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JEMC said:

I can understand that coal from different places have different dross or unwanted minerals that can affect its combustion, but enough to shut down a plant? Unlikely.

What is more likely is that China hasn't found another source that sells them as much coal as Australia did, and that lack of coal is what makes China shut plants down.

A different composition can affect it's efficiency, but depending on what they were using as fuel since there's more than one type of coal: Brown coal, the youngest and cheapest, but also the least efficient and most polluting; black coal, which is quite a bit more efficient and has less polluting elements in it (things like sulfur for instance), but also more expensive, and anthrazite, which is almost pure carbon, but also by far the most expensive.

A power plant built for brown coal could overheat on Anthrazite, and a power plant built specifically for Anthrazite will potentially not get enough heat from brown coal. In most cases however, you can get by by using less heating material with those that were built for brown coal, while those built for Anthrazite would get less Steam and thus less power and very bad efficiency, but should still be able to work. That is, if the plant was working with a subcritical steam turbine.

The problem can however be very real if the steam generators are supercritical or ultra-supercritical, which often need high grade coal powder (coal gets powdered these days to burn better and hotter) to reach the necessary temperatures to work properly. Those are also the among the most modern and efficient designs, and most coal power plants built in the last 40 years are either of those types or co-generate both electricity and heat for heating buildings, So it's not impossible that the power plants really needed to be shut down if the coal available to them ain't of high enough grade for those power plants to reach supercriticality.