Clean Coal Technologies in Japan

4C2. Coal Water Mixture Production Technology (CWM)
Technology overview

1. Coal slurry overview
The fact that coal is a solid causes some challenges; it requires

and can be handled as an easy-to-treat liquid. Conventional coalmore complex handling than a fluid does, environmental

slurry, without additives could be piped but, due to anmeasures must be undertaken to prevent the dispersion of its

approximate 50% water content, exhibited poor long-run stability,

dust, and a lot of space is required to store it. Utilizing it in aand required dehydration before firing. High-concentration CWM

slurry form offers the potential to make the use of coal cleaner,

can now, as a result of studies on the particle size distribution ofcomparable to heavy oil. One type of coal slurry, coal oil mixture

coal and the development of a dispersant and other additives, be(COM), is prepared by adding heavy oil to coal. This is different

kept fluid as well as stable even when less water is added,

than another slurry, coal water mixture (CWM), a mixture of coalallowing it to be directly combusted without being de-watered.

and water. While COM is convenient for burning and while it was

The inclusion of only a small amount of additives stabilizes thetested before CWM, it failed to catch on because of the demandcoal-water slurry in which coal particles of a certain sizefor heavy oil for other uses. CWM, on the other hand, poses nodistribution are uniformly dispersed, constituting a weightproblems relative to spontaneous combustion or dust dispersion

concentration of approximately 70%.

2. Characteristics of high-concentration CWM
The typical characteristics of high-concentration CWM (Chinese3) Particle size distribution

coal CWM) are shown in Table 1 below.

For higher concentration and stability CWM, the size of thepulverized coal should preferably be distributed over a wide range

Table 1 Characteristics of high-concentration CWM
Coal concentration (wt%)
Higher heating value (kcal/kg)
Lower heating value (kcal/kg)
Apparent consistency (mPa-s)
Specific gravity (-)
Ash content (wt%)
Sulfur content (wt%)
Grains of 200 mesh or less (%)

rather than narrowly distributed. Standard particle sizes used are

68-70
5,000-5,200
4,600-4,800
1,000
1.25
6.0
0.2
80-85

roughly as shown in Table 2.
Table 2 Grain size distribution of high-concentration CWM
Maximum grain size

150-500 m

Average grain size

10-20 mGrains of 74 m or less

80% or more

Fine grains of several micrometers or less

Around 10%1) Coal type4) Rheological characteristics

In general, high-carbonization, low-inherent moisture (about 5%

CWM’s fluidity has characteristics of a non-Newtonian fluid butor less in approximate analysis), and low-oxygen content (about

can be characterized as approaching a Bingham fluid. The fluidity

8% or less in ultimate analysis) coal is suitable for high-characteristics also change, depending upon the type of coal,concentration CWM.

concentration, additives, and flow state. The apparent viscosity is2) Additives

roughly 1,000mPa-s (at room temperature and shear speed ofAdditives consist of dispersants and stabilizers. A dispersant

100/s).

functions to disperse coal particles into slurry, using electrostatic5) Heating value

repulsion effects or steric repulsion effects; sodium sulfonate fromThe heating value depends upon the type of coal used. Annaphthalene, polystyrene, polymethacrylate, polyolefine, and the likeaverage lower heating value is 4,600-4,800kcal/kg.

are used here. Additives, including the stabilizers CMC and xanthangum, are used to prevent coal particles in the slurry from settling.
3. Manufacturing process for high-concentration CWM
High-concentration CWM can be

Receiving/storage of feed coalproduced by pulverizng coal intoCrushinga particle size distribution suitablefor CWM, selecting the correct
additives (a dispersant and astabilizer),

andappropriately

blending the coal, water, andLow concentration
wet pulverization
De-watering
Pulverized coal/water/additives mixing

High concentration
wet pulverization

Dry pulverization

Coal pulverization andmixing with water/additives
by a pulverizer
Pulverized coal/water/additives mixing

additives to manufacture a highlyconcentrated, low-viscosity, highlystable, and good-quality CWM. A
diagram of the CWM manufacturing
process is shown in Figure 1.

Enhancement of mixing
CWM storage
CWM shipmentFig. 1 CWM manufacturing process70

Coal water mixture (CWM)