Exploratory studies on wire mesh fast pyrolysis of biomass has been carried out to derive fundamental kinetic data and compare this approach to that of ablative pyrolysis at the University of Aston (q. v.) (86). The advantage of this approach is that very small quantities of biomass can be processed under very high and controllable heating rates which makes derivation of basic reaction kinetics more reliable. There is little data available.

5.20.1 Summary

The objective of this research is to convert biomass such as sewage sludge, agricultural wastes and refuse/MSW into fuels or raw materials for the organic chemicals industry as an alternative solution to landfill and incineration of sewage sludge (87,88). Several batch and continuous laboratory scale plants up to 5 kg/h have been built and tested. The principle is to use low temperatures of less than 350°C and long reaction times of up to an hour to achieve low oxygen content oils and high yield of fine chemicals. The concept has been licensed to several organisations in Europe (Stenau q. v.), North America (Wastewater Technology Centre q. v.) and Australia (Wastewater Technology Centre q. v.) and several plants of up to 2 t/h are planned or operating based on sewage sludge.

5.20.2 Description

Figure 5.17 shows one configuration of the auger reactor system based on dried sewage sludge.

At a laboratory scale, both batch and continuous screw reactors have been used. Sludge dewatered to about 20% wt solids, or other biomass, is heated slowly to 300-350°C in an oxygen free environment for about 20 minutes, and the liquid product collected in an ice-cooled bath. No additives are needed as the silica, silicates and heavy metals present in the sludge are claimed to act as catalysts. The vapour is condensed and collected. Feedstocks tested include sewage sludge, rape, lupine and Euphorbia.

Four types of equipment have been used on a larger scale for low temperature conversion. There are: a rotary furnace (up to 80 kg/h), a fluidised bed, (up to 400 kg/h), a reactor with a transport belt for MSW conversion (up to 200 kg/h) and a cone screw converter for the conversion of agricultural wastes.

5.20.3 Products

Oil yields ranging from 18-27 wt% (feed basis) and char yields from 50-60 wt % (feed basis) have been achieved. Table 5.19 shows the elemental analysis of the products.

Liquids with very low oxygen content (less than 5% wt oxygen) have been reported from a sewage sludge feed. The low oxygen level and chemical intermediates are claimed to be due to low reaction temperature, natural catalysts in the feed and slow reaction times. The oils contain aliphatic hydrocarbons and fatty acids as the main components (89).

The sewage sludge derived oil has been reported as being used as fuel for a diesel engine.