Sunday, February 6, 2011

A Matter of Semantics – Clean Coal

Clean coal technology is now perceived as a coal combustion cycle that includes carbon capture and sequestering. Clean coal technology is actually any combustion technology that reduces harmful emissions and/or significantly improves combustion efficiency.

A one percent increase in combustion efficiency produces a two percent reduction in carbon dioxide emissions. So if you double the efficiency of a coal fired power plant you halve the carbon dioxide emissions per unit power. That is a cleaner coal technology that does not involve carbon capture and sequestering. It is an imperfect solution as there are still carbon dioxide emissions.

Co-generation or combined cycle heat and power, is a method of increasing the thermal efficiency of power plants fueled by any form of fuel. Co-generation is the use of waste heat from the combustion process, which is over 60 percent of the energy conversion, to provide useful work. Less waste equals greater efficiency.

Waste heat use is common in industrial applications where both heat energy and electrical energy are needed in the processes. In public utility applications, waste heat is most often used as a means of providing steam or hot water for building heating. The overall efficiency of co-generation in public applications is limited by the means of delivering the heat energy to the consumer and the average need of the consumer for the heat energy or demand.

Demand varies with season which places limits on many co-generation designs. Electrical demand can be steady or inverse to heating demand and vice versa. Maximum efficiency in co-generation can only be obtained where the demands remain relatively equal. In new designs, co-generation can take many forms. Gas turbine power plants that use waste heat for low pressure steam turbines for example. This allows for balanced loading. Utilizing existing power plants with useful life requires more creative co-generation options.

Pairing public utility generation of either form of energy with industrial use that can vary as needed to maintain high efficiency, is an option for many co-generation projects. Since these pairings do not always exist, many clean coal applications include hydrogen production that can vary with the demands allowing excess heat or electrical energy to be stored in the form of fuel, hydrogen, that can be consumed as needed.

While hydrogen generation and storage has its own inefficiencies, it greatly increases the efficiency of the overall combustion cycle if the hydrogen can be used in a reasonable time frame. Hydrogen is not easily stored which precludes long term storage as an option. Hydrogen applications would be peak demand use or conversion to synthetic fuels which are more easily stored for longer time periods.
To optimize combustion efficiency, balanced demand for all forms of energy generated or stored would be needed.

Hydrogen enriched natural gas is a relatively simple means of balancing hydrogen demand. Hydrogen is added to natural gas that is piped for use by normal natural gas consumers. This application utilizes the existing natural gas infrastructure. Use of the enriched natural gas reduces the carbon dioxide produced by natural gas consumers depending on the percentage of hydrogen in the enrich product.

As hydrogen production increases, expansion of the gaseous fuel infrastructure would be needed to improve the overall efficiency of the energy co-generation system. Conversion of internal combustion engines from petroleum products to enriched natural gas will increase with the expansion of the gaseous infrastructure if the cost of gasoline equivalent gallons offset the cost of conversion. Fuel cell technology, which has a higher efficiency than internal combustion, would become more attractive as the gaseous infrastructure expands.

Since all forms of public energy generation have to adjust to varying consumer demand, generation forms that are not well suited to demand adjustment would benefit from using hydrogen production during off peak periods should the gaseous infrastructure be cost effectively accessed. Condition dependent energy sources, wind and solar primarily, would benefit despite the lower efficiency of hydrogen production via electrolysis. Nuclear and coal fired power plants would benefit by using sulphur-iodine thermolysis hydrogen production to improve co-generation efficiency in areas where more common co-generation applications do not exist. Wikipedia provides a long list of hydrogen production methods.

In a perfect world, non-fossil fuel based energy options would be abundant and easily matched to our current energy delivery infrastructure and energy lifestyle. Dealing with our past imperfect choices to move into a more environmentally friendly energy future will require compromises to avoid economic chaos. Utilizing existing infrastructure and generation sources in the transition to a sustainable, independent future will require increased thermal efficiency of existing components while alternate energy sources are improved and brought online. Clean coal, meaning high efficiency coal use, is unfortunately a part of the progression to an energy independent future.

No comments: