Both the OPW in the form of solid and liquid residues can serve as a source of raw material for biofuel production. In order to enhance the physical and chemical com­position of OPW fibres for energy production, gasification, pelletisation (or briquet­ting), torrefaction and liquefaction have been applied. The heating value of OPF presents it appropriate for use as pelletised fuel which could be mixed with glycerol to enhance its heating value (Azuan 2008). Research on the production of high-quality briquette fuel produced by the mixing of 100% pulverised EFB with sawdust or PKS (also called palm kernel expellers) have been carried out (Nasrin et al. 2008) . High-energy solid fuels have been produced from pelletised PKC (Razuan et al. 2011), briquetted PKS and PPF (Husain et al. 2002) and torrefacted EFB (Uemura et al. 2013) . The energy content of torrefacted fuels from wheat straw, reed canary grass and willow as reported by Bridgeman et al. (2008) are 77%, 78% and 86%, respectively, which are lower than that for EFB (85-95%), PPF (96%) and PKS (100%) (Uemura et al. 2013). PKS is widely used traditionally as solid fuel (or pelletised fuel) by black and goldsmiths in most part of Africa. It can also be mixed with other grades of biomass for co-firing with steam or used in bio­mass power plants.

The BOD of POME could be reduced (to <100 mg/l) when treated in ponds and digesters for the production of biogas. For every ton of POME generated in the palm oil mill, about 12.36 kg of methane could be generated from it (Basri et al. 2010).

In Malaysia and Thailand, large quantities of generated POME are used as feed­stock for biogas production (Basri et al. 2010). In order to promote sustainable utili­sation of OPW in the palm oil mills for value-added products, the feasibility of biogas production from EFB and its co-digestion with POME have been investi­gated by O-Thong et al. (2012) . Co-digestion of EFB with POME is found to enhance microbial biodegradability and increase-methane yield (by 25-32% higher) at mixing ratios of 0.4:1, 0.8:1 and 2.3:1 on volatile solid basis than digest­ing EFB alone or POME alone (O-Thong et al. 2012). A further optimisation study on the improvement of biogas and methane yields from the digestion of a mixture of POME and EFB has been done by Saleh et al. (2011). The optimal conditions to obtain about 25.6% methane were 47.8°C with 50.4 ml POME and 5.7 g EFB. EFBs again have been improved (through NaOH pretreatment for 60 min) to produce biogas with methane yield of 0.404 Nm)kg (volatile solids) which accounts for about 97% of the theoretical yield of methane from carbohydrates (0.415 NmVkg carbohydrates) (Nieves et al. 2011; Davidsson 2007).

Bio-oils are found to be promising candidates for petroleum fuel replacement which are applicable in various thermal devices. The production of bio-oil from OPW such as EFB (Misson et al. 2009; Abdullah 2005; Abdullah and Gerhauser 2008; Lim and Andresen 2011; Sulaiman and Abdullah 2011), PKS (Abnisa et al. 2011; Kim et al. 2010; Salema and Ani 2011), OPF (Lim and Andresen 2011), PPF (Salema and Ani 2011) and PKC (Razuan et al. 2010) by pyrolysis has been reported. Bio-oil yields of 46.1 wt% (at 500°C pyrolysis temperature for 1 h with a particle size of 1.7 < dp < 2 mm), 80-90 wt% (pretreated with NaOH and H2O2) and 42.3% (with sub/supercritical treatment with 1,4-dioxane at 290°C) have been pro­duced from PKS (Abnisa et al. 2011), EFB (Misson et al. 2009) and PPF (Mazaheri et al. 2010), respectively. Abdullah and Gerhauser (2008) have concluded that the fast pyrolysis of washed EFB with a low ash content produced bio-oil which had similar yields as that mostly obtained from wood. Again, the characterisation of EFB bio-oil by Pimenidou and Dupont (2012) indicates better hydrogen yield (15.9 wt%) via steam reforming compared to that of pinewood (13.7 wt%).

Various effective catalysts such as calcined dolomite have been proven to be a viable catalyst which improves hydrogen production and reduces the amount of tar generated in syngas produced from OPW solid residues (Mohammed et al. 2012). The low moisture, high volatile matter, low fixed carbon and ash contents of EFB make it highly volatile and reactive (Demirbas 2004) and highly appropriate for the production of gas fuel. The pyrolysis of PKS for the production of hydrogen using nickel and La/Al2O3 at900°C in a fixed bed reactor yielded 37.28 vol% and 38.45 vol%, respectively (Yang et al. 2006). Bio-hydrogen production from the hydrolysate of microwave-assisted sulphuric acid-pretreated OPT has been reported (Khamtib et al. 2011) . Steam gasification (at 800°C) of OPT yielded more syngas (50%), energy and hydrogen (60%) (Nipattummakula et al. 2012) than those from mangrove wood, paper an. food waste (Nipattummakula et al. 2012; Ahmed and Gupta 2009). The EFB generated as wastes from the palm oil mill after processing about 60 tonnes of FFB every hour is estimated to be capable of producing about 3 MW of electricity (from controlled gasification process). Pattanamanee et al. (2012) have concluded that EFB presents an efficient OPW for producing bio-hydrogen by anaerobic photo-fermentation with an isolated photosynthetic bacterium R. sphaeroides S10.

The production of bio-alcohols such as ethanol from EFB (Tan et al. 2010), etha­nol from OPT (Yamada et al. 2010), ethanol from PKC (Cervero et al. 2010), buta­nol from EFB (Noomtim and Cheirsilp 2011), butanol from POME (Hipolito et al. 2008), butanol from PPF (Ponthein and Cheirsilp 2011) and ethanol from POME (Alam et al. 2009) through various processes prior to various pretreatment methods such as acid and enzymatic pretreatments. A million tonne of EFB would have the potential to produce about 81 x 103 kl of ethanol (Shinichi et al. 2009). Also, OPT has the potential of producing higher ethanol yield (9.5-10.3 kl/ha) compared to that of sugarcane bagasse (4.5-7.2 kl/ha) (Mori 2007). An average-weighted OPT could produce about 107.8 kg and 123.5 kg fermentable sugars from the sap and solid fibres, respectively, which can produce about 69.8 l and 41.4 l bioethanol, respectively (Mori 2007). Simultaneous saccharification and fermentation (SSF) of OPT fibre for bioethanol production gave a yield of 78.3% making OPT a potential source of raw material for bioethanol production (Jung et al. 2011). PKC which is readily available from palm kernel crushing units contains large amount of mannan (which is about 35.2% of the total carbohydrates in PKC) which can be easily hydrolysed into sugar for the production of bioethanol (with yield of 125 g/kg PKC) without any pretreatment (Cervero et al. 2010).

Biogas, bio-oil, bio-hydrogen, etc. which are produced from OPW can be used to generate electricity in order to reduce the dependency on fossil fuel used by power plants as well as safeguarding the environment. For instance, in Malaysia, it is estimated that for a million ton of FFB processed, about 16,000 GW/year of elec­trical energy can be generated from the OPW produced (Low 2011).