**5. Co-liquefaction of coal and biomass**

raw MB coal. Thus, based on Equation 2, it is estimated that about 89% of raw MB coal would

In order to reduce operation cost and capital, and also to increase coal liquefaction efficiency, the liquefaction processes were usually carried out at less severe conditions. However, in most of coal liquefaction, the temperature used was higher than 400 °C and at relatively higher pressures. Thus, many attempts have been made by researchers to establish and develop methods of liquefying coals at lower temperature and pressure. Mukah Balingian, a low rank Malaysian coal, was extensively used in liquefaction study [29]. In the investigation, three different types of pretreatments, i.e. solvent swelling, in-situ solvent soaking and heating and

be converted during liquefaction process.

**4. Coal liquefaction**

194 Biofuels - Status and Perspective

**Parameters Results** Maximum reflectance of vitrinite 0.40%

**Figure 4.** Correlation of coal conversion with petrofactor [37].

Total reactive maceral content (Vitrinite + Liptinite) = 91% Petrofactor 1000 x (0.40%) / (91%) = 4.40

Estimated coal conversion 100 – 2.5 (4.40) = 89%

**Table 3.** Petrofactor value and estimated conversion of raw MB coal [29].

One of the solution to the above-mentioned situations encountered with coal and biomass is by the co-liquefaction of both of them. This can also maintain the stability supply of the materials. Moreover, one of the advantages of using biomass is that it accelerates the thermal decomposition of coal macromolecular structure by reducing the severe reaction conditions, especially the temperature during liquefaction. By producing smaller molecular fragments, it enables the combination with the large fractions produced from coal decomposition and simultaneously terminate the cross-linking reaction between them (this is to avoid the production of larger molecular weight products). Thus, the yield of low-molecular-weight products will be increased, oil quality improved and eventually, will reduce the yield of the residue during co-liquefaction process [25].

Most of the research works done for alternative energy sources have shown that, in general, co-processing or known as co-liquefaction of coal with biomass-type wastes has a positive effect on the liquefaction yields and product quality [14, 15]. Co-liquefaction of coal with biomass has gained increasing research interest due to the growing concerns over greenhouse gas emissions [28]. Because biomass materials also contain abundant hydrogen, co-liquefaction of coal and biomass waste is one of the best and feasible option to reduce the consumption of hydrogen as well as to avoid severe reaction conditions during co-liquefaction. Till today, however, very limited research work has been carried out in this field.

The mechanism of co-liquefaction of coal and biomass is believed to be a radical process. Thus, the unstable free radical fragments which are formed during pyrolysis of coal and biomass will be stabilized by active hydrogen contributed from biomass to form hydrogenation/lowermolecular-weight products. Researchers anticipated that there is a synergistic effect (synergy is two or more things functioning together to produce a result not independently obtainable) in the co-thermolysis process, and the yield of solid products from co-thermolysis is different from the arithmetic calculated value [1].

Hua *et al.* [38] carried out co-liquefaction of coal and rice straw and believed that there exists a synergistic effect during co-pyrolysis of Shenfu coal and rice straw. Shui *et al.* [39] investigated the co-liquefaction behavior of a sub-bituminous coal and sawdust. They found that the thermolysis of Shenhua coal was accelerated by sawdust and more volatile matter was released from the coal molecular structure during the co-thermolysis process. In another study, Guo *et al.* [25] also investigated the synergistic effect existence in the co-liquefaction of coal and biomass. They found that a positive synergistic effect during the process actually existed. Thus, they concluded that the synergistic effect depends on several factors, i.e. (i) coal rank, (ii) liquefaction conditions and (iii) different liquefaction products.

The liquefaction process of coal and biomass materials, which is known as "Co-liquefaction", has not been developed in Malaysia. The yields and quality (especially H/C ratio) of the liquid products obtained from coal under less severe liquefaction conditions (at lower temperature and pressure) can be improved with co-liquefaction of coal and biomass. Therefore, the cost of oil produced from direct coal liquefaction can be reduced significantly. The process can make full use of hydrogen in biomass, thus decreasing the consumption of hydrogen and moderating the conditions of DCL [1].

Some important parameters for co-liquefaction are the materials used, the design of the reactor, pressure, extraction solvent, temperature, holding time and catalyst used. Hua et al. [38] reported that the rice straw contains 68.3 w/% of volatile matter and resulted in 60.3% of oil at 420 °C. However, rice straw contains high amount of silica. Shui et al. [39] reported that fir sawdust contains 78.2 w/% of votalite matter and results in 55.2% of oil at 420 °C. However, fir occurs in mountains over most of the range. Guo et al. [25] reported that poplar sawdust contains 80.27 w/% of volatile matter and results in 59.19% of oil at 360 °C. However, high tannic acid content is present in poplar. Basic properties of crude rubber seed oil and crude palm oil blend as a potential feedstock for biodiesel production with enhanced cold flow characteristics were studied by Yusup et al. [40] and the inspections determined that the rubber seed oil can be used in the current diesel machines with no alteration required, confirming the adaptability of the produced biodiesel to the current standards. This shows that the charac‐ teristic of rubber seed as a biofuel material is more suitable than rice straw, fir sawdust, poplar sawdust and other biomass that contains less oil.

Hua et al. also [38] suggest that the FeS catalyst used has a good catalytic hydrogenation activity on rice straw, but the drawback of this catalyst is that it is low in basicity to reduce carboxylic acid present in the vegetable oil. An alternative for the FeS usage as a catalyst is by using dolomite. Dolomite is a natural rock found abundantly in certain areas of Malaysia and Thailand. Due to its very low cost to produce and being easy to obtain, the main domestic usage of dolomite is currently in the landfill site and in cement manufacturing. CaCO3 and MgCO3 are the major components of dolomite with a small amount of silica and ferrite. In a simple calcination process at high temperatures, the CO3 group in pure CaCO3 will decompose to produce CaO (which is highly basic) and MgO [41].

Guo et al. [25] state that the temperature is the most important factor during liquefaction. Shui *et al*. [39] reported that the main pyrolysis temperature range of Shenhua coal (a sub-bitumi‐ nous coal) (362 – 750 °C) is much higher than that of sawdust (260 – 420 °C), and the releasing rate of volatile matter of Shenhua coal is much lower than that of sawdust. Hua et al. [38] reported that the pyrolysis temperature of Shenhua Coal is in the range of 360 – 750 °C; however, the pyrolysis temperature of biomass is in the range of 250 – 400 °C.

Hence, an attempt should be made for co-liquefaction of low-rank Malaysian coal and biomass wastes such as rubber tree wastes (rubber seed, rubber seed pod or rubberwood) for the production of alternative fuels and other important purposes such as chemical feedstocks. Working on the area of energy has now become the priority in most of laboratories worldwide. In Malaysia particularly, under the New Economic Model (NEM), innovation and research is regarded as crucial factor to propel the industry. The research in this area certainly contributes to the increases in energy sector that part of National Key Economic Area (NKEA). Further‐ more, a competitive domestic economy can be created as part of Strategic Reform Initiative (SRI) [42].
