The Titanic's boilers contained 159 furnaces. The ship had 29 boilers in total: 24 were double-ended and 5 were single-ended, producing the steam needed for her engines and turbine.
Coal was burned in these furnaces and fed by hand by the firemen working in the boiler rooms. Titanic carried thousands of tonnes of coal in her bunkers, and the furnaces consumed more than 600 tonnes on a typical day at sea. The work was continuous, hot, dirty, and physically demanding.
Steam from the boilers powered two four-cylinder reciprocating engines, which drove the outer propellers. Exhaust steam then passed to a low-pressure turbine driving the central propeller, allowing the ship to use energy that would otherwise have been wasted.
The number 159 refers to furnaces, not boilers. That distinction explains why nearby figures can be misleading: Titanic had 29 boilers but nearly five times as many furnace openings. Her power plant was designed for propulsion, heating, electricity, and the many hotel services required by a large ocean liner.