A good example of the kind of tech that doesn't need to be purely renewable by definition to be making a positive clean energy and environmental difference. Unfortunately it does not appear that plastic waste is going anywhere anytime soon, so finding a solution for what to do with that and extracting back out some of the energy that went into its original production is of course valuable. 

Charley - it's worthwhile to do a little investigative work before getting too excited about  "breakthrough" technologies involving the production of hydrogen.

Polyethylene, like most other materials we call plastic, is made up of molecules structured as a chain:

"C" indicates a carbon atom in the diagram; "H" is hydrogen. The image above is shortened - the chains have thousands of "links" (H-C-H) making up a long, stringy molecule which, mixed with trillions of other ones, creates a material useful for making drinking straws and other products which need to be flexible, durable, and inexpensive.

The hydrogen-carbon chemical bond - the little lines between H and C on the diagram - is one of the most useful in nature. When broken, it gives off energy in an exothermic chemical reaction (breaking similar bonds in gasoline is what powers internal combustion vehicles) . After two bonds are broken, two H atoms quickly bind together to form an H2 molecule which, unless it's harvested by an entrepreneur like yours, quickly combines with oxygen in the air to form water (H2O).

The first question to your entrepreneur should be: "What happens to the carbon?"

The carbon, as you'll see, is a problem. If it's ejected into the air, it also quickly combines with oxygen to form carbon monoxide (CO) or carbon dioxide (CO2), two potent greenhouse gases. It can be collected, but that process always requires more energy, and the energy almost always comes from breaking more hydrocarbon bonds (burning, or oxidation). The problem isn't solved, it gets bigger. There are no shortcuts.

Hydrogen doesn't exist in nature by itself, and the easiest way to make it is by breaking it away from carbon atoms. So with any hydrogen produced from a hydrocarbon, whether a plastic or a fossil fuel (gasoline, natural gas, diesel fuel, kerosene, propane, butane, etc) your first question should be: "What happens to the carbon?".

onedit: the ash cake you describe likely held some of the carbon given off by burning plastic, the rest goes into the air. Because hydrocarbons are so durable, from an environmental standpoint you're better off tossing it into a landfill - there, it will be sequestered for up to 30,000 years!