5.24.1 Summary

The objective of this process (Oil From Sludge, OFS) is the pyrolysis of raw or digested sewage sludge to produce synthetic oil, gas and char (103., 104). Two liquid fractions are produced; a light fraction and a heavy fraction. The process is based on a continuous laboratory scale plant (5 kg/day) for the conversion of sewage sludge to synthetic oil and a solid fuel developed at Tubingen University, Germany for which the Wastewater Technology Centre (WTC) have a license to use the patented technology in combination with the WTC process (104, 105).

Bench scale research work commenced in 1982 (106) leading to the first 1 tonne/day pilot scale plant in 1986 (106). Bench scale results were confirmed and various parameters such as operating temperature, pressure and solid and gas retention times were optimised (106). A second 1 tonne/day pilot plant was built by Campbell Environmental Ltd. (CEL) of Perth, Australia in 1988 (106). Two one tonne/day pilot scale plants have been constructed (Canada and Australia) while a commercial scale plant consisting of three 24 tonne/day process trains is planned for Highland Creek, Toronto, Canada in 1993 (105). Several plans to commercialise the process in Canada have been announced but no plants have yet been built (107).

5.24.2 Description

Sewage sludge consists of a mixture of organic materials and inorganics. The sludge (either raw or digested) needs to be dried from approximately 35% solids to approximately 90 to 95% solids to minimise the production of water which has to be burnt (105). Dried sewage sludge is fed from a live bottomed feed hopper mounted on load cells to the pyrolysis reactor by a variable speed (plug ended to seal the reactor system) screw feeder. If the process is not used continuously, this plug may partially pyrolyse and forming a solid mass which could block the feeding system.

The pilot reactor has an internal diameter of 254mm and is constructed from stainless steel. The reactor operates at a pressure of approximately 2.5 cm water and is heated by hot gases (at 700-800°C) produced by the combustion of propane in an external burner which pass through a jacket surrounding the reactor. Heat is directed to the first third of the reactor by a single inlet pipe and to the second two thirds of the reactor by two inlet pipes. The flow of hot gases to the reactor and hence the heating rate is controlled by baffles. The best oil yields are obtained using a reactor temperature of 450°C and a solids retention time of 20 minutes. The solids retention time in the reactor can be controlled by varying both the sludge feed rate and the reactor operating conditions. It is aimed to quickly heat the feedstock to 450°C in the first third of the reactor and to maintain this temperature along the whole length of the reactor. Waste flue gases from the reactor exit the reactor heating jacket through two flues and are discharged to atmosphere.

Sewage sludge moves through the stainless steel reactor on two conveyors. A screw conveyor moves feed through the first third of the reactor and a series of paddles move the feed through the final two thirds of the reactor. In the first stage of the reactor, the sludge is heated in the absence of oxygen (105). Approximately 50% of the sludge is vaporised in the first stage. In the second stage of the reactor, the pyrolysis gases are contacted with the residual char in either a counter-current or co-current mode to catalyse vapour phase reactions to convert lipids and proteins to hydrocarbons (105, 108).

The char product leaves the end of the reactor through a chute and enters a water cooled plug end screw. The screw conveyor transfers the char into an ash collection skip which is purged with nitrogen to avoid fires.

The vapour phase products enter a condenser which extracts a heavy oil fraction from the product vapours. The condenser is cooled using electrically heated air enabling a small temperature gradient to be maintained permitting control of the heavy oil yield and quality. Vapours leaving the first condenser enter a water scrubber (spray tower) which extracts further oils. The removed oils and water are separated using a disc centrifuge and the water is cooled and recycled back to the spray tower. Following a run, the dirty water is recycled to the water processing plant on site. The non-condensable gases from the system leaving the water scrubber are flared, A flow sheet is given in Figure 5.22.

Flowsheet of 1 tonne/day Oil From Sludge Pilot Plant 5-1 02

The total yield of oil from the pyrolysis of digested sewage sludge is shown in Figure 5.23. The yield of oil from raw sewage sludge is reported to be 27% of the feed input (basis not reported) while the oil yield from digested sludge is reported to be 11.5% of the mass feed input (105). The exit temperature of the product oil from the system is not reported but it is reported to be cool (105).

The pilot scale oil from sludge process produces two oil products; a light oil fraction and an heavy oil fraction. Both oil products are reported to have very low oxygen contents. The heavy oil product is removed from the first condenser following the reactor. This oil is thick and viscous and flows very slowly at 15°C. The water content of the heavy oil is approximately 1% by mass. The light oil product is removed from the disc centrifuge. The light oil has a caramel colour, has a low viscosity and contains approximately 5% water by mass. The heavy oil does not distil while the light oil is entirely volatile. The as-received (including water) higher heating value of the light and heavy oils is between 35 and 40 MJ/kg. The heavy oil product accounts for approximately 80% of the liquid yield while the light oil product accounts for the remainder. Oil char separation is reported to be efficient — char levels in the heavy oil are not measurable.

The heavy oil product has been found to be suitable as an anti-stripping compound in asphalt as the properties which make the oil a poor quality fuel make it a good anti stripping compound. The use of the product oil in asphalt production would also prevent CO2 emissions from the use of the product oils. Pilot scale experiments are now, therefore, being optimised towards specific high value non­fuel products. Canmet are currently investigating possible end uses for the products from the oil from sludge process. Liquid composition data for the heavy and light oil fractions is not currently available. The composition data shown in Table 5.22 shows the composition of a single liquid oil product produced by the oil from sludge system before the two separate oil stages were installed. Some heavy metals are carried over from the sludge to the oil although the majority of the heavy metais remain with the char (108).

Estimated mass and energy balances for a 180 tonne/day pyrolysis plant are shown in Figure 5.23. The process thermal efficiency (ratio of oil chemical energy to the chemical energy of the dry sludge) calculated using the figures shown in Figure 5.23 is 52.2%. Although the yield of oil varies with the type of sludge processed, the effect of reaction temperature on oil yield is the same for all sludges (108). Peak oil yield is obtained at a reaction temperature between 425 and 450°C. Accurate reactor temperature control is, therefore, required (108). Above 450°C, conditions favour the production of product gas. The data indicate that the heating value of oil produced from digested sludge is higher than the heating value of oil produced from raw sludge (105).

5.25 WORTHING INDUSTRIES INC.

A 200 kg/h fluid bed reactor derived from the University of Waterloo work was originally constructed in 1986 by Encon for wood and peat pyrolysis. The unit was constructed as a self contained plant on the back of a trailer for transportation to test sites for demonstration. This is now used for processing old telegraph poles. These have their outer skin of treated wood removed and which is then pyrolysed in a 50 kg/h fluid bed pyrolyser for recovery of chemicals including creosote and PCP as well as bio-oil. Up to 30,000 poles per year are processed. The skimmed

poles are re-treated and recycled (109). No information is available on performance or costs.