Most efficient milling installation for biomass boiler

fed with wood chips Álvaro González Fontana

Paweł Rodak

Scope of the presentation

Introduction

Wood chips biomass

Milling (or grinding) biomass

Milling installations

Different configurations for Hammer Mills

Different strategies for pulverization

Economic analysis

Conclusions

Introduction

Biomass represents wide variety of fuels

Introduction

Biomass nowadays plays important role in energy sector, as it is considered renewable energy source with zero CO2 emission

Zero emission??

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.

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

Wood chips biomass

Milling (or grinding) biomass

Wood chips, like any other ligno-cellulosis material, is difficult to grind, the process requires significant amount of energy.

Milling (or grinding) biomass

Apart from the energy requirement, the choice of the best grinding solution also depends on economic factors: investment and operational costs.

What is the price?

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

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 requirement of knife mills was found in all cases to be less than that for other mills used for milling of dry biomass

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

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?

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

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

Different strategies for pulverization

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

Economic analysis

Key factors

Energy needed for the process

Operational costs – labour

Maintenance costs – ex. the price of working parts

The investment cost

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.

Economic analysis

Disc mills

Applied only in cases when the other solutions, such as knife

or hammer mill cannot produce the desired results.

Higher energy requirements.

Higher investment costs.

More expensive to maintain.

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 applying these solutions must be based on NPV and IRR

model.

Taking into account only the biomass quality, all the solutions would be

applied.

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 developing a biomass installation with this kind of technology an analysis of proximity to resources would be 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.

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.

Q&A