3.3. Toxicity categories (human, marine and freshwater) and eutrophication
The coal scenario result in the human toxicity category more than triples the impact of the other scenarios. Impacts in toxicity from the coal case scenario derive primarily from the disposal of coal mining spoil, which is associated with high concentrations of contaminants, especially heavy metals [44] and [45]. That activity is associated with 65% of the marine ecotoxicity impact for the whole life cycle of the plant and 66% of human toxicity, but also the disposal of coal combustion ash contributes to the toxicity categories with 8.8% and 6.1% respectively. Eighty-seven percent of the freshwater eutrophication impact for the coal scenario (which is more than five times higher than in the others scenarios) is also derived from the disposal of mining spoil.
Wood pellets are the second scenario and have the highest impacts on human toxicity. This is mainly due to the emission of contaminants during pellets combustion, which contributes to 46% of the impact in the human toxicity category (for the whole life cycle of the plant), but also because of the manufacturing of pellets, with a contribution of 15%. This last process is also responsible for 24% of the impact in marine ecotoxicity.
The biomethane scenario presents similar impacts than the wood one in marine ecotoxicity and freshwater eutrophication. Main impacts in these categories for the biomethane scenario are due to the upgrading of biogas to biomethane, which contributes to a 34% (in marine ecotoxicity) and 35% (in freshwater eutrophication).