Our coal is roughly half the caloric value of imported coal ( that differential is already accounted for in fuel cost/kwh, writing this for those who might confuse and say its cheaper because its caloric value is lower and use that to discredit like happened in the past).
I can not say for certain but I think these imported coal plants are designed to run at higher temperatures to attain the rated efficiency ( as bitumen by inherent properties burns hotter than lignite, also lignite is harder to burn). These coal plants have boiler sizes and running lignite vs high grade coal can not achieve the optimal output. Will require significant modifications in boiler size and boiler turbine arrangement ( if that is even possible).
Lignite coal plants are specifically designed are larger in size and are more expensive.
Maybe 20% is the most that we can mix to make these plants to run ( this shows government has considered this).