CLP Power is to assess whether the two oldest coal-fired generating units at its Castle Peak plant should be closed. Environmentalists said decommissioning of the units would be a great help in reducing air pollution. The assessment is part of an estimated $30 billion spending plan by the company approved by the Government. The plan, for 1999-2004, also covers transmission and distribution of power, new substations and strengthening the existing system. A government spokesman said the company's key generating development proposal was to refurbish all eight existing coal-fired units at Castle Peak power station in the next two years. The Government has also accepted a proposal by the company to sell a diesel-fired turbine at Castle Peak as it has become obsolete and difficult to maintain, the spokesman said. However, disposal of the unit is subject to a joint review by the firm and the Government. The spokesman said the Government also accepted in principle a company proposal to divert part of its natural gas supply for use in studies on use of the fuel as a substitute for diesel for buses and heavy vehicles. '[We] will consider each specific proposal on its own merits,' he said.