Syngas Cleanup: Emissions Advantages of Gasification
Low Emissions of Gasification
When considering the advantages of gasification over competing technologies, one of the most significant is unquestionably the low pollutant emissions typical of gasification processes. To quantify the advantages in reduced emissions, the following discussion considers a comparison of coal-fired Integrated Gasification Combined Cycle (IGCC) and pulverized coal (PC) power plants, representing a balanced comparison of gasification to combustion in the context of power production.
Criteria Emissions Control
Gasification has inherent advantages over conventional combustion due to the ability of the technology to achieve extremely low emissions of sulfur and nitrous oxides in addition to low levels of particulate matter. The higher temperature and pressure process streams involved in gasification also allow for easier removal of carbon dioxide for geological storage or for sale as a byproduct.The major difference between gasification and combustion is that in combustion, air and fuel are mixed, combusted and then exhausted at near atmospheric pressure. In gasification, on the other hand, oxygen is normally supplied to the gasifiers and just enough fuel is combusted to provide the heat to gasify the rest. Since air contains a large amount of nitrogen along with trace amounts of other gases, which are not necessary in the combustion reaction, in addition to oxygen, combustion gases are much less dense than syngas produced from the same fuel. Pollutants in the combustion exhaust are at much lower concentrations than the syngas, making them more difficult to remove.
The Clean Air Act, enacted by Congress in 1963, requires the United States Environmental Protection Agency (EPA) to create National Ambient Air Quality Standards (NAAQS) for any pollutants which effect public health and welfare. As of 2007, the EPA had established standards for ozone, carbon monoxide, sulfur dioxide, lead, nitrogen dioxide, and coarse and fine particulates. These standards are reviewed and updated every five years.
These NAAQS, known as Title I, are administered by each state in conjunction with the EPA. Each state must submit a State Implementation Plan (SIP) to the EPA for approval which details how the state will comply with the NAAQS. The SIP may be more stringent than the Federal requirements, but must meet them at a minimum.
The complications of varying state and local implementation plans generally translate into great variation in the permitting process for new power plants based on their proposed sites. Various state and local regulations and whether or not those areas meet the NAAQS play a large role in the negotiation process for emissions requirements at new plants. Also, the future of emissions regulation is cloudy and more stringent regulations, along with the inevitable increase in worldwide electrical demand, could play a substantial role in determining the eventual market penetration of gasification technology for electrical production.
The National Energy Technology Laboratory (NETL) published a detailed performance comparison of three different IGCC technologies along with subcritical and supercritical pulverized coal power plants (Natural Gas Combined Cycle (NGCC) was also included, however since coal is not the feedstock in that scenario it is not discussed here) entitled Cost and Performance Baseline for Fossil Fuel Plants1 in 2007. The following pages use the findings of this report to highlight the magnitude of the emissions reductions possible with currently available technology. Design principles for the IGCC systems were based on best current design practices listed in the Electric Power Research Institute's CoalFleet User Design Basis Specification for Coal-Based Integrated Gasification Combined Cycle (IGCC) Power Plants: Version 4, while the PC plants were modeled based on incorporating the best commercially available technology that could be implemented in a plant to start operation in 2010. The results of the report are summarized below in the following subsections detailing each of the major emissions related to electricity production from coal. The three IGCC technologies far outperformed both subcritical and supercritical PC plants in minimizing these criteria emissions.
Carbon Dioxide (CO2)
CO2 is becoming the focus of attention as its relation to global climate change becomes clearer. Government regulations in the near future could severely limit or impose cap and trade scenarios on emissions of CO2.
The following figure, based on data from the NETL baseline study mentioned above, demonstrates that even without taking into account the advantages IGCC has in carbon sequestration, it still emits less carbon dioxide emissions than the coal-fired PC plants.
If future regulations place economic incentives on capturing and storing CO2, then the benefits of IGCC over PC plants become even more pronounced. The ease and effectiveness with which carbon capture technology is added to IGCC systems is shown in the following graph. The economic advantages that PC has over IGCC without carbon capture are made up for and even reversed slightly, with IGCC being slightly less expensive on a mills/kWh basis. In addition, the capture technology used with IGCC is able to remove approximately 90 percent of CO2, giving it a slight edge against both subcritical and supercritical coal-fired PC power plants.
Sulfur Dioxide (SO2)
SO2 causes a wide variety of health and environmental problems due to its reactivity with other airborne substances. Among these are:
- Respiratory Problems - During peak levels of airborne gaseous SO2 those with asthma and who are active outdoors can experience difficulty breathing and long term exposure can result in respiratory illness and can aggravate existing heart disease. In addition, airborne SO2 can interact with other chemicals to form sulfate particles which when inhaled are associated with respiratory disease, breathing difficulty, and premature death.
- Visibility Reduction - Airborne sulfate particles also scatter and absorb light to cause haze and reduce visibility.
- Acid Rain - Along with nitrogen oxides, SO2 reacts with airborne substances to form acid which damages forests, crops, soil, lakes, and streams when the acid falls to earth in the form of rain, fog, snow, or dry particles.
- Corrosion - SO2 causes premature decay of paint and building materials, monuments, statues, and sculptures.
Fossil fuels such as coal contain sulfur in two inorganic forms, pyritic sulfur (FeS2) and sulphates (Na2SO4, CaSO4, FeSO4), and organic forms such as sulphides, mercaptans, bisulfides, etc. Typically coal contains anywhere from 0.2 to 5 percent sulfur by dry weight.
The sulfur in the coal is converted to H2S and COS in the gasifiers due to the high temperatures and low oxygen levels. These "acid gases", as they are called, are removed from the syngas produced by the gasifiers by acid gas removal equipment prior to the syngas being burned in the gas turbine to produce electricity.
The graph above shows that when all else is even, the current IGCC technologies provide nearly an order of magnitude reduction in SO2 emissions compared with their PC counterparts.
Nitrogen Oxides (NOx)
NOx refers to both nitric oxide (NO) and nitrogen dioxide (NO2). The environmental effects of releasing too much NOx into the atmosphere are listed below.
- NOx is a main constituent in the formation of ground-level ozone which causes severe respiratory problems
- Respiratory problems may result from exposure to NO2 by itself, but also of concern is NOx reacting to form airborne nitrate particles or acid aerosols which have similar effects.
- Along with SOX, NOx contributes to the formation of acid rain and causes a wide range of environmental concerns.
- NOx can deteriorate water quality by overloading the water with nutrients causing an overabundance of algae.
- Atmospheric nitrogen-containing particles decrease visibility.
- NOx can react to form N2O, which is a greenhouse gas, and contribute to global warming.
Coal usually contains between 0.5 and 3 percent nitrogen on a dry weight basis. The nitrogen found in coal typically takes the form of aromatic structures such as pyridines and pyrroles. The feedstock flexibility of gasification allows for a wide variation in the nitrogen content of coal.
During gasification, most of the nitrogen in the coal is converted into harmless nitrogen gas (N2) which makes up a large portion of the atmosphere. Small levels of ammonia (NH3) and hydrogen cyanide (HCN) are produced, however, and must be removed during the syngas cooling process. Since both NH3 and HCN are water soluble, this is a straightforward process.
NOx also can be formed downstream by the combustion of the syngas with air in the gas turbine to produce electricity. However, known methods for controlling NOx formation keep these levels to a minimum and result in NOx emissions substantially below those associated with other coal-fired electrical production technologies as seen in the following figure.
Particulate matter consists of microscopic solid particles or liquid droplets which are small enough to enter the lungs and cause health problems. Both NOx and SOx are associated with the formation of particulate matter, but other processes can contribute to their formation. Aside from health concerns, particulates cause reduced visibility and haze when released in the atmosphere.
Ash is formed in coal combustion and gasification from inorganic impurities in the coal. Some of these impurities react to form microscopic solids which can be suspended in the exhaust gases in the case of combustion, or the syngas produced by gasification.
Gasification offers two main advantages in particulate control over combustion processes. First, gasification of coal provides the capability of removing most of the ash as inert slag or bottom ash for disposal or sale as a byproduct. Second, since the syngas leaving the gasifiers is much more dense than combustion exhaust gases, the particulate matter can more easily be removed. The following figure gives an indication of the advantage that gasification has for particulate emissions.
The high temperatures and pressures within the gasifier have the potential to turn inorganic substances within the feedstock into slag instead of the ash that is produced in combustion. The slag produced as a byproduct of the gasification process captures heavy metals and does not allow them to leach out of the material, while heavy metals in the ash produced by combustion can find their way into groundwater and surrounding soils once they are buried for disposal. In addition, as with the pollutants mentioned above, the reduced volume of the syngas versus the combustion exhaust gases allows for more economical clean up of any remaining heavy metals or other contaminants.
In summary, gasification has inherent advantages over combustion for emissions control. Emission control is simpler in gasification than in combustion because the produced syngas in gasification is at higher temperature and pressure than the exhaust gases produced in combustion. These higher temperatures and pressures allow for easier removal of sulfur and nitrous oxides (SOX, and NOX), and trace contaminants such as mercury, arsenic, selenium, cadmium, etc. Gasification systems can achieve almost an order of magnitude lower criteria emissions levels than typical current U.S. permit levels and +95% mercury removal with minimal cost increase.2
Syngas Clean Up