麻烦各位解释一下“红题字”吧 ...困惑...谢谢！！

yibuyibu

In several countries they have become interested in the possibility of using fuel produced from crop residues either as a replacement for petrol (as ethanol) or as fuel for power stations (as biomass), Such fuels produce far less carbon dioxide than coal or oil, and absorb carbon dioxide as they grow. They are therefore less likely to contribute to the greenhouse effect. But they are rarely competitive with fossil fuels unless subsidised - and growing them does no environmental harm than other crops.
恳请各位解释一下红色字体的意思，分析一下结构！谢谢！！rarely，unless，does no，感觉好多否定词啊。。困惑。。

yibuyibu

自己顶一下

yibuyibu

yibuyibu

和化石燃料相比 他们缺乏竞争力 除非。。。。