Solis: an Italian combo showcase
Biomass combined heat and power using organic ranking cycle. In 2016, Solis inaugurated its 5 MW combined heat and power (CHP) plant in Settimo Torinese, Italy. The EUR 9 million greenfield project is an archetypal showcase of a CHP concept with Organic Rankine Cycle (ORC) gaining favour across Europe. A concept that combines two core Italian technology providers.
Consisting of green waste wood from sawmills, forest residues and ”top’n’lop” from parks and field verges, the fuel is supplied to the plant as chipped material. The fuel is sourced from within 70 km and delivery is by truck weighed on arrival and on exit after unloading. A wheeled loader is used to fill a 400 m3 fuel bunker with moving floor. The total onsite storage is enough for 5-6 days of operation.
Thermal oil boiler
From the fuel bunker the woodchips fall into a drag-chain conveyor that brings the fuel up to the boiler’s in-feed. A double valve sluice that prevents back-burn meters the fuel into the hydraulic in-feed of the 4 MWth step-grate thermal oil boiler.
Supplied by Italian boiler manufacturers Uniconfort, the boiler enables the efficient combustion of biomass with a high-moisture content.
Due to ground conditions, the entire boiler unit is elevated approximately 200 cm from the floor to accommodate the bottom ash removal system. The bottom ash is recycled as soil enhancer whereas the fly ash, removed from the flue gas using bag filters, is sent to landfill. Ammonia is used to reduce NOx emissions.
The hot oil from the boiler is transferred to the 1 MWe ORC block, supplied by Italian ORC specialists Turboden. The unit is 10 CHP Split meaning it uses two circuits, a high temperature and a low temperature. In the former, the hot gases in the boiler combustion chamber heat the thermal oil from incoming 250°C to 310°C.
In the latter circuit heat exchangers capture heat from the flue gases heating the incoming thermal oil from 130°C to 250°C. Silicon oil is used as the working medium as, according to Turboden, this results in better electrical performance with slower turbine rotation, lower pressure, no erosion of metallic parts and blades and no water consumption.
The ORC turbogenerator uses the medium-to-high-temperature thermal oil to preheat and vapourize the silicon oil in the evaporator. The vapour turns a low-revolution turbine in an asynchronous generator. The exhaust vapour flows through the regenerator, where it preheats the liquid silicone oil that is en route to the evaporator. The vapour is condensed in the condenser, which is cooled by the 60 – 70°C incoming water from the district heat network heating it to the outgoing temperature of 90°C.
The liquid silicon oil is pumped into the regenerator and evaporator, completing the closed-cycle operation. A thermal oil emergency circulation system is in place to cool the oil to a safe temperature in the event of a power failure or other emergency situation.
As heat supply to the Settimo Torinese district heat network is the priority for the plant, an ORC by-pass is in place. This means the boiler can operate via a heat exchanger and cooler, directly against the incoming district heat water circuit. The entire plant is automatic and run by off-site staff.