Influence of Sewage Sludge on Ash Fusion during Combustion of Maize Straw

[NNadir]

The paper I'll discuss in this post is this one: Influence of Sewage Sludge on Ash Fusion during Combustion of Maize Straw (Liu et al, Energy Fuels 2019, 33, 10, 10237-10246)

As things stand right now there is nothing "green," about the combustion of biomass. Biomass combustion is responsible for slightly less than half of the 7 million air pollution deaths we accept each year without a whimper of protest, although such deaths are largely - but hardly entirely - found in the third world as a result of the combustion of things like straw and garbage indoors in the absence of suitable stoves. The extent to which this practice is "renewable" is a function of the depletion of soils in which the biomass is grown. The real "green revolution" of the 1950's was dependent on the fertilization of soils with fixed nitrogen, which nevertheless a threat to the planetary atmosphere - and, certainly of as much or possibly even greater concern, phosphorous, an essential largely mined resource which is very much subject to depletion.

Despite the above statement it does seem to me that the combustion of biomass under oxyfuel combustion - that is in an atmosphere of pure oxygen - does have much to recommend it. It is entirely possible, it seems to me, to do this in a closed system, one with no material exchange to the environment under any but entirely controlled conditions - which would include useful materials, including relatively pure carbon dioxide available for reduction to solid forms of carbon. Any carbon so obtained would be effectively removed from the atmosphere, and thus the process would not be carbon neutral but rather carbon negative.

I've thought a great deal about such systems, and daydream about them quite frequently, but a purely technical issue is the material nature of the reactors which might do this. Biomass contains a number of inert materials, some of which at high temperatures in the presence of oxygen can be quite corrosive. A material for accomplishing this must therefore able to withstand high temperatures while avoiding corrosion. I believe modern materials science can meet the challenge, but it is in no way a "slam-dunk." Another important issue is heat exchange. Slags can form on the walls of reactors that are difficult to remove, and also prevent free heat exchange, representing an engineering difficulty for the recovery and use of energy in these kinds of processes.

I once read a book called "The Big Necessity" by a woman named Rose George which was a wonderful, um, not exactly "popular" book but written at a level not requiring a scientific education, a rumination on human shit, and by human shit I am not referring to the racist orange thug in the White House, but rather that brown stuff, human feces.

One of the greatest waste disposal on this planet, short only of the problem of dangerous fossil fuel waste, is precisely that, human shit.

Actually though, sewage sludge might well, if regarded correctly, represent a resource, inasmuch as it contains water, carbon, and the aforementioned phosphorous, a very serious matter.

The aforementioned paper points to some possible advantages to including sewage sludge in the combustion of biomass, and it caught my eye.

This excerpt from the introduction to the paper describes in more detail describes some of what I've just said, although I would regard the first two sentences as being highly questionable as practiced:

As a green renewable energy source, biomass has a zero-greenhouse gas emission characteristic and can convert solar energy and carbon dioxide into useful chemical energy. The rational use of biomass energy can not only reduce the consumption of fossil fuels but also effectively reduce environmental pollution. Therefore, the development of biomass energy is important for heat and power generation.(1,2) In the past few decades, woody biomass has mainly been used to produce electricity and heat. Due to the ever-increasing need for woody biomass in other fields (chemical products and liquid biomass fuels), the price of woody biomass has risen.(3?5) As a result, more attention is paid to agricultural waste.

The ash content of agricultural waste is usually much higher than that of the woody biomass, whereas the composition of ash is also more complex and varied.(6) Agricultural waste contains a large amount of alkali metals (potassium and sodium), as well as related inorganic elements including calcium, magnesium, chlorine, and sulfur.(2,6,7) During the combustion process, most of the potassium in the fuels reacts with silicon to form potassium silicates with a low melting point. These potassium-containing compounds with low melting points exist in a molten state and lead to sintering and slagging at the bottom of the furnace.(6?8) When using a fluidized bed as combustion or a gasification reactor, potassium may also react with the bed material to form low-melting eutectic compounds, which results in the agglomeration of particles, hinders fluidization, and even causes failure of the fluidization.(9?11) Some of the potassium-containing compounds evaporate in the gas phase (such as KOH, KCl, K2CO3, and K2SO4) and condense or deposit on the solid or liquid phase on a low-temperature heating surface, eventually destroying the heating surface.(2,6,8,12) Straw is the most common agricultural waste and has considerable potential for development in terms of combustion for heat and electric power.(13) During the combustion process, the chemical reaction mechanism and the theoretical knowledge of straw (mainly wheat, cotton, and maize) ash have been extensively studied...

....In recent years, a variety of chemical additives have been commonly used in industry to alleviate the problems of ash sintering and slagging. However, this may require high investments and may reduce the economic viability of using these additives in industrial applications.(16,17) Therefore, it is necessary to find a new low-cost, environmentally friendly, anti-slagging additive. The co-combustion of a suitable amount of sewage sludge (SS) and potassium-enriched biomass can alleviate the corrosion of the heating surface, which is a good alternative to using chemical additives.(18?22) SS contains a large amount of silicon, aluminum, phosphorus, iron, and calcium. It was found that SS can capture the alkali metals in the straw and react with it to form high-melting-point compounds, reducing the formation of low-melting-point potassium compounds.(23) Therefore, SS can effectively alleviate the problems of sintering and slagging of biomass ash...

...Li et al.(25) studied the reaction mechanism of phosphorus in SS and potassium in wheat straw. The results showed that the reaction formed high-melting-point potassium aluminosilicate and alkali metal phosphate, which increased the potassium fixation rate of mixed ash. Skoglund et al.(26) conducted a cofiring experiment between biomass and municipal sludge. It was found that the alkali-chloride in biomass ash transformed into alkali metal sulfate after adding SS, which could reduce the risk of alkali metal chloride-related corrosion and slagging.

In general, SS can be used as an anti-slagging additive for the combustion of maize straw (MS), but the scientific evidence for evaluating engineering application feasibility and conducting cost comparison analyses was necessary. The main objective of this work is to study the effect of SS on MS alkali metals’ release characteristics and slagging. The potassium retention rate and the sodium retention rate of MS, SS, and their blends, as well as their slagging characteristics and ash characteristics, were obtained. The results obtained in this experimental study can provide data and theoretical references for sewage sludge’s use as an anti-slagging additive.

This is a Chinese paper and the sewage sludge in this case was dried. (I'm personally not sure that drying the sludge would be a good idea in the long term, as this incurs an energy penalty, but this is their process, not mine.)

They usefully, show the form of the maize straw (MS) and by was of eliminating the puerile silliness and squeamishness associated with this nevertheless important waste form, show a picture of the dried sewage sludge used in their experiments:



The caption:

Figure 1. (a) MS raw material and (b) SS drying raw material.

The maize straw was locally grown:

2.1. Samples. The molding MS used in the experiments originated in Jilin province, China, and is a major crop in northeast China. MS is directly processed in the farmland; a small amount of black soil may be mixed in MS. The MS is first crushed and then compression to form molding MS. SS from Jilin sewage treatment plant was selected, as shown in Figure 1. First, the molding MS and SS were naturally dried and then dried in an air-drying oven at 105 °C to a constant weight. Finally, they were pulverized to obtain particles with a size less than 200 ?m. Ultimate and proximate analyses of raw materials are presented in Table 1. The raw samples were ashed in accordance with ASTM/E1755-01. The chemical compositions of the MS and SS ashes were analyzed using X-ray fluorescence (XRF) (ZSX Primusll RIGAKU), and the results are listed in Table 2. To study the effect of SS as an additive on MS ash slagging, the blends of MS?SS with sludge mass ratio in maize straw-sludge mixture of 10 and 20% (by weight) were used and were denoted as M9S1 and M8S2, respectively. The amount of additive was selected based on two factors: (1) the amount of mixed additive should be sufficient to meet the expected reaction requirements, and can significantly reduce the issues related to ash melting and slagging, and (2) the mixing ratio should be practical and feasible. When the additive is mixed, the increased ash content after combustion should not be too high. This is because it becomes difficult for the combustion equipment to remove massive amounts of ash.

2.2. Combustion Process. The combustion experiments were conducted in a muffle furnace. The door was kept semiopen to ensure that the sample was completely burnt in the air. The experiments were conducted at temperatures of 700, 800, and 900 °C. When the furnace temperature reached the set value, four samples (MS, M9S1, M8S2, and SS) were sent to the muffle furnace. To ensure the burning of fuel, each experiment lasted 60 min. After this, ash was collected for subsequent analysis.

Table 1:



Table 2:



The raw materials were analyzed by atomic absorption spectroscopy (AAS) after digestion whereas the ash was measured by XRF. In my opinion ICP/MS is the "go to" technology for elemental analysis and is preferred to AAS, but AAS has a long history and is generally satisfactory for use except where very sensitive analysis is required. (ICP/MS might have picked up things like cadmium and lead, the former being a big problem in agriculture, albeit in Southern as opposed to Northern China.) XRF has the advantage of being able to say something about speciation and also the capability of picking up chlorine, a very important element when one is considering issues in corrosion, a topic I may discuss when discussing other papers that have recently caught my eye in the carbon capture via biomass schemata.

This is how the ash looks when prepared at different temperatures:



The caption:

Figure 2. Morphology of ash at different temperatures.

There is a tendency for the alkali metals to migrate during the combustion process, via volatilization. Corrections were applied to reflect the differences between the starting material and the ash.

The following graphics touch on that point and the ability of sewage sludge to mitigate this migration.



The caption:

Figure 3. Effect of SS on the ability to fix alkali metals. (a) Potassium retention ratio; (b) sodium retention ratio; (c) potassium retention growth rate; (d) sodium retention growth rate.

XRD (X-ray diffraction) analysis of the speciation observed:



The caption:

Figure 4. XRD pattern of ash mixed with MS and SS under different conditions (a: MS; b: SS; c: M9S1; and d: M8S2). (1) SiO2, (2) KCl, (3)K2SO4, (4) KAlSi3O8, (5) K2SiO3, (6) Ca7Mg2P6O24, (7) Fe2O3, (8) Ca2P2O7, (9) Al2SiO5, (10) KAlSi2O6, (11) KAlSiO4, (12) CaAl2Si2O8, (13) KCaFe(PO4)2.

The caption:

Figure 5. Micromorphology of MS at different temperatures (magnification of 500×).

The following graphics refer to the XRD diffraction Energy Dispersive Spectroscopy, where the composition of the marked particles is determined.



The caption:

Figure 6. Micromorphology of SS at different temperatures (magnification of 500×).

A table of results:



Similarly:



The caption:

Figure 7. Micromorphology of M9S1 and M8S2 at different temperatures (magnification of 500×).

And finally:



The caption:

Figure 8. Micromorphology of MS and M9S1 at 800 °C (magnification of 5000×).

Some commentary:

3.5.2. Evaluation of Slagging Tendency on the Basis of the Chemical Compositions of the Ash
The difference in the ash chemical composition is the root cause for the difference in biomass ash melting temperature. The most commonly used indicators for determining the degree of biomass slagging are the ratio of alkali to acid, the ratio of silicon to aluminum, and the ratio of iron to calcium. The alkali acid ratio refers to the ratio of the sum of the alkaline components (oxides of iron, calcium, magnesium, potassium, etc.) to the sum of the acidic components (oxides of silicon, aluminum, and titanium) in the biomass ash. The ratio of silicon to aluminum refers to the ratio of the oxide of silicon to the oxide of aluminum in the biomass ash; the ratio of iron to calcium refers to the ratio of the oxide of iron to the oxide of calcium in the biomass ash.

The paper is interesting because it tells us a great deal about the properties of biomass both in the form of straw and in the form of sewage sludge, the latter being a material that represents a huge environmental problem but also may prove to be an important resource.

This is an air based combustion system, and differs from other alternatives to processing, for example, high temperature steam reforming, or dry (CO2) reforming, and oxyfuel combustion.

The paper does not address the suitability of these ashes for the recovery of phosphorous, for example, and other elements, nor does it specifically address the materials science issues connected with, for example, corrosion and scaling.

Nevertheless, this is very valuable information in defining a path forward for future generations to recover from what we have done to them.

I trust you're enjoying your work week.

[blm]

1. Much more interesting shit than the orange shit.

And more useful.

[NNadir]

2. You know, except for having made things worse, at the end of the day, the orange asshole...

...really doesn't matter.

He's an unfortunate blip, or fart, in human history, even less interesting than Caligula.

Caligula of course, put a horse in the Senate, and Trump's pet is not a horse but rather Lindsay Graham, a man with less self respect than Caligula's horse, and far less honest than the horse.

The fact that Trump with the help of his pet "not a horse but worse" Senator has accelerated the fall of the United States will not matter all that much, any more than Romulus Augustus presided over the fall of Rome. Nations fall of course, even powerful nations, and it's pretty clear that our country is in the process of committing suicide by Putin.

The issue of the environment, by contrast, is with humanity pretty much forever. The authors of this paper are Chinese, and it may fall them to save what is left to save, for all China's faults. The Chinese do pretty good science, in general, not perfect, but well funded and well thought out in my opinion.

[blm]

3. Lololol

Orange Caligula