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Biofuels and Biomass
Sewage sludge gasification under hydrothermal condition: Phosphorus behavior and its kinetics Apip Amrullah, and Yukihiko Matsumura Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b04289 • Publication Date (Web): 20 Feb 2019 Downloaded from http://pubs.acs.org on February 21, 2019
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Energy & Fuels
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Sewage sludge gasification under hydrothermal condition:
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Phosphorus behavior and its kinetics
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Apip AMRULLAH1,2, Yukihiko MATSUMURA1*
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1Department
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Kagamiyama, Higashi-Hiroshima, 739-8527 Japan.
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of Mechanical Science and Engineering, Hiroshima University, 1-4-1
Original affiliation: Department of Mechanical Engineering, Lambung Mangkurat
University, Banjarmasin, South Kalimantan, Indonesia.
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*To whom correspondence should be addressed. 1-4-1 Kagamiyama, Higashi-
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Tel: +81-82-424-7561, Fax: +81-82-422-7193, E-mail:
[email protected] Hiroshima 739-8527 Japan,
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Abstract
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This is the first report to model the behavior of phosphorus in sewage sludge under
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hydrothermal condition for the ranges of temperature (300‒600 °C) and reaction time
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(5‒30 s) using a continuous reactor. The pressure was fixed at 25 MPa. The H2, CO2 and
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CH4 were components of the product gas. In short reaction time less than 10 s, organic
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phosphorus (OP) was almost completely converted to inorganic phosphorus (IP) under
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supercritical conditions. Then, part of IP precipitated in the reactor. Kinetics parameters
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for the reaction from OP to IP was determined by assuming the first-order reaction.
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Precipitation rate in proportion to oversaturation fitted the experimental data well.
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Keywords: sewage sludge; supercritical water gasification; phosphorus; reaction rate
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1. Introduction
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Fossil fuels are limited vitality energy sources, and their utilization is related to
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an environmental issue. These problems have driven researchers to develop renewable
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energy technologies including gasification of biomass. One of the potential biomass that
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is a low-cost material and available in a large amount is sewage sludge. A large amount
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of organic matter [1], nitrogen, and phosphorus is contained in sewage sludge. Thus,
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sewage sludge can be utilized to generate gas for energy, as well as to recover phosphorus
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which is also a valuable commodity material. In pioneering studies, Adam et al. [3] found
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that the P-bioavailability was considerably inflated throughout thermochemical treatment.
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Unfortunately, sewage sludge has a high content of water. The challenge of exploitation
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of biomass with a high content of water is that pre-drying is often needed for the most
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ancient thermochemical treatment method to recover energy [2].
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Meanwhile, the technology of sub- and supercritical water gasification utilizes
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water as the medium where various reactions of biomass can take place. The term
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“supercritical” is to denote the region of temperature on the far side the critical point, i.e.
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374 ºC. At this condition, the hydrogen bonding between molecules in water are
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weakened and contributes to H2 production [4,5]. In distinction, the term “sub-critical” is
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to denote the region of the temperature higher than 100 ºC and less than critical
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temperature (374 ºC) with high pressure (5−22 MPa). Under subcritical condition, the
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liquid state of water is maintained [6]. Recent works of supercritical water gasification
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(SCWG) [7–11] demonstrated that under supercritical water condition, all of the fluids
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become one phase. Therefore, water is a suitable solution for organic materials and
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gaseous products. The gasification reaction of wet biomass in water can be carried out
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using SCWG, and the pre-drying process can be abolished [12].
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Studies on SCWG of sewage sludge, focusing on gas production, have been
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carried out. Supercritical water has been confirmed to be able to convert SS into H2, CO,
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and CH4 with the less solid product than traditional thermal processes [13]. Another study
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reported that by using a high-pressure autoclave the effect of chemical composition on
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gas production was reported. At the higher reactant composition, the total gas production
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increased [14]. Also, Reddy et al. [15] pointed out that the gasification efficiency was
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affected by feed biomass concentration.
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Sewage sludge contains a high amount of phosphorus that makes it an important
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resource for phosphorus recovery. Phosphorus has been produced using traditional
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techniques to be a marketable product, like mineral plant food, fodder, pure type of
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phosphorus, and phosphoric acid [16]. Therefore, a deeper understanding of P recovery
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from sewage sludge is necessary. Several studies have been recently performed
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concerning recovery phosphorus from sewage sludge. As reported by Yuan et al. [17],
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the EBPR method is an attractive method for phosphorus recovery from wastewater
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treatment. Arakane et al. [18] reported that under subcritical water condition the
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magnesium ammonium phosphate (MAP) is a common technique for phosphorus
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recovery in excess sludge. Most recently, Zou and Wang [19] claimed that crystallization
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as hydroxyapatite (HAP) is often used of phosphorus recovery in domestic wastewater.
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These results show the recovery rate of total phosphorus was 0.593 mol/mol. The removal
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efficiencies were 0.826, 0.875, and 0.916 for COD, PO43--P, and NO3 -N, respectively.
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Considering that sub- or supercritical water gasification gasifies organics, leaving
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phosphorus behind, this technology should be effective for phosphorus recovery in
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sewage sludge. It is also critical for energy generation from sewage sludge because
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inorganics removal results in no production of ash to plug the reactor. In addition,
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phosphorus recovery helps improving the economic aspect of the process by value added
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by-product formation. However, as far as we know, studies on phosphorus behavior
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combined with gas generation as well as the reaction kinetics of P transformation of
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sewage sludge under hydrothermal condition are still limited. These information should
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be important for phosphorus regeneration process from sewage sludge using sub- or
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supercritical water. Thus, we herein aim to elucidate the behavior of phosphorus together
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with sewage sludge gasification characteristics and to determine the kinetics of
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phosphorus change during hydrothermal gasification.
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2. Materials and methods
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2.1 Feedstock
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The sewage sludge in this work was the same as the one previously reported [20].
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Briefly, it was active sludge produced from Higashi-Hiroshima wastewater treatment
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plant. The particle size of 40 μm was obtained after the pulverization process. The
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properties of the sewage sludge were obtained by our research group [21] as shown in
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Table 1. The water deionizer (Organo, BB-5A) produced deionized water (
