Biomass cogeneration plant

The amount of biomass available to the client company prompted it to look for a way to optimise it, replacing almost entirely the use of fossil fuels.

PAL intervened in the process of disposing of the old system and installing the new structure, which consists of three parts:

1. Biomass combustion plant
2. Thermal oil plant for heat recovery and ORC turbine power generation
3. Hot water cooling plant and waste heat recovery from the turbine used for the pellet drying line and district heating for external industrial users

The power delivered by the plant is 995 kW gross, which after internal self-consumption becomes 825 kW transferred to the grid. The thermal energy is instead 4,000 kW in the form of hot water at a temperature of 80/90°C, which can be used to power the new belt dryer for pellet production and, in part, for industrial district heating to an external utility. The annual quantity of biomass required to reach these targets is 16,000 t/year.

The cogeneration plant consists of:

• Storage bunker with hydraulic rake unloading system;
• Redler transporter;
• Hydraulic pusher;
• Sophisticated devices to ensure safety and autonomy.

The characteristics of the plant are as follows:

• 2 t/h of plant biomass input
• Biomass lower heating value 2,580 Kcal/Kg or 3 kW/Kg
• 6,000 kW thermal power input as biomass
• 995 kW gross electrical power generated (825 kW net)
• 8,000 operating hours per year
• Approx. 6,700,000 kWhe/year net electricity generation
• Approx. 30,000,000 kWht/year net heat output

It is managed by a central unit located in the boiler's electrical panel, and combustion takes place inside a high-tech firebox that maximises the thermal efficiency of reactions.

The process consists of three phases:

1. Drying the material in the initial part of the combustion chamber
2. Distillation in the combustion chamber
3. Reactions in the gas phase with the formation of the main compounds of total oxidation, namely CO2 and H2O.

The biomass used, which is virgin, natural and untreated, comes from plant material from dedicated cultivation, forest maintenance or pruning, plant material from mechanical processing such as bark, sawdust, shavings, chips, roundwood, etc.

The process of generating energy from biomass has the following advantages:

• High cycle efficiency
• Very high turbine efficiency (up to 85%)
• Low mechanical stress on the turbine due to low peripheral speed
• Low turbine speed, allowing direct connection of the electric generator to the turbine without the interposition of a speed reducer
• No vane erosion, due to the absence of liquid formation in the nozzles during expansion
• Long component life
• Automatic operation possible

The water superheated by the process has characteristics such that it can be used for civil and industrial heat transfer uses such as district heating, cooling and drying.