Biomass Combined Heat and Power for Industrial Hosts

Using the heat that power plants waste

A biomass plant that only makes electricity rejects most of the fuel’s energy as low-grade heat to cooling towers or air coolers. A combined heat and power plant sells part of that energy as steam or hot water to a host: a sawmill drying timber, a food processor, a sugar mill, a paper mill or a district heating network. Total efficiency rises sharply, and the host often supplies the fuel as well.

What makes a good host

• A steady heat demand through the year, ideally around the clock.
• A long expected life for the host operation.
• Biomass residues produced on site or nearby.
• Space for the plant and fuel storage close to the heat users.
• Creditworthiness, because the host is also a customer.

Designing around heat and power

The feasibility study must decide whether the plant follows the heat demand, the power demand or a balance of both. Back-pressure turbines suit hosts with large, steady steam needs. Extraction-condensing turbines offer more flexibility when heat demand varies. Organic Rankine cycle units fit smaller hosts with hot water needs. The choice affects capital cost, efficiency and how the plant responds to changes at the host.

Contracts with the host

A heat supply agreement sets the volume, quality, price and reliability of heat, and what happens when either side has an outage. Where the host also supplies fuel, a fuel agreement links the two. These contracts are central to the bankability of the project.

Backup and reliability

Hosts usually need heat even when the biomass plant is down for maintenance. Auxiliary boilers or a grid connection for power keep the host running. The study should include these in the design and the cost estimate.