Biodiesel: An Overview

by: Associate Prof. Dr. Ramli Mat & Dr. Lau Tian Fwu

The concept of using vegetable oil as an engine fuel dates back to 1895 when Rudolf Diesel (1858-1913) developed the first engine to run on peanut oil, as he demonstrated at the World Exhibition in Paris in 1900. Unfortunately, R. Diesel died 1913 before his vision of a vegetable oil powered engine was fully realized. After R. Diesel death, the petroleum industry was rapidly developing and produced a cheap by-product "diesel fuel" powering a modified "diesel-engine".

Early in this century, work directed at renewable diesel fuels had been initiated. It was immediately recognized that the use of whole vegetable oils was not acceptable in diesel engines. In year 1938, Walton reported on pioneering work with vegetable oils and suggested an early concept for biodiesel. Three oils were examined in a diesel engine, which utilized 0.416 Ib/bhp-hr of fuel, similar to a modern engine in efficiency. Steady state testing showed that soybean oil, palm oil, and cottonseed oil all gave fuel economies of 90-91% compared to petroleum diesel at wide-open throttle and various speeds. Whole oils reported to form carbon deposits and exhibit pour point problems; palm oil corroded copper and brass significantly. Because of the difficulties experienced, Walton suggested splitting of the triglycerides and using the resulting fatty acids as fuel. The ideal suggested by Walton evolved slowly and considerable effort was devoted to investigating vegetable oils (Graboski and McCormik, 1997).

In year 1944, Martinez de Vedia reported on tests with 20 and 40% linseed oil/diesel blends. With these blends, lube oil properties including acidity, ash, asphaltene precipitates, and Conradsen carbon increased during use, and increased more rapidly than for diesel alone. The author found that decanting the fuel off of the precipitate before use could minimize fouling of injectors and clogging of fuel filters. Carbon deposits on engine combustion chamber parts were significantly greater with the blends than with diesel alone. The author stated that linseed oil blends would be troublesome if used in commercial equipment for extended operating periods (Graboski and McCormik, 1997).

In year 1951, Huguenard investigated fuel economy and indicator card traces for cottonseed oil/diesel blends at various injection timings in two laboratory scale IDI diesel engines. Cottonseed oil and blends with diesel could be run with greater timing advance than diesel fuel. After a modest period of operation, the first engine failed to make rated load. The engine was dissembled and found to be heavily carbonized and to have ring damage (Graboski and McCormik, 1997).

In year 1980, Bruwer and coworkers reported that sunflower seed oil is inferior to diesel in terms of fuel economy and power. Furthermore, as the injectors coked, unburned fuel began to dilute the crankcase oil causing ring sticking and general engine failure. The investigations described here are supported by many other studies that point to engine endurance issues associated with the use of whole vegetable oils or vegetable oils blended with diesel. Bruwer and coworkers, in one of the first reported studies on fatty acid esters, found that using an ester of sunflower oil seemed to resolve the problems associated with the whole oil and in fact produced less carbon deposits in a test engine than petroleum derived diesel. Smoke opacity was also lower with the ester than with diesel (Graboski and McCormik, 1997).

Engler and coworkers examined blends of sunflower oil and sunflower oil ethyl ester prepared by converting 38, 68 and 98% of the initial oil. Unacceptable combustion chamber deposits were observed for the 38 and 68% conversion fuels. For the nearly pure ethyl ester these deposits were not significant. These positive initial findings with regard to engine performance, engine deposits, and emissions have prompted considerable recent research into fat and vegetable oil ester based diesel fuels (Graboski and McCormik, 1997).

The increasing consumption of fuel in the previous decades has led to a rapid decrease of Earth’s fossil reservoirs. Therefore, since traditional fossil energy resources are limited and greenhouse gas emissions are becoming a greater concern, research is now being directed towards the use of alternative renewable fuels that are capable of fulfilling an increasing energy demand. For this reason the possibility of developing alternative energy sources to replace traditional fossil fuels has been receiving a large interest in the last few decades. Fuels derived from renewable recourses, such as biomass, are favorable alternatives.

Biodiesel is an environment friendly liquid fuel similar to petro-diesel in combustion properties. Increasing environmental concern, diminishing petroleum reserves and agriculture-based economy of our country are the driving forces to promote biodiesel as an alternate renewable transportation fuel. Because of having similar properties to petroleum based diesel fuel, biodiesel fuel, is considered as the most promising diesel fuel substitute.

Continuously increasing use of petroleum will intensify local air pollution and accelerate the global warming problems caused by carbon dioxide (CO₂). If pure or blend biodiesel is used as fuel, the net production of CO₂ can be highly suppressed and reduce the global warming because carbon dioxide emitted during combustion is recycled in the photosynthesis process occurring in the plants used as raw materials for biodiesel production (Vicente et al., 1997). Therefore, biodiesel has become more attractive recently because of its environmental benefits and the fact that it is made from renewable resources.

Biodiesel is usually prepared in the presence of homogeneous base or acid catalysts. The acid-catalyzed process often uses sulfuric acid or hydrochloric acid as catalysts; however, the reaction time is very long (48–96 h) and a high molar ratio of methanol to oil is needed (30–150:1, by mol) (Siler-Marinkovic and Tomasevic, 1998). Potassium hydroxide, sodium hydroxide and their carbonates, as well as potassium and sodium alkoxides such as NaOCH₃, are usually used as base catalysts for this reaction. As the catalytic activity of a base is higher than that of an acid and acid catalysts are more corrosive, the base catalysis is preferred to acid catalyzed routes, and is thus most often used commercially.

However, in this conventional homogeneous method, removal of these catalysts after reaction is technically difficult and a large amount of wastewater was produced to separate and clean the catalyst and the products. Therefore, conventional homogeneous catalysts are expected to be replaced in the near future by environmentally friendly heterogeneous catalysts mainly due to environmental constraints and simplifications in the existing processes.

The replacement of homogeneous catalysts by heterogeneous catalysts would have various advantages, most important being the application of easier working up procedures, the easy catalyst separation from the reaction mixture and the reduction of environment pollutants.

References:

Graboski, M.S. and McCormik, R.L., 1997. Combustion of Fat and Vegetable oil Derived Fuels in Diesel Engines. Prog. Energy Combust. Sci , 24: 125-164.

Siler-Marinkovic, S. and Tomasevic, A, 1998. Transesterification of Sunflower Oil in Situ. Fuel , 77: 1389-1391.

Vicente, G., Coteron, A., Martinez, M., and Aracil, J., 1997. Application of the Factorial Design of Experiments and Response Surface Methodology to Optimize Biodiesel Production. Industrial Crops and Products , 8: 29-35.