Product problems

The known problems with the liquid product that may require attention are suspended char, high viscosity, alkali metals, and potential inhomogeneity of the oil. The more significant are discussed below while a more comprehensive summary is given in Table 4.4.

4.4.3.1 Char

Direct combustion of the bio-oil is probably the least demanding application and a variety of tests have established that combustion does not present any significant problems. Some problems to date include char particles which cause filtration and pumping problems, atomisation problems and "sparklers" in combustion giving poor burn-out rates and high viscosity which contribute to the handling and pumping problems. More severe problems can be anticipated with atomisation for injection into turbines and engines where precise control of flow rates is essential.

The optimum solution to char suspensions interfering with bio-oil injection and atomisation is to reduce it at source in the vapour phase before condensation. Some success has been achieved with this approach by Ensyn, but only for limited periods of time. There is clearly considerable potential for development in this area.

Char filtration from the liquid bio-oil is known to be difficult due to the nature of the suspension, the high viscosity and the unusual relationship between the char and heavier components of the bio-oil. Dilution with water adversely affects heating value and may cause phase separation. Addition of diluents such as methanol or ethanol reduces viscosity and improves filterability but at a financial and energetic penalty. Heating is not advisable due to the temperature sensitivity of the bio-oil

with some changes occurring at around 50°C and significant modification of the oil above 100°C. Pressure filtration is effective for short periods but costly due to the unusual relationship between the char and heavier components of the bio-oil, and may result in unacceptably high bio-oil losses.

Table 4.4 Characteristics of Bio-oil and Methods for Modification (її)

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Characteristic Effect

There are complementary measures that may be taken such as reduction of the injection pressure into the boiler or engine/turbine, thus creating a less arduous environment and greater tolerances for the injector system. This will require careful matching with the atomiser and application characteristics. Also as application size increases, this problem reduces as more bio-oil is required to be injected and nozzles are increased in diameter.