The way to do calculation is as following:
1.1tph=1000kg/ hr= 5/18 kg/sec. It is mass per sec of steam.
2.Now take a steam parameters that is pressure and temperature.draw Rankine cycle . Find the enthalpy h1. The data will come for 1kg/sec.
3.Then find enthalpy for 5/18 kg/sec. So you get enthalpy for 1 tph.every coal has GCV that is gross calorific value.For every grade there is different GCV.
4.As efficiency of furnace is 85–90% that means ratio of heat consumed to form steam by heat given by coal is 0.85–0.90. so find the heat that is given by coal.
5.Then equate that heat that is heat given by coal= (mass of coal)*GCV. You have value of heat and GCV. You will get mass.