Cogeneration for Mine Water Management

Mitigation of mine methane streams and management of mine water are two important environment issues for coal mines in Australia.  Technologies such as gas engines using high concentration mine methane and the Megtec Vocsidizer using low concentration methane are being applied as alternative power generation methods.  Commonly, there is an exhaust gas stream from methane mitigation plants that has an elevated temperature and could be potentially used as a source of energy for mine water management via desalination.  This study examines two case studies based on current mine methane mitigation technologies to determine if thermal desalination technologies could feasibly be applied to utilise the waste heat.  These are generically based on the 7MW gas engine plant operating at Tahmoor Mine and the 5-6MW Megtec Vocsidizer plant being installed at West Cliff Colliery.

A review of thermal desalination technologies suggested that reliable performance and costing data could only be acquired for conventional plants based on either evaporator units or heat exchanger-flash tank units connected in series.  Vapour compression can be used to increase the heating value of vapour from the evaporator and crystalliser units can be incorporated to produce a high solids content product for disposal.  The study indicated that the use of waste heat from mine methane mitigation processes for desalination of mine water is technically feasible.  Multiple effect evaporation (MEE), multi-stage flash (MSF) and mechanical vapour compression (MVC) technologies for desalination are mature technologies and can be coupled to waste heat from either gas engine or Vocsidizer plants using mine methane.  These types of thermal desalination technology have even been proven at the Debiensko mines in Poland and on coal contaminated water at the Bayswater site in the Hunter Valley NSW.  However, at both these sites there have been issues with poor performance due to water chemistry and plant arrangement problems. There are alternative thermal desalination technologies, but these have only been demonstrated on a small scale and insufficient data is available to indicate cost and feasibility of an industrial size installation.

Despite the apparent technical feasibility of the process, there are a number of significant issues with the use of thermal technologies in this application that need to be considered.  Desalinated water production rates are very low when using waste heat from power plants of 5-7MW output and there is a poor economy of scale and high unit cost for production from the associated desalination plant.  The best financial performance is for a 2-stage desalination and crystallisation process with mechanical vapour compression using the exhaust gas from gas engines as a heat source, which should produce approximately 0.338ML of water per day at $3601/ML.  It is expected that a cost of production of approximately $1000/ML or less is required for economic viability of a process configuration.  In addition, impurities in the water are likely to result in contamination of the solid salt products of the process, limiting the potential value for sales to provide an additional income stream.  This would have a significant impact on the financial viability of the plant and reducing the contaminants to acceptable levels is likely to require additional pre-treatment stages that will further damage the economic viability of the process.  All performance characteristics determined are influenced by the water quality and reliability of the methane mitigation plant.