section morphologies of the oxide films on Incoloy 800 was observed by scanning electron microscope. Corrosion will be obviously accelerated by increasing temperature. And the addition of oxygen in corrosion medium will cause the formation of various inorganic salts, forming a salt deposit layer on sample surface.