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Sat Sep 29, 2018, 12:02 AM

Recovery of Phosphorous from the Hydrothermal Gasification of Sewage Sludge.

The scientific paper I'll discuss in this post is this one: Phosphorus Transformation in Hydrothermal Pretreatment and Steam Gasification of Sewage Sludge Feng et al Energy Fuels, 2018, 32 (8), pp 8545–8551. The text, which I will quote verbatim without the use of the parenthetical (sic) notation, shows the linguistic seams of having been translated from Chinese into English, but the ideas are far more important than perfectly grammatical English.

I have read somewhere - it may have been Vaclav Smil's outstanding rumination on nitrogen fixation, Enriching the Earth - that the main reason that China agreed to meet with Henry Kissinger, and then Nixon, was not to achieve peace but was rather concerned with the practical need to have access to American Haber-Bosch technology for the fixation of nitrogen. 1972 was, after all, only 23 years after Mao's victory over the nationalists, and coupled with the disaster of the cultural revolution, the ideological attacks on intellectuals, and the continuous strain on China's soils, it was not clear that China could feed its massive population, and it may not have been clear to the Chinese leadership that they could survive a massive famine associated with the total collapse of agriculture.

The words "green revolution" have come to mean something quite different in the 21st century than what they meant in the mid-20th. From my perspective the 21st century meaning consists largely of a specious lie we tell ourselves to avoid reality, but in the mid 20th century the meaning was very much connected with the growth of a major technology connected with farming: the agrochemical fertilization of soils and (like or not) the development of chemical pest control, including, but hardly limited to, the control of weeds.

Without these developments, Thomas Malthus would not be quite so forgotten nor quite the subject of intellectual bemusement he seems to have become, if he is considered at all by anyone other than old people like me.

It wasn't all that long ago that in many places it was impossible to feed huge swathes of the world's population

In 1943, in the Bengal, between 2 and 3 million people starved to death; for perspective, this is roughly 7 to 10 times the number of British combat deaths in all of World War II.

Worse, during the period of Mao's "Great Leap Forward" (1958-1961) - the numbers are controversial and will probably never be accurately known, it is estimated that tens of millions of people, maybe as many as 40 million, starved to death. While Mao and Zhou Enlai politically survived the "Great Leap Forward," according to the account I read, it wasn't clear to them that they could survive a second round. (Effectively their style of Goverment didn't survive them; if China is a socialist state, it is comparatively an ersatz version of what Mao was trying to create.)

Nixon, the way I heard it in such a way as to believe it, could open the door to technology transfer to China of fertilizer production (and other technologies) so China opened the door to Nixon.

A desire for peace had nothing to do with it. (Nixon's war like purpose was to further isolate the Soviet Union.)

I will not discuss chemical pesticides here further, nor will I talk any further about nitrogen fixation, a necessary component for there to have been an agricultural "green revolution" spreading rapidly in the 1950's, but will instead point to the cited paper's discussion of a critical agricultural commodity that is far more subject to depletion than fixed nitrogen is, phosphorous.

From the introduction of the paper:

Among the traditional treatments of sewage sludge (SS), landfill and incineration might cause secondary pollution and anaerobic digestion has a long processing period; (1) thus, gasification was introduced as a novel method with less secondary pollution and shorter processing time.(2) Meanwhile, hydrogen-rich syngas could be produced when using steam as the gasification agent.(3) The high moisture content in SS calls for an effective and economical dehydration method before its thermal conversion. Hydrothermal treatment highly enhances mechanical dewaterability of sludge with low energy cost.(4) Besides, it also reduces N emission and improves syngas quality during steam gasification.(5,6)

Despite producing high-quality hydrogen-rich syngas, as a thermochemical disposal method, gasification results in a highly concentrated phosphorus (P) content in sludge ash.(7) P is an essential element for all living organisms. Phosphate rock is the primary source of P fertilizer, while it will be depleted in 1 century.(8) In National Mineral Resource Planning issued by the Ministry of Natural Resources in China (2016–2020), P was listed in the category of strategic minerals. SS produced from water treatment is rich in P. In China, there was approximate 125 000 tons of P of 24.186 million tons of SS production in 2012.(9) Furthermore, the P concentration in sludge is still increasing with the improvement of the water treatment technology.(10) Therefore, it is promising to develop P recovery technology from SS, which is recognized as the second largest source of P.(7)

Gasification need not be, even if sewage sludge contains considerable carbon, be carbon negative, nor carbon neutral. Au contraire it can be a source of carbon pollution inasmuch as gasification requires considerable heat.

In the author's work, they treat sewage sludge from the Shanghai municipal sewage treatment plant, at a range of 4 temperatures between 200C and 260C - subcritical water/steam - for a half hour, with the extrema being performed both in the absence and presence of 10% calcium oxide.

Following this treatment the resulting hydrochars were gasified with steam at 900C.

The carbon impact of this treatment is thus entirely dependent on the source of heat. If the heat source is coal, this process will be from a carbon perspective, disastrous. Were the heat sourced from nuclear energy - and it wasn't here, although China has built advanced high temperature research nuclear reactors - the process can be carbon negative, depending on how the resulting carbon originating from the sludge is used.

In any case, the problem with the gasification of sewage sludge and other biomass sources is very much connected with the fate of various other inorganic constituents, notably sodium and potassium, but silicon and metals can also be problematic. Depending on conditions, sulfur can be oxidized to species resulting in sulfuric acid, nitrogen to nitric acid, although it is also possible to transform these elements - under other conditions - to reduced species such as sulfides, sulfur, ammonia and organic amines.

Phosphorous can also be corrosive under the right conditions and hence this work. Phosphorous speciation is reported to consist of inorganic and organic phosphorous compounds. Among the inorganic phosphorous species are phosphoric acid, phosphorous pentoxide (P2O5) and phosphates of aluminum, manganese, and iron, as well as calcium phosphate where calcium oxide was added. Phosphorous pentoxide can volatilize at high enough temperatures; in this form it is an allotrope that is a dimer, P4O10 with an adamantane type structure having alternate oxygens and phosphorous atoms, with each phosphorous bound to a doubly bonded oxygen external to the ring structure:

Some graphics from the paper relating to the recovery of phosphorous from sewage sludge:

The caption:

Figure 2. P concentrations and recoveries of hydrochars.

The caption:

Figure 3. P recoveries and concentrations in gasification ash of dried sludge and hydrochars.

The effect of temperature on the volatilization of phosphorous:

The caption:

Figure 4. Conversion of P2O5 with temperature at 20 bar.

Some graphical representation of speciation in the presence of CaO additions:

The caption:

Figure 6. Evolution of Al/Ca/P compounds with CaO addition into the AlPO4–water system.

Some of the authors' concluding remarks:

This study discussed the transformation of P during the integrated thermochemical process combining hydrothermal treatment and steam gasification. The results showed that OP was significantly removed from solid during the treatment as a result of destruction, while IP was mostly recovered. Part of OP was converted to IP in disposal from 220 to 240 °C. At 260 °C, OP was emitted to the gaseous phase and, thus, not captured by minerals. Meanwhile, NAIP tended to be transformed into AP with the increase of pH, and CaO addition significantly promoted this transformation. In the gasification process, OP was completely removed, while IP was almost recovered with the substantial conversion from NAIP to AP. The highest total P recovery (84.92%) was achieved from GA-200-CaO, while P was the most concentrated in GA-240 (30.39 mg/g).

My personal sense of disgrace is not that so much that I live in a country increasingly ruled by sybaritic mindless pigs - although this is troublesome - but is more connected with the obvious fact that we do not care a wit for future generations; our contempt for them is not even barely disguised.

(I want, I need a Tesla car!!!!)

At least these scientists are leaving them with some important information; one wishes one could feel that they will have the resources to employ this information not just for survival, but in order to have the ability to do the good and great things that properly treated, women and men can do.

Have a nice weekend.

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Reply Recovery of Phosphorous from the Hydrothermal Gasification of Sewage Sludge. (Original post)
NNadir Sep 2018 OP
Eko Sep 2018 #1

Response to NNadir (Original post)

Sat Sep 29, 2018, 12:04 AM

1. How is it that someone gets to keep bashing green energy

and is not banned from here?

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