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Sun Feb 24, 2019, 05:35 PM

Recovery of Phosphorous From the Supercritical Water Gasification of Dried Sewage Sludge.

The paper I'll discuss in this post is this one: Behavior of Phosphorus in Catalytic Supercritical Water Gasification of Dewatered Sewage Sludge: The Conversion Pathway and Effect of Alkaline Additive (Chenyu Wang†§ , Wei Zhu*†‡, Cheng Chen†, Hao Zhang†, Yujie Fan§, Biao Mu†, and Jun Zhong†, Energy & Fuels, 2019, 33 (2), pp 1290–1295)

One of the big problems humanity faces in the long term, not that we're particularly interested in the lives of future generations, unlike the generations that preceded us, is the availability of phosphorous, which drove the "green revolution" of the 1950's which wasn't all about happy talk about so called "renewable energy" but was more about feeding humanity, that is about agriculture.

As a fan of the possibilities connected with supercritical fluids, in particular supercritical water, this paper about phosphorous recovery caught my eye.

From the introductory text:

Phosphorus is an essential element for all life forms and it is estimated that the remaining accessible reserves of phosphate rock on the earth will run out in 50 years if the growth of demand for fertilizers remains at 3% per year.(1) For this reason, the recovery of phosphorus is very necessary. Dewatered sewage sludge (DSS) is an inevitable by-product of sewage treatment. It is difficult to dispose and is a source of environmental pollution risks because of its high moisture content and complex organic components. However, because of the large amount of phosphorus enriched in sludge during the sewage treatment process,(2) it has a high phosphorus recovery potential.

Supercritical water gasification (SCWG) of sewage sludge has been receiving widespread attention in recent years,(3) because it is a method that can decompose pollutants in sewage sludge and, at the same time, can produce syngas (hydrogen, methane, carbon monoxide, and so on), a clean energy resource.(4) However, DSS contains many macromolecular substances such as lignin and humus, which inhibit gasification to some degree. In addition, the reaction conditions of SCWG are harsh, requiring a significant amount of energy for the water to reach a supercritical state. Thus, it is difficult to justify the high cost of operation if the only product obtained from the process is syngas. However, if large amounts of phosphorus can be recovered simultaneously with syngas, then, the product value of SCWG of DSS will be improved effectively.

To achieve high phosphorus recovery from DSS, it is necessary to study the regulation of the transformation of phosphorus during the gasification of DSS in supercritical water. In our previous work,(5) the DSS was treated in an autoclave at a reaction temperature of 400–500 °C without adding a catalyst. The organic phosphorus in the sludge was almost completely converted into inorganic phosphorus after the reaction, yielding a large amount of phosphorus that reached 20 mg/g in the solid residue...

The problem with phosphorous in the solid residues is that it is not easy to recover.

The authors reference a number of other publications discussing this problem, noting that a scheme for gasifying algae produced an enriched phosphorous liquid phase in the presence of alkaline salts.

...Therefore, knowledge based on the current research rests on two common areas of understanding. The first is that after hydrothermal treatment, organic phosphorus will be converted into inorganic phosphorus. The second is that inorganic phosphorus is mainly enriched in the solid phase products after hydrothermal treatment. However, the regulation of phosphorus transformation and the pathway it takes under catalytic conditions in the SCWG of DSS are still poorly understood. In this work, Na2CO3 and K2CO3 were used as homogeneous alkaline catalysts to further study (1) the transformation of phosphorus during sludge gasification in supercritical water and (2) the effects of alkaline additives on phosphorus behavior and the mechanism involved in such effects. On the basis of our results, we propose a strategy for the recovery of phosphorus from gasification products of DSS in supercritical water.

They gasified sewage sludge obtained from the Nanjing Sewage Treatment plant.

After drying, portions were removed as samples were burned at 550C for 4 hours to obtain total organic carbon.

The remaining portions were gasified in supercritical water at 400C, 23 MPa pressure, for 30 minutes.

Some graphics about their results:

The caption:

Figure 1. Effect of the amount of alkaline additives on the phosphorus content of the liquid product (400 °C, 30 min).

The caption:

Figure 2. Distribution of total phosphorus in solid residue (S-TP) and liquid product (L-TP) (400 °C, 30 min).

Phosphorous in the solid phase was determined to exist in a number of forms:

Phosphorus in the solid residue exists in either organic or inorganic forms. The inorganic phosphorus includes exchangeable phosphorus (Ex-P), aluminum-combined phosphorus (Al-P), iron-combined phosphorus (Fe-P), occluded phosphate (Oc-P), self-ecological phosphorite, and debris phosphorus. Self-ecological phosphorite and debris phosphorus both belong to calcium-combined phosphorus (Ca-P). The respective contents of the various forms of phosphorus in dry raw sludge and solid residues after SCWG with different alkaline additives were determined. The results are shown in Figure 3. In the raw sludge, phosphorus was mainly in the form of inorganic phosphorus, and the content of organic phosphorus was only 0.14 mg/g. The value we obtained for the content of organic phosphorus in raw sludge is lower than the values measured by other scholars,(5,7) which may be mainly due to the difference in sludge properties and sewage treatment processes.

Figure 3:

The caption:

Figure 3. Effect of the amount of (a) Na2CO3 and (b) K2CO3 on phosphorus forms in solid residues (400 °C, 30 min).

The addition of alkali metal carbonates increases the liquid fraction. It also works to change the aluminum speciation:

Quartz (SiO2) was mainly detected in raw sludge and in the solid residues without alkaline additives. When Na2CO3 was added, the sodium ions combined with aluminum ions and SiO2 to form analcime (NaAlSi2O6). On the other hand, when K2CO3 was added, the potassium ions combined with aluminum ions and SiO2 to form kalsilite (KAlSiO4). Aluminum ions tend to combine with alkali metal ions, and the phosphate ions that were originally bound to the aluminum ions are released into liquid products. The results of XRD indicate that the reduction of Al-P is related to the addition of alkali metal ions. Moreover, the addition of K2CO3 is related to a weaker detection peak signal of SiO2 compared to the addition of Na2CO3

The XRD spectra:

The caption:

Figure 4. XRD patterns of (1) raw dry sludge and solid residue, (2) without additive, (3) with 4 wt % Na2CO3, and (4) with 4 wt % K2CO3 (400 °C, 30 min).

The caption:

Figure 5. (a) Olsen-P content of the solid residue and (b) amount and proportion of DRP in the liquid product (400 °C, 30 min).

Olsen-P refers to an analytical method, DRP refers to "dissolved reactive phosphorous."

The control of aluminum speciation is believed by the authors to be one of the mechanisms allowing for the release of phosphorous into the liquid phase.

In the presence of alkaline additives, the transformation of phosphorus occurs not only in the solid phase but also between solid and liquid phases. The conversion of phosphorus in the solid residue follows the same pathway that operates without alkaline additives, and the pathway between solid and liquid phases mainly follows two routes.

In the first, as shown in eq 3, the phosphorus that was originally combined with calcium releases into liquid product under action with alkaline additives

In the second route, as shown in eq 4, alkali metal ions combine with Al to form analcime or kalsilite, and phosphorus, which was originally combined with aluminum, is released into the liquid phase.

This graphic summarizes the author's view of the mechanism:

The caption:

Figure 6. Proposed pathway of phosphorus transformation in the SCWG of sewage sludge with alkaline additives (M+ represents the Na+ or K+ and m = 0, 1, 2).

Phosphorous distribution:

The caption:


The behavior of phosphorus during sludge catalytic gasification with alkaline additive in SCW was studied. Without an alkaline additive, the dominant reaction process is the conversion of different forms of phosphorus in solid phase, and 98.9% of the phosphorus enriches in the solid residues. Adding an alkaline additive can effectively promote the transfer of phosphorus from the solid phase to the liquid phase. Alkaline additives combine with Ca2+ and Al3+ to form calcium carbonate, analcime and kalsilite, and the phosphorus that was originally combined with Ca2+ or Al3+ is released to the liquid phase in the form of phosphate. The highest content of phosphorus in the liquid product reached 2214.5 mg/L, which is equivalent to the yield of other phosphorus recovery methods by chemical extraction. Direct production of liquid products with a high phosphorus concentration can simplify the exaction steps during subsequent phosphorus recovery. Therefore, the recovery of phosphorus from municipal sewage sludge by SCWG has great potential.

My view is that supercritical water oxidation is a critical technology if we ever hope to be serious about climate change, although there is little evidence we will ever be so. We hold the future in contempt.

Critical problems with supercritical water oxidation of biomass, of which sewage sludge is a subset, however involves issues of corrosion of reactors, driven in part by potassium interactions. Nonetheless I believe this materials science problem has a solution.

I trust you will have a pleasant Sunday evening.

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Reply Recovery of Phosphorous From the Supercritical Water Gasification of Dried Sewage Sludge. (Original post)
NNadir Feb 2019 OP
littlemissmartypants Feb 2019 #1

Response to NNadir (Original post)

Sun Feb 24, 2019, 06:29 PM

1. I missed your posts!

This is a good one!

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