Observer: Thousands of activists were converging on Washington this weekend in the biggest-ever protest against the use of coal, which they argue is the dirtiest fossil fuel and yet provides half of the US’ electricity. The focus of the demonstration is what might, in other circumstances, seem a quaint relic from another era: a coal plant built 99 years ago to assure Congress an independent source of power. But the Capitol Hill Power Plant, which environmentalists say should have been …