Suicide by catalytic converter and deaths at Lake Nyos; Is Carbon Monoxide the toxic agent? Implications for leakage risks from CO2 pipelines.

摘要

A key aspect of the Lake Nyos incident was the exceptionally large quantity of CO2 that was abruptly released in the incident. The exact quantity is uncertain however there is a consensus view that it amounted to between 1.0 and 1.6 million metric tons of CO2. In the context of CO2 sequestration this minimum amount of CO2 corresponds with approximately four months emissions from a 275 MW FutureGen type IGCC or over a month of CO2 captured from a large, but not atypical, 1000 MW coal fired power plant. This volume of CO2 is also equivalent to weeks or months of gas that would be transported in the largest pipelines contemplated for a future sequestration project. Given the isolation valves that would be activated the CO2 released at Lake Nyos was 2 to 3 orders of magnitude larger than the largest release plausible from CO2 pipelines. Carbon monoxide poisoning utilizing automobile exhaust in an enclosed space for decades was a classic suicide strategy. Since the development of catalytic converters the carbon monoxide has been in large part converted to carbon dioxide and a nominally non-lethal level of carbon monoxide. Unfortunately suicidal acts continued unabated due to the powerful effects of mixed CO2 and CO. Although the medical teams reported no definitive evidence of cause of death, they did note that many victims had prominent skin bullae (blister like features) or has succumbed to comas, and that neither of these prominent symptoms can be related to CO2. The US medical team concluded that "doubt must remain that all the findings can be attributed to [CO2] alone". After the Lake Nyos incident, evaluation of the symptoms documented in medical studies from the foreign scientific expeditions lead to a conclusion that the Lake Nyos victims died from carbon monoxide poisoning. Although these assertions have been dismissed by geologists studying the Lake Nyos event, their reasoning appears flawed. A plausible model for the causative agents for the Nyos disaster was a combination of CO2 (quite possibly at non-lethal concentrations), reduced oxygen, and the critical deadly agent, CO. In the absence of carbon monoxide neither the gas in exhausts from catalytic converter nor the gas at Lake Nyos, would be capable of killing. This together with the magnitude and circumstances of the release has significance for risks associated with CO2 transportation by pipeline. Mixed gases are known to be deadly at concentrations at significantly lower than the lethal levels of either component alone. Perhaps the real lesson of the Lake Nyos incident for carbon capture and storage is to draw attention to the inherent danger from mixtures of toxic gases. For example mixtures of H2S with CO2 will increase the toxic effects of H2S. The risk profile for a sequestration project changed a decision is made to inject mixed streams of gas such as CO2 + H2S rather than essentially pure CO2. A more useful analogy between natural CG2 seepage and the worst-case scenario for leakage from CO2 sequestered in deep brine reservoirs is provided by the volcanic/geothermal terrains in Italy, the Azores, and Africa. In this paper the available information and analysis of the presumed CO2 releases at these sites and the related deaths are reviewed and the implications for evaluating the risk posed by either slow or rapid leakage from CO2 sequestration are considered.