Ash content

Figure 2-5. Hardgrove index of different coals, bio-coal, and torrefied pellets (Bagramov, 2010)

The process of size reduction is mostly done in several steps. First bio-coal is crushed into small pieces (less than 1cm) by the help of a hammer in very small scale or by the means of an industrial crusher in larger scale. Then grinding is done by means of different devices such as high speed crusher. The final step is micronizing of bio-coal which could be done by the help of micronizing devices (for example, Wiley mill or burr mill or ball mill (Long, 2014)). More detailed information about grinding devices could be found in Papachristodoulou and Trass (1987).

However, some other processes of grinding were tested by some researchers like Atesok et al. (2005), who used wet grinding of coal particles and also investigated the effect of two different dispersant on the grindability of coal and achieved good results. Cui et al.

(2008), also investigated a novel process for grinding the coal, utilizing a high pressure water-jet mill and froth flotation. They observed 50 to 70% reduction in energy used for size reduction of coal compare with the traditional mechanical mills.

2.4.5 Ash content

Considering the composition of the solid in slurry fuels results in low amount of impurities exist in both coal and bio-coal. This small amount of components does make big problems

when operating in diesel engines. For instance, ash is responsible for the corrosion in exhaust, piston wear and rings and cylinder wear. According to Ellem and Mulligan (2012), ash components are mostly in the form of oxides such as Silicon (SiO2), Aluminum (Al2O3), Iron (Fe2O3), Calcium (CaO), Magnesium (MgO), Sodium (NaO2), Potassium (K2O) and Phosphorus (P2O5). Silica is the most abrasive component in coal, which is responsible for wear in different parts of the engine (Patton et al., 2009), while other components of ash are not abrasive, they should also be removed because they create some deposits on the engine parts which cause problems (Patton et al., 2009). Silicon dioxides have also the potential of making glass in high temperatures. (Long, 2014) believed that by the addition of calcium oxides and potassium oxides, melting point of silicon dioxide decreases by 75% from 2000ºC to around 500ºC thus silicon converts to glass sludge when meeting high temperature in the engine and will cause undesired problems in engine parts.

The best way he had suggested to tackle this problem is to remove the ash content from the original biomass.

Figure 2-6. Normal diesel wear and accelerated wear caused by slurry fuels (Soloiu et al., 2011)

The major parts of the engine being destroyed by the ash are injector nozzle and orifices (Soloiu et al., 2011). (Soloiu et al., 2011) reported that engine ring wear in coal water slurries is 20 to 100 times higher than ordinary diesel fuel when utilizing the same engine component material. They argue that in order to minimize the wear caused by ash, special high strength material should be used which are too expensive. They used bio-coal slurry

fuel in a diesel engine and observed 4-8 times more wear in the injector’s conical seat body shown in Figure 2-6. They also observed corrosion on the injector’s needle.

Despite all the above mentioned facts, it should also be highlighted that the particle size of the solids also affects wear in a diesel engine. (Cui et al., 2008) set an investigation of the coal particle size on wear problems and observed double wear in piston ring and cylinder when increasing coal mean particle size from 10.2 to 16.3 µm while the maximum particle size were 32 µm and 47 µm respectively.

Generally, the amount of ash in bio-coal is less than that of coal. However, most of the ash components in bio-coal are water-soluble and easy to remove but silica removing is much harder (Patton et al., 2009). The optimum amount of ash which Soloiu et al. (2011) reported for their coal water slurry in a four-stroke diesel engine was 1 wt. %. They found the major problem of coal slurry fuels in the injector’s nozzle erosion.

Table 2-6 indicates the ash content in bituminous coal, hardwood, and softwood. Here, two important issues should be noted. Firstly, the ash content in bio-coal is higher than in wood in term of mass weight since wood losing a major part of its mass during pyrolysis but the ash remains in coal. Therefore, the same amount of ash remains in coal while bio-coal has around 30% mass of the initial wood and secondly, huge amounts of ash in wood, maintain in the bark because most of the ash in wood is the result of transport by wind and they mostly remain on the bark.

Esnouf (1991), had mentioned ash content in poplar wood bio-coal equal to 2.4-3.3% and pine bio-coal 7% while mineral coal contains 10 wt. % ash.

Table 2-6. Impurities in coal and wood without bark (% of ash) (Patton et al., 2009)

Sio2 Ca K Mg Mn Fe Al

Bituminous

Coal 20-60 1-20 Trace 0.3-4 Trace 5-35 10-35

Hardwood 5 40 27 7 3 0.8 0.7

Softwood 5 30 13 5 5 0.8 3

More information about the details of the amount of components could be found in (Papachristodoulou and Trass, 1987).

Ash content in bio-coal depends on the biomass resources used to make the bio-coal, but commonly it creates less problems than coal because firstly, in most of the cases it contains less ash and secondly, most of ash in coal is in the form of silica, which is hard to remove while bio-coal has a small amount of silica and more amount of water soluble which are easy to remove (Patton et al., 2009). According to American national standard (ASTM, 1999), the ash content in diesel fuel oil must be lower than 100 ppm and sulfur content should be less than 15 ppm. Patton et al. (2009), set up a lot of experiments to test washability of pine bio-coal by using the distilled water, dilute hydrochloric acid (HCl, 1 wt. %), and acetic acid (10 wt. % AcOH) for wood pretreatment and char treatment. They obtained the best result by combining DW pretreatment of wood followed by DW treatment of bio-coal, which removes ash up to 96% from 1.64% to 0.065%.

In the similar study, Esnouf (1991), examined two separate methods for ash removal process. The first method which is called chemical treatment uses hydrochloric acid (HCl, 0.5 M) to remove ash from fine bio-coal particles with average particle size of 200 µm.

The process was done by mixing char with the acid solution for 1 h at ambient temperature.

Although they claim this method removes ash content from 20 to 70 wt. % depending on the initial ash content, the result was not satisfying while ash was only reduced from 3.4 to 1.2 wt. %.

The other method was the selective agglomeration treatment which uses large amounts of water-oil slurry and micronized bio-coal. Char is dispersed in water-oil mixture and when the slurry meets stirring vigorously, the hydrophobic parts of the bio-coal remain in the oil phase while hydrophilic minerals go through aqueous phase. The optimum amount of this method removes the ash content from 4.1 to 1.2 wt. %, which do not meet the standard content. Esnouf (1991), suggested a combination of the chemical and selective agglomeration treatment and claims ash reduction to 0.8 wt. % by this method. Schematic of the process is shown in Figure 2-7.

Figure 2-7. Process of ash removal by combination of chemical (Acid) and selective agglomeration treatment (Esnouf, 1991)