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Hydrogen from coal + ccs with energy penalty just 7 per cent
Production, Dec  12  2009 (The Hydrogen Journal)

- Robert Williams, senior research scientist with Princeton Environmental Institute at Princeton University (USA) has calculated how hydrogen can be made from coal with carbon capture using just 7 per cent more energy than making electricity from coal without carbon capture.

The system he proposes will also actually take carbon dioxide out of the atmosphere, avoid the need for coal-rich countries to import oil, provide extra clean vehicle fuels, and cut the amount of water used to generate electricity from coal by 81 per cent.

He was speaking at the December 9th conference “Which Technologies to Diversify Transportation Fuels” organised by IFP in Paris.

The plant he suggests has a feedstock of coal (90 per cent) and biomass (10 per cent) - both put into separate gasifiers to convert them into hydrogen, carbon monoxide and carbon dioxide (syngas). The syngas can be used to make liquid fuels (via the Fischer Tropsch process), create electricity and also (if needed) provide a supply of hydrogen.

If 66 per cent of the energy is used for liquid fuels and 33 per cent used for making electricity, and the value of the liquid fuels (gasoline or diesel) calculated as equivalent to oil refinery gate prices (ie untaxed gasoline price), then the overall additional cost of creating "clean" (ie with carbon capture) electricity from coal works out at just 7 per cent of the energy.

This compares to the (optimistically calculated, he said) 36 per cent of the energy used for carbon capture storage if you go about it the way which is conventionally discussed - retrofitting a carbon capture and storage unit onto the flue gas stream of a conventional pulverised coal plant.

A relatively small number of such plants could generate enough liquid fuel to cover the entire US light vehicle fleet, Mr Williams calculates, avoiding the US’ dependence on foreign oil.

Since the biomass takes carbon dioxide out of the air when it grows and this carbon dioxide ends up being sequestered underground, you could reasonably argue that the whole system actually reduces the net carbon dioxide in the air.

The vehicle fuels produced with this process are cleaner than gasoline because all the impurities have already been removed.

Approximate capital expenditure for a carbon capture unit retrofit on a 540 MW coal plant could be calculated at $430m, with an increase in water requirement of 33 per cent.

The problem with retrofit carbon capture is that it has to take carbon dioxide out of the flue gas, and it is at a very low partial pressure – if the entire flue gas stream is at 1 atmosphere, and carbon dioxide is 15 per cent of that, then the partial pressure of carbon dioxide is just 0.15 atmospheres.

Altogether it can mean an increase in the cost of electricity of 1.5 times what it was for a standard pulverized coal plant (which is old enough to have the capital costs of building the plant written off), he said.

By comparison, using IGCC (pre combustion carbon capture), without also creating liquid fuels, the overall energy penalty is around 19 per cent. This also means that there is much less carbon dioxide to deal with – 0.86 tons of CO2 per mWH for IGCC compared to 1.2 t CO2 per mWH for a retrofit.

It also uses much less water than a standard pulverized coal plant, and has much lower emissions of sulphur dioxide, nitrogen oxide, particulates and mercury.

Capturing carbon dioxide is easier because it can be removed from a high pressure stream of hydrogen and carbon dioxide mixed together – the partial pressure of CO2 is much higher – 11 atmospheres.

The overall costs are a lot higher though – Mr Williams estimates that a new IGCC plant will cost 3 times more than a carbon capture and storage retrofit on an existing coal plant (with costs of the existing coal plant written off).

Mr Williams calculates on the basis of a plant which takes in 1 ton of biomass for every 9 tons of coal as input fuel, and biomass costing about 3 times as much as coal in dollars per gigajoule.

The whole plant is based on well known technologies – so there would not need to be a great deal more research and development to build it.

The biggest challenges with building it could be co-coordinating sale and purchase of the 5 different commodities involved – buying coal and biomass, paying for carbon dioxide storage, and selling liquid fuel and electricity.

“We need new strategic -- power and oil industry partnerships, even in government,” he said.

There have been concerns expressed about the reliability of IGCC plants based on first efforts, but these can be mitigated to a certain extent by adding redundancy and having bigger plant units (such as gasifiers) than you might install if you had complete confidence in the plant reliability.

There has already been a 250 MW IGCC plant built in Buggenum, Netherlands which has a feed of 30 per cent biomass and 70 per cent coal, he said, with coal and biomass put through the same gasifier.

Robert Williams website






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