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Biomass System Selection – Start with the Fuel

5 March 2023

The number of possible biomass fuels and fuel mixes utilizing waste “opportunity” fuels for a biomass energy system is large and continues to change and grow. When you include the variabilities imposed by varying soil and climate conditions for agricultural wastes and varying feed for animals that results in changing animal waste fuels, the variability is almost limitless.

One approach that is consistently tried is to take a combustion technology and try to make that combustion technology work for all fuels. This “one solution fits all” approach often results in unsatisfactory results. We would argue that the best way to design and optimize biomass energy systems is to start with the fuel and then to smartly apply the best fuel conversion technology.

Successful conversion of biomass fuels includes all of the elements that go into an efficient and smoothly functioning plant, including but not limited to, the areas of fuel receiving, sizing, possible densification, storage, reclaim, delivery to the boiler and metering into the boiler followed by controlled combustion or gasification, ash handling including disposal or sale, and finally air emission controls. These separate systems make up a unified system where if any one of them is failing to perform it usually means the plant is down.

In order to design these systems, one needs to know as much information as possible about the fuel itself, including its size distribution (dust to large pieces), shape (evenly shaped particles to strings, sticks or leaves), structure (crystalline, amorphous), bulk density and chemical analysis, as well as information about where it was produced.

Analysis of the fuel including proximate and ultimate analysis that includes elements and compounds often not reported such as chlorine and detailed mineral analysis of the ash. These results should also be reported in both “as-received” and “dry” condition. While published information is very useful as a general guide and for initial discussions of possible energy conversion projects, it should be noted that standard published information is not adequate for initial feasibility studies, and certainly not for final design. The possibility for wide variations from standard published information is vast. For example, biomass fuels grown in the lateritic soils of West Africa will vary significantly from the same species of plant grown in Europe or Asia.

A recent example would be East Energy Renewables in North Carolina. The challenge here was to convert hundreds of thousands of annual tons of poultry litter from numerous North Carolina poultry growers into steam, renewable energy and heat.

POULTRY LITTER FUEL CONSIDERATIONS:

  • Moisture and Heating Value – Conservative sized single pass furnace with lower heat release and furnace exit gas temperature, along with heated air for combustion
  • Size Distribution – Effectively proven stoker grate combustion and individual feeders
  • High Alkali (Na + K) – Fouling considerations including wide tube pitch on heat recovery surface components, low flue gas velocities, and strategically placed sootblowers
  • High Ash Content and Characteristics – Traveling grate, oversized bottom ash chute and collection conveyors, and some design considerations as high alkali
  • The effects of fouling and corrosion influence – all facets of the boiler design, including the furnace sizing, flue gas velocities, tube spacing and materials
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