What economic factor allowed the Newcomen engine to be widely adopted in coal mines despite its high fuel consumption?

Unsaleable small coal (slack) was abundant and effectively costless at the mine site

The widespread adoption of the Newcomen engine, even with its noted inefficiency in fuel use, was heavily dependent on the economic context of coal mining locations. For coal mines, the fuel required to run the engine—often unsaleable or unusable small coal, frequently termed slack—was abundant and available at virtually no cost on site. In this scenario, the massive economic benefit derived from accessing deeper seams and increasing overall output far outweighed the thermal inefficiency of the engine. This reality—that cheap fuel at the point of use superseded poor thermal efficiency—was key to its success until the engine moved to locations where fuel was expensive, like the tin mines of Cornwall.