|Oxidative Hydrothermal Dissolution|
|By Ken B. Anderson, Southern Illinois University Carbondale and Thermaquatica Inc.|
|Most strategies for producing high value products from coal are based on first converting it into something equivalent to oil. After all, the better part of a century has been spent investing in developing ways to refine oil into a myriad of high value products. |
|Converting coal into syncrude has the advantage that it can then be fed into that pipeline (almost literally), taking advantage of that experience and the vast global petrochemical infrastructure that is already in place. |
The problem with that approach is that coal, especially low rank coals like Victorian Brown Coal (VBC) are not at all like oil. Their underlying chemistries are fundamentally different.
Compared to oil, VBC is low in hydrogen and very high in oxygen. As a result, trying to convert VBC into something equivalent to oil is like trying to force a square peg into a round hole – it’s an uphill battle from the start and the result isn’t going to be pretty.
VBC is, however, a very reactive material. For example, decades of experience have shown how readily it reacts with oxygen, often resulting in self-heating and even spontaneous combustion.
Recently, a promising new technology called Oxidative Hydrothermal Dissolution (OHD) has emerged that takes advantage of the coal’s inherent reactivity, and it now appears that OHD may provide a novel, environmentally friendly route to production of high value products from VBC.
OHD is a simple process. It works by reacting coal with small amounts of dissolved oxygen in moderately high temperature (200-350oC) high pressure (80-180 bar) liquid water (Figure 1).
Under these conditions, pulverized VBC reacts in a few tens of seconds, resulting in essentially complete conversion of the coal to low molecular weight products.
No exotic catalysts or solvents are required, no gaseous nitrogen or sulfur oxides are produced, the raw aqueous product – called liquor – is non-toxic (as seen by the fact that algae and fungi readily grow in it) and importantly, the process generates very little CO2.
Typically, more than 90 per cent of the original carbon can be recovered as soluble materials that can then be refined into a variety of products including: liquids that could potentially be used as transportation fuels; and chemicals that can be used to manufacture high value end products.
The major products of OHD of VBC (up to 50 per cent of the extractable organic product) are simple aromatic compounds called Vanillate and para Hydroxy Benzoate (pHB), which are derived directly from lignin, one of the major structural components of the original wood from which VBC is derived.
Vanillate and pHB, are high value materials that can be used to make synthetic polyesters that are essentially direct substitutes for polyethylene terephthalate (PET), the plastic widely used to make (for example) polyester fibers and drink bottles.
| Annual global production of PET, which is petroleum-derived, exceeds thirty million tonnes, with a gross market value of over $30 billion. Polyethylene Vanillate (PEV) is produced by a process similar to that used to produce PET and it has very similar properties, except that because its structure preserves aspects of its biological origins, PEV breaks down in the environment, whereas PET does not. |
PEV has not previously penetrated the polyester market because current prices for Vanillate make it uncompetitive. That could change, however, with production from an abundant, low cost raw material like VBC.
|Figure 1: OHD process|
|The OHD process has been successfully demonstrated for VBC at laboratory and small engineering scale, and work is now in progress to move ahead towards a commercial demonstration-scale facility. Once demonstrated at commercial scale, OHD has the potential to be at the core of a new utilization strategy for VBC that produces high value environmentally friendly products. |
OHD is an example of how innovative R&D can change entrenched paradigms and result in potentially game changing opportunities for VBC. It shows that we need to expand beyond strategies we have been pursuing for the last half century. Even a few years ago, who would have imagined using VBC as a raw material for the production of eco-friendly plastics? What other opportunities have we not yet explored?
We need to continue to look at VBC as an extraordinary resource, but not just for the uses it is put to now. We need to focus on high value products that provide strong economic returns to investors and that provide economic opportunities for Victorian communities.
We need to focus on environmentally acceptable routes to produce those products and to build community support by showing that focus. OHD is one promising new approach that has the potential to accomplish these objectives, building a better brighter future for VBC and the communities that depend on it.