Miljøprojekt Nr. 1185 – Pyrolyse af CCA-imprægneret træ

Pyrolyse af CCA-imprægneret træ

Summary

The purpose of the project was to investigate and develop a treatment technique for CCA-pressure-treated wood by means of low-temperature pyrolysis to secure the recovery of the energy content of the wood and processing of the metals.

CCA-pressure-treated wood contains the heavy metals copper, chromium and arsenic. Copper and chromium are toxic to the environment and also important resources, that should be recycled. Arsenic is toxic to the environment, but is of no importance as a resource today, as its use has been banned. Arsenic must be stabilized and disposed of in a safe way.

According to the waste strategy of the government in Denmark for 2005-2008, the quantity of CCA-pressure-treated wood for disposal is expected to rise to approximately 100,000 tonnes annually in 2010.

Kommunekemi a/s has developed a method for gasification of CCA-pressure-treated wood. During the method a gas is generated, containing approximately 80% of the arsenic in the wood, while the rest of the arsenic together with copper and chromium remain in the inorganic residual ashes. The 80% arsenic can be removed from the gas, but it would be preferred to completely avoid evaporation of the arsenic into the gas.

Laboratory tests, carried out abroad, indicated that by low-temperature pyrolysis it should be possible to retain all the metals in the coke left from pyrolysis. Pyrolysis is a process of heating without oxygen. During the process the wood is transformed into coke (charcoal), tar and gas. A great part of the amount of energy from the wood is retained in the coke, and it is necessary to separate the metals and coke to make the coke suitable for use in plants without specific measures for flue gas cleaning.

Pilot plants for low-temperature pyrolysis

In the present project a pilot plant for continuous pyrolysis of chipwood at approximately 300 °C was developed and tested. The plant consists of a ø300 mm reaction chamber, that is heated by the flow of hot nitrogen. At the beginning we used hot flue gas, but it proved too difficult to control. The pyrolysator is provided with a hermetic feeding system for chipwood and outlet system for coke to make it possible to feed in chipwood and take out coke continuously during operation. The gas generated is led into a scrubber system and then into Kommunekemi’s incineration plant.

The plant is provided with a number of temperature sensors in the reactor and at other places in the system. In addition the contents of O₂, H₂, CO and CO₂ in the gas are measured and logged directly into a gas analyser. Sampling equipment for gas tests was installed in the exhaust outlet from the pyrolysator. A type S Pitot tube was also installed in the exhaust outlet to calculate the gas flow.

Samples of the gas were taken and the contents of metals in the gas were collected in gas washing bottles and subsequently analysed as regards to Cu, Cr, and As. Based on the analyses the contents of these specific metals in the gas were calculated.

After a great deal of development work, we succeeded in constructing a pilot plant for continuously pyrolysating CCA-pressure-treated wood at low temperature.

The subsequent tests with CCA-pressure-treated wood showed that only 1-5% of the arsenic in the wood exists in the gas. As regards copper, this is less than 1% and for chromium less than 0.5%.

Finishing treatment of the coke

The coke generated was examined by different methods:

DTU-BYG investigated, if electro-dialysis could be used to separate coke and metals, but it was only possible to extract 28% copper and 2% chromium by this method. For arsenic the results were not precise.

In the same connection DTU-BYG examined the coke by electron microscopy and also investigated the possibilities for chemical extraction. In the chemical extraction the efficiency for copper and arsenic was not more than 30% and for chromium it was not more than 10%.

Kommunekemi investigated the possibilities to separate metals and coke in a cyclone, but it was not possible with the type of cyclone used. Neither was it possible to separate metals and coke by sedimentation in liquids.

Conclusions

We succeeded in developing and constructing a pilot plant for low-temperature pyrolysis of CCA-pressure-treated wood.
By pyrolysis it is possible to retain more than 95% of the contents of arsenic in the wood in the coke.
It is has not been possible to demonstrate a method to separate metals and coke.