No two biomass facilities are the same. When it comes to designing a wood yard and fuel-feed system, one size decidedly does not fit all.
Every facility has different needs and capacities when it comes to fuel handling. A study of 20 biomass power plants conducted by the National Renewable Energy Laboratory documented a range of tons burned per year from 98,000-846,000 tons – meaning that a plant on the high end can burn 8.6x as much as a plant on the low end.
Produced by a host of factors, these dramatic differences demonstrate that prefab or cookie-cutter fuel-feed designs could be unnecessarily costly, and even overkill, for many greenfield installations or conversion plants, whether dedicated exclusively to biomass or established for co-firing.
The burning questions
Principals responsible for any biomass projects have major governing considerations to weigh when evaluating wood yard design, including:
- Throughput volume and storage volume: Perhaps the most compelling metrics for determining whether manual or automated handling should be used for the fuel-feed chain
- Physical size of land and facility: Automation can require significant excavation and construction, and typically has a larger footprint than a manual operation
- Upfront capital budget vs. operating budget: Fully automated systems require considerably higher capital outlays to implement, but offer lower operating costs than manual systems
- Climate: Seasonal extremes can force a facility toward greater automation, which can be easily housed in enclosed systems
- Local regulations and public relations: In addition to federal emissions regulations, plants must take into account local requirements governing byproducts such as dust, ash and noise
Stock turnkey fuel-feed systems designed exclusively on a computer without any on-site assessment can overlook the impact of these important factors, resulting in an inefficient or unsuitable system.
Fueling the fire
Effective fuel-feed design has to take into account two primary operations – stackout and reclaiming. The need to shuttle between storage and reclaim piles also can become a major factor, depending on the size, layout and throughput of the facility. Any part of this chain can be designed with various levels of automation.
The rule of thumb is that the greater the automation, the higher the capital costs and the lower the operating costs. The inverse also is true: The less automation in the system, the lower the capital costs but the higher the operating costs.
There are three general classes of site design: fully automated, manual and semi-automated. At the extremes of the scale of possible site designs and costs, the fully automated and manual approaches have distinct pros and cons, as displayed in Table 1.
First degree: Automated
The criteria used to evaluate whether a prospective facility would benefit from a fully automated or manual approach can vary from project to project. A major university in the Midwest recently opted for full automation based on a multitude of factors, including fuel-feed requirements, weather conditions and long-term operational costs.
The university’s entire wood yard system had to fit in a half-acre area and needed to be fully enclosed due to harsh northern winters and regional dust control requirements. Full system enclosure eliminated the need for front-end loaders, reducing both labor costs and operator exposure to dust and weather.
The university’s fuel-feed and storage requirements also heavily influenced the system’s final design. The feed system needed to be capable of receiving up to 60 tons/hour, fed directly into an 110,000 cu.ft. storage unit. From the storage unit, feed would then be delivered to two boilers at a rate of up to 9 tons/hour.