Jeff Mills, Senior Project Manager with Evergreen Engineering in Eugene, Oregon gave an overview of biomass fuel feedstock guidelines and the need to control variability. Not paying attention to this fundamental issue can make even the most sophisticated project grind to a halt. 

There’s too little attention paid to biomass feedstock, said Mills, and his review of four recent cases showed how new facilities and processes struggle with unexpected feedstock characteristics. 

In the first case, an 18 MW power plant in the southwestern US using urban wood fuel found that the character of the feedstock changed when source changed. Fiber length increased (the fiber was stringy), it had a lower fuel density, and dirt and contaminants increased. Effects on fuel handling and infeed equipment included pluggage, limited volume, downtime and slow facility startup  so capacity was not met. Ash handling systems saw capacity limitations and increased disposal costs. The fixes included modification of the reclaimer and conveyor systems to handle a wider range of properties. Lesson learned: “Urban wood” is a broad category with widely varying properties from different suppliers.

The second project was a pulp mill process in the US northwest planned to use 200 tpd of urban wood. Feedstock variations included urban wood, pallets and hog fuel. Problems were increased dirt and other contaminants and uneven fiber length. The basic lessons learned were similar to the case cited above. In addition, it became clear that quality urban wood in sufficient quantity costs more due to higher processing cost and competition for fiber. Also, at the bottom of his summary slide, Mills emphasized “DIRT” in bold caps. Enough said.

The third project used wood biomass from green forest thinning to fuel a 25 MW power plant in the southwestern U.S.  Biomass was processed in the field and changed with the use of a tub grinder versus a mill grinder. Ground wood was stored on the ground and reclaimed with a front end loader. The feedstock contained many larger pieces, was too stringy and had many rocks and stumps. The impact on feedstock handling equipment included plugging, higher wear and downtime. Ash handling equipment experienced pluggage and damage as did baghouse equipment. The fixes included a change in fuel handling – ground wood went directly into trucks with no ground storage. Screening and rejects handling in the ash system were also changed and a higher bag cycle cleaning rate instituted in the baghouse. The impacts on the power plant were increased downtime, slowed startup and increased cost. The lesson learned: biomass processing greatly affects quality. 

Project case study number four was the use of paper mill sludge as fuel for a 25 MW power plant. The issues here werre unforeseen chloride residuals and unplanned air pollutants from the sludge. The plant had to rewrite its facility air permit and modify emissions measurement equipment. The lesson learned: verify your feedstock. Trust but verify, is Mills’ mantra.

A summary of lessons learned in all four cases is that not all feedstocks are created equal. Buyers need to research suppliers and verify the supply. It’s important to specify properties for the feedstock, evaluate the supply, estimate the true cost, evaluate the feedstock market and institute quality control. 

Timing is also key: this should all be done after basic process development, during supply sourcing and site selection and before final facility design.