Why did water-gas reactions using coal and coke once provide much of the world’s hydrogen?

Before 1940, the water-gas reaction was the primary industrial method for producing hydrogen because it provided an efficient way to extract the element from readily available fossil fuels like coal and coke.1

In this process, red-hot coke is reacted with steam to produce a mixture of carbon monoxide and hydrogen ($H_{2}O+C\to CO+H_2$). This mixture, often referred to as synthesis gas, could be further processed through a water-gas shift reaction—passing the gas with additional steam over a catalyst—to convert more carbon monoxide into carbon dioxide and yield even more hydrogen.123

This method was favored because it allowed for the production of hydrogen in flexible proportions, which was essential for various industrial applications, including the Haber-Bosch process for ammonia synthesis. However, by 1970, the use of coal-based water-gas reactions for hydrogen production had declined significantly, largely displaced by more modern, efficient methods such as the catalytic steam-hydrocarbon process.12