Waste Heat Recovery From Boiler Stack

Abstract

Steam is generated in pressured vessels called boilers. The cost to industry to generate steam is substantial, typically up to 40% of total expenditure. In operating the boilers, efforts are made to have and keep the boiler‟s efficiency as high as possible and to recover any waste heat. Most of the heat loss from the boilers occurs through stack flue gases; accounting for up to 30% of the total energy input into the boiler. For large boilers and those that use natural gas, economizers and air – preheaters are installed to recover waste heat from flue gases. However, these heat exchangers are not suited for small boilers whose capacity is less than 6,800 kg/h and also those that use Heavy Fuel Oil (HFO). Most small boilers do not recover waste heat from flue gases. The small capacity boilers lose a significant amount of energy through the flue gases. To quantify these loses, as part one of this project, a short survey of ten stacks was conducted. The survey found that all the ten stacks had no heat recovery systems. Five were insulated of which in only two was the lagging effective. The other five had no insulation at all. The stacks with well-maintained lagging had the lowest temperature drop (inlet 223°C outlet 210°C). The flue gases retained most of the heat energy and exited the stacks still at high temperature. The flue gas in other un-insulated stacks and with poor lagging most of the energy through the stack surface and exited on average, at 67°C. To recover some of the waste heat the stack can be modified to act as a heat exchanger to recover waste heat from the same flue gases. In the modification of the stack, the double pipe coil heat exchanger design was adapted. The model stack was a 101.6mm (4 Inches) diameter pipe and 1280mm height. On the external surface of the pipe was brazed a helical 9.53mm copper tube with a 101.6mm pitch. The assembly was then insulated. The flue gases were conveyed in the normal way but lost heat to the water flowing in the copper tube. Tests on the stack were performed representing three scenarios. In the first scenario, hot gases generated by an oil burner were passed through a bare stack pipe. The temperature of 7 | P a g e the hot gases at entry to the stack was on average 385°C and exited at 122°C. The second scenario involved passing the hot gases through the stack with the copper tube brazed on the stack outer surface and then covered with a 35.3mm fiberglass insulation. This represented an insulated stack but with no heat recovery. In this scenario, the exit temperature was on average 185°C. The third scenario was similar to the second except that water was passed through the helical tube. This represented the modified stack with heat recovery. The exit gas temperature averaged 136°C. As the water passed through the helical tube, its temperature increased. For example, at a water flow of 0.0114 kg/s, the temperature increased from 22.7 to 48.7°C. The energy absorbed by water was found to be about 30.6% of the recoverable energy in the flue gases. This project has therefore shown that a significant amount of waste heat from small capacity boilers can be recovered with some limited modifications on the boiler stacks.