Life cycle emissions and energy study of biodiesel derived from waste cooking oil and diesel in Singapore

Abstract

Background, aim and scope

Biodiesel derived from Waste Cooking Oil (WCO) is considered highly environmentally sustainable since WCO is a waste product from domestic and commercial cooking processes and then recycled to a transportation fuel in Singapore. In addition, it avoids the conversion of land use for crop production. This is a strong advantage for Singapore which has relatively smaller land space than other countries. The import of virgin oil as feedstock into Singapore is also avoided. Therefore, the more appropriate feedstock to produce biodiesel in Singapore context is WCO. According to the National Environment Agency, diesel vehicles in Singapore contribute 50% of the total particulate matter smaller than 2.5 μm (PM_0.25) emissions to air ambient. Hence, the aim of this life cycle assessment study was to compare the environmental performances of biodiesel derived from WCO and low sulphur diesel in terms of global warming potential, life cycle energy efficiency (LCEE) and fossil energy ratio (FER) using the life cycle inventory. The results of this study would serve as a reference for energy policy makers and environmental agencies.

Materials and methods

ISO14040 and ISO14044 (ISO14040 2006; ISO 14044 2006) are used as the method for implementing this study. This comparative study between biodiesel derived from WCO and low sulphur diesel is done by comparing the life cycle inventory results. The tailpipe emission tests were done using a gas analyser.

Results

Biodiesel production data are collected from a local facility. The production of main ingredients, types of transportation, conversion of WCO to biodiesel and the usage of biodiesel were considered within the dataset. Production of low sulphur diesel was modelled according to several references. The phases include foreign crude oil production, refining, transporting of diesel to station and finally the usage of diesel. The testing vehicle for both transportation fuels is an ISUZU pickup truck with engine capacity of 3,059 cc and in direct injection combustion chamber type. The functional unit in this study is output of 1 transportation-km.

Discussion

In this section, two types of emissions are discussed. First is the net life cycle emission. The second is the exhaust tailpipe emissions. Highest amount of reduction on a life cycle basis is PM_2.5 and PM₁₀ with a significant reduction of 99.99%. On the exhaust tailpipe emission basis, the reduction for total particulate matter is 94.80%. The LCEE of biodiesel produced from WCO is calculated as 86.93%. This is higher than biodiesel reported from US studies (using soybean as feedstock) which is 80.55%. The low LCEE value of 71.09% for low sulphur diesel could be attributed by the fact that Singapore depends greatly on foreign crude oil production and imports. The FER is calculated to be 9.39. The life cycle of biodiesel produced from the recycling of WCO produces more than nine times as much energy in its final fuel product as it uses in fossil energy. This is three times higher than biodiesel derived from soya oil in the USA.

Conclusions

The emission results and the life cycle energy efficiencies have indicated that the replacement of low sulphur diesel with biodiesel derived from WCO as a transportation fuel is favourable.

Recommendations and perspectives

In Singapore, the potential substitution percentage of diesel by biodiesel if all of the WCO can be collected and processed to biodiesel is 1.42%. There is a need for recyclers to convince the food establishments and users of cooking oil of the benefits of recycling cooking oil, which in turn obtains a steady source of WCO as feedstock for biodiesel production. In addition, as the biodiesel life cycle defined is very much dependent on WCO as a feedstock, it is recommended to optimise the WCO collection network.