Scrubbing Syn Gas From Municipal Solid Waste.

[NNadir]

All of modern humanity's efforts to address climate change - few efforts have actually been serious - have failed.

Thus it will fall to future generations not merely to stop dumping dangerous fossil fuel waste directly into the atmosphere, but also to recapture it.

From my perspective, one avenue for doing this is pyrolysis of organic matter in particular, biomass.

To some extent, one source of biomass is municipal solid waste, which contains paper, cardboard and food wastes.

Incineration of biomass - which has been utilized for quite some time - is pyrolysis, strictly speaking, however often involves the release of toxic substances, in some cases some very toxic substances.

In closed systems, pyrolysis - given a source of clean heat - can be used to create "syn gas" a mixture of carbon oxides (generally monoxide) and hydrogen. Modern chemistry can basically make almost any large scale high production chemical from syn gas, or at least an alternative chemical having superior or equivalent materials. (DME, dimethyl ether, for example is a superior fuel to both LPG, gasoline and diesel fuel, and it can easily be made from syn gas.)

Thus it is with some interest that I read this interesting paper from Chinese scientists on cleaning up syn gas from municipal waste:

Treatment of Volatile Compounds from Municipal Solid Waste Pyrolysis to Obtain High Quality Syngas: Effect of Various Scrubbing Devices (Dezhen Chen et al, Energy Fuels, 2017, 31 (12), pp 13682–13691)

Some excerpts from the paper beginning with the introdution:

The thermochemical waste-to-energy (WtE) technologies are crucial for sustainable waste management, and the innovative WtE technologies are being constantly pursued by scientists.(1-3) Thermochemical processes, which are known as incineration, pyrolysis, and gasification, convert the organic components of municipal solid waste (MSW) into different energy carriers and reduce their negative impacts on the environment. Compared to conventional incineration technologies, pyrolysis and gasification have higher potentials for reducing the emissions of acidic gases (SOx, HCl, HF, NOx, etc.), volatile organic compounds (VOCs), PCDD/Fs, and leachable toxic heavy metals.(4) Public concerns about the pollutant emissions that could be related to MSW management support an increasing demand on gasification and pyrolysis technologies, especially in developing countries where the MSW management facilities are not fully developed yet. Some pyrolysis and gasification technologies have been developed at commercial scale in recent years. In particular, pyrolysis is the process of thermal degradation of organic material in an oxygen-free atmosphere, which produces pyrolytic liquid (a mixture of organic chemicals and water), syngas, and charcoal.(5) However, the products from MSW pyrolysis should be improved to be of any practical use.(5, 6) In a recent MSW pyrolysis process involving char reforming of volatiles, energy can be concentrated in the syngas with a little oil of improved quality left.(7) The syngas, which usually has a higher heating value (HHV) larger than 15 MJ/m3N, is of much higher quality than the syngas from gasification processes, and a promising product for feeding gas engines, gas turbines, or just a fuel gas. Syngas from both gasification and pyrolysis processes is usually contaminated with particulates, tars (defined as the organic compounds with molecular weight higher than benzene(8)), gaseous pollutants such as H2S, HCl, NH3, and alkali, and alkaline earth metals (AAEMs).

They utilize municipal waste from the Shanghai city dump, which is about 15% kitchen waste, 6% paper, 27% "cloth and fiber," 3% wood, with the rest being residue, probably glass and ceramics.

They hand separated these components - one can imagine automated systems to do this - and gave an analysis indicating that the carbon content, overall, of this residue was about 43% carbon, 6% hydrogen, and 48% oxygen - probably represented by carbohydrate oxygen.

They then set up apparatus according to this schematic:



Here's a description of the apparatus:

The treatment of volatile compounds includes a hot char filter to remove particulates in the volatiles; two Allihn condensers with ice water as condensing medium, to separate liquid products (here simply denoted as tars) from the syngas; an oil scrubber for removing the tar and particulates; and a sodium carbonate (Na2CO3) solution scrubber to remove H2S and NH3. The hot char from MSW pyrolysis, as reported by Wang et al,(7) was adopted to form a filter of 74 mm in height and 40 mm in diameter in a stainless steel tube, operated at 550 °C, which corresponds to an average superficial velocity of 280 h–1. The Allihn condensers have a heat exchanging surface of 680 cm2 in total. The oil scrubber uses 100 mL of cooking oil with a viscosity of 120 cP (Kerry Oils & Grains Co., Ltd., China) as a solvent for trapping liquid product in the volatiles, the amount of oil adopted is comparable to that of a previous research.(21) For the Na2CO3 solution scrubbing step, pH and L/G ratio are the most important factors affecting the efficiency 22) therefore, different concentrations (affecting pH) and amounts of Na2CO3 solutions are utilized to find the proper Na2CO3 concentration and L/G ratio to optimize the scrubbing effect. Table 3 provides the operating parameters of the experimental tests, which also include the temperature of Na2CO3 solution.

Here's a description of the pyrolysis itself:

The experiments were carried out in batches. Each run was performed with the same quantity of MSW sample and char, 100 and 42 g, respectively. Then, the temperature of hot char filter was increased by an electrical heater at a heating rate of 10 K min–1 up to the preset temperature of 550 °C. The pyrolysis temperature was set to 550 °C too, which is a commonly used temperature for pyrolysis proceses; the hot char filter remained at the same temperature as the pyrolysis reactor therefore avoided its reheating and saved the energy consumption in practice. As the temperature of the hot char filter reached 550 °C, the pyrolysis reactor was heated with the same heating rate to 550 °C and then kept at this temperature for 30 min. Simultaneously, the volatile products were forced to pass through the hot char filter, driven by nitrogen gas, and then went to the ice water condensers and scrubbers.

The key point here is the cooking oil scrubber and the sodium carbonate scrubber. A key feature of the pyrolysis is the hot char filter.

They obtain, in a 27% overall yield on a fairly clean gas stream consisting of carbon dioxide, hydrogen, carbon monoxide, methane, ethane and ethylene (ethene).

After scrubbing, they are able to clean up the syn gas considerably giving a mixture of hydrogen to carbon monoxide that has a mol to mol ratio of 1.46.



The above figure shows the reduction in the contaminants of the gas.

Their conclusion:

Step-wise cleaning of the volatile compounds produced from pyrolysis of MSW was investigated, with a specific attention to the cleaning effects of hot char filtration, condensation, oil, and Na2CO3 solution scrubbing. The combination of this stepwise cleaning system can improve the syngas quality in both syngas components conditioning and pollutant removal. Hot char filtration removed about 71% of the particulates in the volatile and converted about 32% of the oil phase into gases. Condensation was a simple but effective cleaning step, able to separate 67% of the oil phase and more than 14% of particulates in the original volatile compounds and greatly reducing the burden for the successive scrubbers. Oil scrubbing removed 100% of the particulates and 96% of the tar entering the system, especially the tars belonging to the Classes 2 and 4, and greatly reduced their dew point, ensuring the cleaning effects but the oil scrubber only took in slight shares of the total particulates and oil in the volatile, therefore ensuring its long life span at the same time. A Na2CO3 solution scrubber operating at 70 °C appeared highly effective in removing NH3, H2S, HCl, and tar, and guaranteed a high quality syngas for practical applications.

Personally, I like scrubbers of this sort for pyrolysis, but as opposed to carbonates, I prefer hydroxides.

One of my favorite clean up hydroxides would be cesium hydroxide, in particular radioactive cesium hydroxide, but that's entirely another matter.

This is an interesting, if obscure paper, and it was nice to see. Future generations will need this sort of thing, since we've been so diligent about screwing them over.

Have a nice evening.

[caraher]

1. What are the advantages of hydroxides over carbonates?

And...

One of my favorite clean up hydroxides would be cesium hydroxide, in particular radioactive cesium hydroxide, but that's entirely another matter.

Why radioactive Cs? What is the relevant radiochemistry?

[NNadir]

2. Hydroxides remove carbon dioxide in a reversible way.

In solution, they are easily regenerated as hydroxides by the addition of calcium (or strontium or barium) hydroxides for reuse, since the carbonates of these three elements are all insoluble.

Evaporated they may also be decomposed by heat.

Cesium, and to a lesser extent, rubidium and potassium, on heating of the carbonates can generate (with appropriate separation schemes) the free metals and equimolar mixtures of oxygen and carbon dioxide gas, an excellent tool for reformation reactions.

Radioactive cesium is of particular interest for side products containing halogen containing contaminants which may appear in the combustion of municipal wastes, and will quantitatively destroy the most problematic of all nitrogen oxides - the most persistent - nitrous oxide, N2O. (In fact, N2O gas can be utilized as a radiological dosimeter.)

It is also self heating.

In an ideal world, the radiocesium available - regrettably there isn't that much - might be utilized to make the insoluble compound cesium titanate, possibly nanostructured to include some barium fluoride. This material - which has been evaluated as a "waste form" for disposal, disposal being a very bad idea in my view - has the interesting property when irradiated of producing electron holes that can decompose some otherwise intractable halides. Exposed to air, it would remove N2O, SF6, residual CFC's, HFC's with air flow being maintained with convective heat from the radioactive decay.

If located in cities, this would destroy the important ground level pollutant ozone, by generating the same chemistry as been worrisome in the upper atmosphere where the radiation source is high energy solar radiation, gamma, x-rays and UV. Of course, in the upper atmosphere, ozone is an important shielding agent, but at the ground level, it is a serious pollutant, responsible for a fair share of the unacceptably large death toll from air pollution.

Exposed to water it could remove contaminants like the horrible PCB problem that has plagued the Hudson River, among other places, and I believe some of the serious perfluoroalkanoic acids, the most prominent such molecule being PFOS, perfluorooctanoic acid.

If one looks at old literature from the 1950's and 1960's, one can realize that many scientists then recognized the potential of fission products to be utilized for a number of very interesting applications. Regrettably, probably as a result of open air nuclear testing, these scientists ideas were lost owing to mass hysteria of the type that continues to plague the planet in an extremely destructive way.

There are no fission products, none, that cannot be put to use. I have spent 30 years convincing myself of that. They are wonderful and remarkable materials, many of which can do things that other radiologically stable materials cannot.

Regrettably, stupidity prevents us from recognizing this, in part because our culture has a "waste mentality" that believes in dumps. Of course the worst dump is our planetary atmosphere, and its use as such is killing us. We could shut this dump, and in fact clean it up and remediate it by embracing nuclear materials, but again, as a culture, we are too stupid to do so.