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Sub-bituminous coal is the mid-grade of coal most people are used to in New Zealand. The dull black rock is the most common coal burned for electricity and industrial heat within NZ, but is also considered a significant polluter and contributor to greenhouse gas production. It does not contain recognisable plant fragments.
THIS PRODUCT SHOULD NOT BE USED FOR HEATING OR COMBUSTION EXPERIMENTS INDOORS. It burns hot and produces significant amounts of smoke if not properly ventilated.
Sedimentary: Formed from the accumulation and burial of dead land-based plant material (therefore technically an 'organic sedimentary' rock).
Source location: Waikato and Southland.
Sizes: Choose from the dropdown menu: Small specimen, Hand specimen, Shelf specimen or Extra large shelf specimen.
Images are examples of specimens only. Sizes and shapes may vary.
Storage: To prevent 'slacking' (crumbling when too dry), store the sample in a zip-top plastic bag. For long term display, coat a clean, dust-free sample with a clear coat of matte acrylic sealer or a PVA glue mix.
More detail
Structure
Sub-bituminous coal is a sedimentary rock composed mainly of carbon-rich plant material such as wood, bark, roots, leaves, etc. (essentially peat). The burial and compaction of this organic matter occurs over millions of years under less heat and pressure than bituminous coal. It can show a slightly layered structure, but often looks more uniform and can develop cracks when exposed to the air.
Type
Organic sedimentary rock.
Physical properties
- Colour: Typically dark brown to pitch black.
- Lustre: Dull or matte, with very little shine.
- Hardness: Brittle and fragile. Quite soft, with a hardness ranging from 1 to 2.5 on the Mohs scale.
- Density: Ranges from 1.1 to 1.4 grams per cubic centimetre. Less dense than bituminous coal.
General properties
- Carbon Content: Has a total elemental carbon content usually ranging from 70% to 76% on a dry basis (with a raw fixed carbon content of roughly 35% to 45%).
- Combustibility: Easily combustible and releases a steady amount of heat when burned.
- Formation: Forms from the compression and light heating of peat, then lignite, over long periods of time.
- Energy Content: Has a moderate energy content, making it a highly useful fuel source for local electricity backup and local factories.
Uses in New Zealand and worldwide
- Energy Production: Used at the Huntly Power Station as a crucial backup fuel to generate electricity for the national grid when hydro lake levels are low.
- Steel Production: Used uniquely at the Glenbrook Steel Mill in NZ as a fuel and reducing agent to convert local ironsand into steel.
- Industrial Processes: Used locally in heavy agricultural industries, including dairy processing factories and meat works, to generate high-temperature steam (but being steadily phased out of use and replaced by electricity).
- Residential Heating: Used in specific historic mining regions, such as Reefton and parts of the deep South Island, for home multi-fuel burners.
- Carbonisation: Can be processed to create specialised industrial char or gases, though it is less commonly used for chemical byproducts than higher ranks of coal.