This project was set out to demonstrate the viability of linking up the central hot water biomass boiler system with number of existing buildings in the centre of a college. The system design called for the construction of a central biomass boiler plant in a carefully selected location, chosen for good access for biomass fuel deliveries and for minimizing the costs of hot water piping distribution. The central heating plant consists of a boiler room, housing the biomass boiler, hot water distribution header and related equipment, and enclosed biomass fuel storage bins. The biomass system is fully automated, so that there is no manual labour associated with fuel deliveries, fuel handling or with conveying it from the storage bins to the biomass combustion system. The system is designed so that gas boilers (located in each of the four buildings) will take over automatically in case of any problem with the biomass system and would be able to provide full backup. The biomass boiler is sized so that it will be able to provide all the required thermal energy needed by the four buildings except in the very coldest weather conditions. In addition the system will provide all the hot water required in the central kitchen and student living building. The use of very efficient piping designed specifically for “district heating” applications was selected. One supply and one return pipe will connect each of the four buildings to the central boiler room, with hot water distribution employing large pumps, connected to the main header, located in the main biomass boiler room. Solid fuel biomass combustion systems used in district heating system applications are more complex than fossil fuel combustion systems and generally require additional components beyond the simple combustor/boiler system. The heating system components must be carefully integrated to ensure successful, trouble-free operation. Although not used in all systems, the main district heating system components generally include:
Heat energy is transmitted from central heating plants to the four individual buildings as hot water (or steam in some applications) in a closed network consisting of two pipes (supply and return pipes). Pipes are laid in the ground, usually at a depth of 4 to 6 feet. The pipes have thermal insulation that prevents heat losses. Various types of piping systems are used for heat distribution. Steel piping has been used by contractors for many years but other types of piping (fibreglass and Pex) are becoming more popular in recent years. On an average, heat losses in the distribution network account for less than 3 per cent of the energy transmitted in the pipes. The water circulating in the pipes delivers the heat to each individual building via heat exchangers, hot water radiators, hot water coils or in-floor heating configuration. The return pipe conveys the water back to the central heating boiler plant for reheating. The temperature of return water from customers to the biomass boiler ranges between 15 and 30 °F. A network of underground insulated fibreglass pipes was used to connect the hot water central system with each individual boiler room located in the existing building. The primary circuit is a 180ºF temperature water (160ºF return), with constant flow, and medium pressure (20 Psig) system. The fibreglass pipe is suitable for use at temperatures of up to 210ºF and pressures of up to 150 Psig. The selected pipe is oxygen diffusion-proof. The insulating material is a closed cell non-absorbent foam with densities over 2 Lbs.