Converting raw biomass to industrially valuable synthetic products.

[NNadir]

The paper I will discuss is this one: Alumina-Supported Spinel NiAl2O4 as a Catalyst for Re-forming Pyrolysis Gas. (Min Song et al, Ind. Eng. Chem. Res. 2019, 58, 27, 11770-11778

The engineering challenge that future generations will face to remove carbon dioxide from the atmosphere, only one of the many messes we are leaving for them in light of our contempt for them, is enormous.

Not only has petroleum waste accumulated in the atmosphere, but the chemicals and materials that petroleum has provided may also be unavailable to them since, one way or the other, petroleum will be depleted, and in fact, should be banned, although the popular approaches to doing so are quite simply - I need to be blunt - stupid.

Recently in this space I discussed a possible method to deal with the sargassum crisis in the Atlantic ocean by converting it into a means of removing carbon dioxide from the air by pyrolysis: Can We Recover Carbon Dioxide From the Atmosphere Using Sargassum Seaweed? The papers to which I referred in those posts dealt with pyrolysis, the heating of biomass to very high temperatures in the absence of oxygene to reform it.

The paper here is also about reforming, but is a route to avoiding the high temperatures that I often suggest, wherein nuclear heat is utilized.

At low temperatures, pyrolyzed biomass forms three components, a gaseous phase, a liquid phase known as bio-oil, and a semi-solid phase known as char (often featuring asphaltenes).

This paper is about converting the gaseous phase into "syn gas" a mixture of hydrogen and carbon oxides that can be used to replace any industrial commodity currently manufactured using dangerous fossil fuels.

From the paper's introduction:

Currently, production methods of syngas (H2 and CO) generation have attracted much attention. At present, industrial production of syngas is mostly from the conversion of fossil fuels,(1) such as coal gasification,(2,3) methane re-forming,(4) and partial oxidation of methane,(5,6) etc. However, in consideration of the nonrenewable nature of fossil fuels and the environmental impact caused by greenhouse gases, it is a global interest to find renewable and clean energy sources as alternatives to fossil fuels. Abundant worldwide biomass can be obtained from various cheap and nonfood resources,(7,8) such as energy crops, agricultural residues, and organic wastes, etc. Therefore, thermochemical conversion technology for biomass,(9,10) e.g., gasification and pyrolysis, has been considered as a sustainable method for syngas production because of the inexpensive and renewable nature of the raw material. Among various biomass thermochemical conversion technologies, a two-stage pyrolysis–re-forming system(11) has been used to convert biomass to syngas, which possesses unique advantages. The biomass is first rapidly pyrolyzed in the first stage, and the derived vapors, excluding the biochar (retained in the first stage), is catalytically re-formed in the second stage. The biomass is not mixed with the catalyst, which can significantly reduce the carbon deposition on the catalyst surface and improve its re-forming performance.

A stable and efficient catalyst has a significant impact on gasification and pyrolysis.(12,13) Among various catalysts, nickel-based catalysts supported on alumina have been typically used because of the high catalytic performance and low cost. Much research has focused on the modification of nickel-based alumina(14) through the cooperation of transition metal,(15) alkaline earth metal,(16) and alkali metal(17) in order to achieve stable catalysts which can resist coking and sintering effectively...

...The effect of the spinel structure on the catalytic performance of nickel-based alumina has now been thoroughly investigated in this study. Four nickel-based alumina catalysts were prepared using the co-precipitation method and calcined at different temperatures. The catalytic performance of obtained spinel catalysts was investigated for the re-forming of simulated pyrolysis gas (CH4 + CO2) and then applied to the re-forming of gaseous products from rice husks pyrolysis. In addition, various characterization methods were applied to illustrate the textural properties and surface characteristic parameters of catalysts so as to reveal the nature of the influence of spinel structure on the catalytic performance of biomass thermochemical conversion...

Lately I've been very interested in nanostructured inorganic species as I've vicariously followed my son's research at Oak Ridge this summer, where he's been working to develop some tools to study neutron diffraction capabilities and so some of my interests are focused on nanostructure. This paper contains the usual TEM graphics...



The caption:

Figure 1. TEM images of four fresh catalysts

...and the usual XRD data by which one knows how one is doing with making what one wants...



The caption:

Figure 2. XRD analysis of fresh catalysts (NiAl2O4, JCPDS No. 78-1601; NiO, JCPDS No. 44-1159; Al2O3, JCPDS No. 10-0425)

A technique with which I have been unfamiliar, TPR, is also applied for characterization:



The caption:

Figure 3. H2-TPR of four fresh catalysts.

Some elucidating text on the technique:

The results are presented in Figure 3. In some research,(19) the nickel species were divided into three types depending on the different reduction temperatures, the free NiO, the “surface NiAl2O4”, and the crystalline spinel NiAl2O4. Other research(21,22) also reported the same phenomenon in nickel-based alumina and named the reducible Ni2+ species with ?, ?, and ?. However, it should be noticed that there is also a reduction peak of nickel species in their results when the temperature is less than 300 °C, which they both neglected. So we make a change to the way the nickel species divided. The nickel species in the prepared catalysts could be divided into four types in Figure 3, whereas the NiO species with a reduction temperature below 350 °C corresponded to the free NiO, the NiO-Al2O3 species with a reduction temperature at 350–600 °C corresponded to the NiO just supported on Al2O3, the bulk NiAl2O4 species with reducibility at moderate temperatures (600–750 °C) were identified as Ni2+ ions that are not completely integrated into the spinel, and the spinel NiAl2O4 species with a reduction temperature over 750 °C corresponded to the NiO in the spinel structure. In addition, no reduction peaks of alumina support were observed in this reduction temperature range.(21)

Pore sizes for the catalysts:



The caption:

Figure 4. Pore size distribution and N2 adsorption–desorption isotherms of the fresh catalysts (solid symbols, adsorption; open symbols, desorption).

Performance of the catalysts with simulated pyrolysis gas:

Figure 5. Catalytic performance of four catalysts reduced at the temperature of H2-TPR results: a, CH4 conversion ratio; b, ratio of H2/CO; c, H2 selectivity (catalyst dosage, 0.25 g; CO2, 50 mL/min; CH4, 50 mL/min; N2, 100 mL/min; GHSV, 48000 mL/(min/g); re-forming temperature, 650 and 750 °C).

The structure of the catalysts and a mechanistic cartoon:



The caption:

Figure 6. Schematic of the three-dimensional structure of spinel NiAl2O4 catalyst (a), schematic of the surface of spinel NiAl2O4 catalyst (b), and reaction mechanism (c).

There is a discussion, in considerable detail of the deactivation of the catalyst and a study of the process of deactivation and possible regeneration.


And here, for the business end is the results of reforming real pyrolysis gas made from rice hulls:





The caption:

Figure 8. Gas composition from pyrolysis–re-forming of biomass (rice husk, 1 g; catalyst dosage, 0.25 g; N2 flow rate, 100 mL/min; water, 4 mL/h; pyrolysis temperature, 600 °C; re-forming temperature, 800 °C).

Note that these processes require high temperatures. If these temperatures are provided by the combustion of dangerous fossil fuels it is useless, and if the heat is provided by the dangerous combustion of biomass, it is far less useful and far more harmful and um, no, trashing pristine deserts with big mirrors for solar thermal schemes will not be an economically or environmentally viable to approach this technology.

It is only useful with nuclear heat.

From the conclusion:

In this work, the relationship between textural properties of spinel in nickel-based alumina and the catalytic performance has been illustrated in two reaction systems of simulated pyrolysis gas (CH4 + CO2) and real re-forming of gaseous products from rice husks pyrolysis with four kinds of nickel-based alumina catalysts. It is indicated that the spinel structure could improve the catalytic performance and the anticoking property of the catalysts. In the actual re-forming of gaseous products from rice husks pyrolysis, the spinel structure catalysts showed better hydrocarbon conversion performance and higher syngas yield. Various characterization results combined with mechanism analysis proved that, among various structural parameters, the spinel structure could improve the surface microporous properties and increase the pore size of the catalyst, which is closely related to the catalytic performance. When a spinel structure is formed in the catalyst, the nickel species will change the existing form, which also has a significant impact on the performance of the catalyst. Generally, the catalyst with spinel has a better catalytic performance and ability of anticoking.

Interesting paper I think.

Have a very pleasant Saturday evening.