The completely water-cooled furnace (Fig. 4.2) provides a cooler envelope for the flame than a refractory-lined front wall or floor and hence produces less NOx. Most NOx is generated at the flame front when combustion is initiated, and a water-cooled furnace absorbs some of the radiation from the flame, which helps cool it, whereas a refractory-lined boiler reradiates energy back to the flame, keeping it locally hotter, thus increasing its potential for forming thermal NOx. The effective projected radiant surface for this design is greater than that of a refractory-lined boiler by 7-15%. Hence the net heat input per unit effective radiant area or the heat release rate on area basis is lower, which also helps lower NOx.