Treatment of Mercury Contaminated Waste in a Two Stage Process, Pre-Treatment, Volume Reduction and Subsequent High Temperature Oxidation
Authors: Bohm J. , Mussig S. 
Published: December 2007

Mercury is known to be present as a trace-gas in the Rotliegend natural gas reservoirs. In the gas reservoirs under consideration levels of between 0.3 – 0.5 mg/Nm3 are found. During the gas treatment process the mercury is removed together with water and higher hydrocarbons. All waste streams, for example sludge, which are accumulated in gas treatment units and equipment e.g. glycol- and coalescence filters and heat exchangers are contaminated with mercury.

In the past this mercury contaminated waste has been disposed of at special dump sites such as in depleted salt mines in Germany.

In order to reduce the impact on the environment, NAM decided to develop an alternative method of disposal. From a feasibility study it was concluded that proper conditioning of the waste streams followed by a thermal oxidation process would be the best option to clean mercury contaminated waste. As there was no suitable thermal treatment process available, an in-house research laboratory was approached.

Their investigation resulted in a "High-Temperature-Oxidation (HTO)" process. On this basis NAM decided to build a facility where all its mercury contaminated waste could be treated. It consists of two separate processes; i.e. the pretreatment and volume reduction facility and the final treatment using the HTO - process.

This paper gives an overview of the required pre-treatment facilities, describes the HTO-process and summarises the achieved treatment results.

Introduction

Produced natural gas especially from Rotliegend reservoirs contains small quantities of gaseous elemental mercury. When applying the low temperature separation technique to bring the gas to sales specification the mercury is also condensed in the metallic form. A process flow scheme of the applied separation technique is shown in figure 1a/b. The majority of mercury may be directly extracted. However, a small proportion of this metal enters the liquid treatment facilities and inevitably leads to the contamination of equipment and waste streams. The resulting materials to be treated can be divided into two broad subgroups, i.e. disposable and non-disposable items. The disposable items comprise of sludge, coalescence-filters, glycol-filters, scrap material, spent activated carbon and occasionally soil. The non-disposable items are equipment such as heat exchangers, personal protection items and trucks used to transport water and condensate.

Until now the waste has been collected centrally in a very basic facility at a production site within the field and prepared for export and storage in a depleted former salt mine in Germany. In 1989 a re-appraisal of the situation, in the light of improving standards for environmental care and personal exposure called for the design of a new facility.

The Netherlands has directed in its National Environmental Policy that the amount of hazardous waste must be reduced and that the export of hazardous waste with the objective of dumping is no longer accepted. In line with this policy the export of hazardous waste has officially been stopped by the authorities since January 1996.

This national policy is in line with the NAM policy, i.e. to reduce the impact on the environment as much as possible.

In the following sections an overview of the development is given, resulting in a two stage treatment process for all mercury contaminated waste.

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