Residues to Riches: Customer-Driven Process

From a woodlands perspective, we often perceive the forest as the start of the supply chain. However, in any marketplace, the customer has the last say. For forest-origin biomass, the mill is the customer, so the mill’s requirements play a key role in the design of supply chain logistics.

Most biomass-based combined heat and power (CHP) or power generation plants in service today were designed for sawmill residues as their main source of biomass.  The supply chain was simple, and the investment in both the supply system and mill infrastructure was minimal because of the low value of the material. Many delivery sites for biomass across Canada do not even have a tipper for full trucks or trailers. Mills may have an in-line grinder, but the capacity is limited to hogging bark. With forest-origin biomass, the supply chain will be more complex and mill improvements will be necessary.

Learn from experience
In the Nordic countries, modern boiler technologies are used such as bubbling fluidized beds or circulating fluidized beds that can tolerate high moisture content and foreign material such as grit, leaves, and needles in the biomass. The mill infrastructure is capable of handling deliveries of biomass in various forms from loose residues and bundles to small trees and even stumps. The ability to receive various forms and qualities of biomass puts less strain on the supply system and allows the involvement of contractors who use a range of technologies and methods. This allows each contractor to optimize his/her system to particular operating and stand conditions. A contractor working close to the delivery site may ship small trees from thinnings, whereas a contractor who is far from the mill may consider using bundles to decrease transportation costs. Most biomass is delivered just in time because mill storage capacity is limited and storing comminuted material results in additional dry-matter (energy) losses.

The ability to receive different forms of feedstock is achieved by the installation of an electric motor-driven grinder. The grinder has a large capacity (metres-wide rotor), and the electricity is often generated at the mill itself. Protected from the weather, with no need to travel among sites, it is a more efficient grinding system than a mobile one driven by a diesel engine. Although a mill can tolerate high moisture levels, it knows the value of dry material; mills carefully monitor moisture content and will pay on an MWh basis, not by the green tonne. Shipments are kept dry with covered shelters and conveyors.

Scale and feedstock considerations
Not all plants are at the scale where a large grinder and sophisticated receiving systems can be afforded. For example, a district heating system will require a small-scale supply system and the delivery of energy chips that are fed directly into the boiler.

Other conversion processes with stricter feedstock criteria will put greater restrictions on the source of biomass and the supply chain. For example, some gasification technologies require a large chip or chunk of wood. A fast pyrolysis plant will require a dry (10% moisture) and  2–6 mm long particle. Thus, particle size alone can limit supply-chain options. Chunking technologies are not common, although some chippers can produce a very large chip and some grinders now have optional rotors to produce finer material. An issue when using harvesting residues is that it will be difficult to get a good yield of the desired particle size.

There is great interest in pellets these days, but with sawmill closures and reduced shifts, the supply of the usual feedstocks is restricted. Again, harvest residues will be considered, but their use can be problematic. Residues contain grit, which will ruin pellet dies. Residues may contain needles, but needles can cause flash fires in drum dryers. Residues will make a lower grade and lower priced pellet from a more expensive supply. One option is to use this material as the heat source for the dryers, not as the prime feedstock for the pellets. In the future, if residues are to be used to make pellets, an additional separation or decontamination stage will be needed to remove grit and fines. These technologies are just emerging.

Carefully review the needs of the mill before designing a biomass supply chain. Hopefully, the mill has also considered the woodlands aspects when it was designed; otherwise, the supply system will be less than optimal. A plant that can handle many biomass sources and forms will result in easier sourcing, lower travel distances, and lower costs than a plant with a strict feedstock requirement.

Mark Ryans is with FPInnovations–Feric Division and can be reached at mark.ryans@fpinnovations.ca . His column is a regular feature in Canadian Biomass.