PLTU Bolok unit 2 merupakan salah satu penopang kelistrikan di Nusa Tenggara Timur berkapasitas 16,5 MW menggunakan batubara dengan spesifikasi nilai kalor 16.748 kJ/kg setara 4.000 kCal/kg. Batubara yang dipasok ke PLTU Bolok memiliki nilai kalor yang bervariasi. Penelitian ini bertujuan menganalisis pengaruh nilai kalor batubara terhadap efisiensi boiler. Menggunakan perhitungan efisiensi boiler indirect method (heat loss) untuk mengetahui losses-losses yang mempengaruhi efisiensi boiler. Sebelum overhaul pada nilai kalor 4.041 kCal/kg efisiensi boiler sebesar 84,866 % dan pada nilai kalor 4.069 kCal/kg efisiensi boiler sebesar 83,333 %. Setelah overhaul, pada nilai kalor 3.797 kCal/kg sebesar 84,237 %, pada nilai kalor 4.013 kCal/kg sebesar 85,064 %, dan pada nilai kalor 4.072 kCal/kg sebesar 85,073 %. Diagram sankey menunjukkan parameter heatloss (kerugian panas) tertinggi adalah akibat moisture pembakaran hidrogen dan akibat moisture di bahan bakar. Kerugian panas akibat pembakaran hidrogen tertinggi pada nilai kalor 4.069 kCal/kg sebesar 8,914 % sedangkan terendah pada nilai kalor 4.013 kCal/kg sebesar 6,918 %. Kerugian panas akibat moisture di bahan bakar tertinggi pada nilai kalor 3.797 kCal/kg sebesar 6,618 % sedangkan terendah pada nilai kalor 4.069 kCal/kg sebesar 5,707 %.

PLTU Bolok Unit 2 is one of the power suppliers in East Nusa Tenggara with a capacity of 16.5 MW, using coal with a calorific value specification of 16,748 kJ/kg or 4,000 kCal/kg. The coal supplied to PLTU Bolok has varying calorific values. This study aims to analyze the effect of coal calorific value on boiler efficiency. The boiler efficiency was calculated using the indirect method (heat loss) to determine the losses affecting the boiler efficiency. Before the overhaul, with a calorific value of 4,041 kCal/kg the boiler efficiency was 84.866% and 4,069 kCal/kg it was 83.333%. After the overhaul, with a calorific value of 3,797 kCal/kg the boiler efficiency was 84.237%, 4,013 kCal/kg it was 85.064%, and 4,072 kCal/kg it was 85.073%. The Sankey diagram shows that the highest heat loss parameter is due to hydrogen combustion moisture and moisture in the fuel. The highest heat loss due to hydrogen combustion was at a calorific value of 4,069 kCal/kg at 8.914%, the lowest was 4,013 kCal/kg at 6.918%. The highest heat loss due to moisture in the fuel was at a calorific value of 3,797 kCal/kg at 6.618%, the lowest was 4,069 kCal/kg at 5.707%.