The document discusses the most efficient milling installation for a biomass boiler fed with wood chips. It analyzes different types of mills (hammer, knife, disc) and milling strategies. Hammer and knife mills are most commonly used due to their lower energy requirements and costs compared to disc mills. Within hammer mills, modifications like an endless sieve can improve performance by reducing particle size and dust. An economic analysis found that while mill types require similar energy, the type of biomass significantly impacts energy needs, so source proximity should be considered. The conclusion is a hammer mill with modifications is best for the boiler, though the biomass type is also important.
4. Introduction
Biomass nowadays plays important role in energy sector, as it is considered
renewable energy source with zero CO2 emission
Zero
emission??
5. Introduction
In boilers biomass can be combusted directly or co-combusted with coal.
Especially for this purposes, it needs to be broken into smaller particles in
order to achieve optimal combustion characteristics, and to make fuel
easier to handle.
6. Wood chips biomass
There are different kinds of wood chips
from different trees
from different sources
from different tree parts
Properties vary from case to case
8. Milling (or grinding) biomass
Wood chips, like any other ligno-cellulosis material, is difficult to grind, the
process requires significant amount of energy.
9. Milling (or grinding) biomass
Apart from the energy requirement, the choice of the best grinding solution
also depends on economicfactors: investment and operational costs.
What is the price?
10. Milling installations
Hammer Mill
Principles of impact and
pulverization
Fragmentation by high velocity
impact
Relatively cheap and produces a
high percentage of fine particles
Heat generated in the impact
process is the cause of the drying
effect
Energy consumption depends on
the initial physical properties
11. Milling installations
Knife Mill
Final particle size depends mainly
on the feeding velocity, rotational
speed of the rotor, and type of
the drum screen
Various types of knife mills are
widely used for biomass milling
Energy requirementof knife mills
was found in all cases to be less
than that for other mills used for
milling of dry biomass
12. Milling installations
Disc Mill
Material is fragmented between
discs and flows to their periphery
Shear and compression are the
predominant comminution
mechanisms in these types of mills
Energy demand is strongly
dependent on the initial
temperature of the biomass. The
higher the temperature, the lower
the energy demand.
Standard particle dispersion
equipment in industries
13. Milling installation
Most suitable equipment
Because of the energy requirement in relation to particle size, hammer and
knife mills are the most commonly used.
Hammer mills are relatively cheap, easy to operate, and produce a wide
range of particles
What can be done to improve a conventional Hammer Mill?
14. Different configurations for Hammer
Mills
Weaknesses
Fineness of the particles produced
depend on the size of the screen
sieve
Milling rates fall as the moisture
content increases
Screen sieves are continuously
changed
Dust particles are usually released
into the atmosphere
Produced particle size is more
than 400μm
Solutions
Endless sieve
Fan
Mechanical separator
Sedimentation chamber
Sieveless screen
15. Different strategies for pulverization
Based on the work of “Luis S. Esteban, Juan E. Carrasco. Evaluation of
different strategies for pulverization of forest biomasses. CEDER-CIEMAT.”
Different processes applied to biomass in pulverization stage
17. Different strategies for pulverization
Great differences in the energy required by the three biomasses.
Proving their unlike physical and mechanical properties.
Greater requirements were recorded for pine chips due to bulk densities,
moisture contents and mean particle sizes higher than in poplar chips.
Regarding the energy requirement, there is no much difference between
the strategies chosen, but between the different types of raw biomass used
18. Economic analysis
Key factors
Energy needed for the process
Operational costs – labour
Maintenance costs – ex. the price of working parts
The investment cost
19. Economic analysis
Hammer and knife mill comparison
Not possible to control the particle
size in hammer mill.
Hammer mill requires similar amount
of electric power.
Both hammer and knife mills have
low investment and maintenance
cost.
Difference in total cost is higher, as
the particle size required is larger.
Knife mill can only process very
clean biomass, increases the
maintenance cost.
20. Economic analysis
Disc mills
Appliedonly in cases when the other solutions, such as knife
or hammer mill cannot produce the desired results.
Higher energy requirements.
Higher investment costs.
More expensiveto maintain.
21. Economic analysis
Solutions to conventional Hammer mills
Increase on the investment cost.
Significant decrease on the maintenance cost.
Decrease in the maintenance cost implies directly a decrease in the labour
cost.
Decision making in applyingthese solutions must be based on NPV and IRR
model.
Taking into account only the biomass quality, all the solutions would be
applied.
22. Economic analysis
Pulverization strategies
There is not a significant difference between the energy requirements for
the different processes within the same type of raw biomass.
Can not be a decisive factor when it comes to economic decision making.
The origin of the biomass woodchips is a crucial factor.
When developinga biomass installation with this kind of technology an
analysis of proximity to resources wouldbe interesting, as well as a further
study on what biomass requires less energy requirements.
Graph theory and the shortest path problem in an heuristic model.
23. Conclusions
Energy required for grinding biomass wood chips or other lingo-cellulosic
material is considered a decisive factor when choosing the most suitable milling
installation.
Hammer mills require the same amount of energy and have similar investment
and maintenance cost than knife mills. Knife mills are more suitable when a
flexibility in particles size is demand, although they can only operate with very
clean biomass.
Due to that facts, the best milling technology for our installation, biomass boiler
fed with woodchips, is a hammer mill.
All solutions for conventional Hammer mills proposed should be applied.
There is not a considerable difference between every process, but between the
different raw woody biomasses used.
