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Lecture 12

Options

• Unconventional Hydrocarbons

– Oil Shale

– Tar Sands

– Gas Hydrates

– Coal-bed methane

à Problems: extraction and removal of S and N

• Conversion of coal

– Gasification

• Above ground

• In-situ

– Liquefaction

à Problems: low H/C ratio and high S and N content

Tar Sands

• Sandstone reservoirs impregnated with very heavy, viscous crude oil

• Oil cannot be removed in conventional way à surface mining necessary

• Retorting similar to oil shale approach

Canadian Tar Sand Deposits Fig. 1

Assessment

• Large, low grade deposits

à Large scale surface mining

• Predominantly heavy oils

à Production of kerosene and bitumen

• High content of S, N and other elements

à Potential for strong negative environmental impact

Coal Bed Methane

• Methane found in coal seams

• Natural Production

– Microbial action

– Thermal breakdown of organic material

• Held in place by water covering the coal seam

CBM in USA (Fig. 2)

Challenges

• Coal beds often act as aquifers for groundwater

• Potential impact on fresh water resources

– Lowering of ground water table

– Pollution

Coal Bed Methane - Review

• Resource of considerable magnitude

• End product: clean gas – methane

• Concerns:

– Small individual deposits à many drill sites

– Associated with ground water aquifers

à competition with agriculture

à contamination of groundwater

à disposal of ‘brines’

à lowering of water table

Coal Gasification

• The idea: convert coal into gas

à improve convenience of use

à decrease the production of unwanted by-products (SO₂; fly-ash etc.)

• Two potential approaches:

– Incomplete burning à CO

– Addition of H à CH₄

• Location

– Above ground

– In situ gasification

Resulting gases

• Main gas ingredients

– CO ‘coal gas’; highly toxic

– CH₄ methane (ànatural gas)

– H₂ hydrogen

– CO₂ ‘final product’; not useful for further energy transport

– H₂S; COS; NO_x etc. ‘pollution gases’

Sulfur: Most sulfur is converted to H₂S; 3 to 10 % to carbonyl sulfur COS,

which needs to be converted to H₂S in a hydrolysis reactor prior to removal of H₂S

The gasification takes place in large facilities, resembling refineries (Fig. 3)

The principle of Underground Coal Gasification – UCG Fig. 4

Coal gasification - assessment

• Large potential resource

• Above ground conversion

– Advantages:

• Improved transport and use

• Removal of pollution ‘in-stream’

– Disadvantages;

• Additional steps à energy losses

• Production of toxic gases (CO)

• In-situ gasification;

– Advantages:

• Elimination of underground mining

• Use of ‘uneconomical’ seams (due to depth; thickness; accessibility)

• Potential for CO2 sequestration

– Disadvantages:

• Underground combustion – control issues

• Release of toxic gases

• Cave-ins

• Contamination of aquifers

•

Coal Liquefaction

• Purpose: Conversion of coal into liquid fuel

• Approach: Hydrogenation, i.e. addition of H to the C of coal

• Currently not used, but major developments occurred in Germany (WW I and WW II) and South Africa (Apartheid Period) in order to alleviate lack of access to oil resources

à Technology available, but not economic under current conditions

Synfuels and unconventional hydrocarbons - assessment

• Large potential energy source

• All of them are hydrocarbons related to fossil fuels and the use will add to the CO2 in the atmosphere

• Most of them require large-scale surface mining operations

• Relative high concentrations of S and other potential pollutants

à at best a short-term solution, but with large potential for severe environmental problems