Chapter 16 Clean Coal Technology
The development of Clean Coal Technology (CCT) is an objective not easy to be performed for different reasons. On the one hand coal is not a uniform source due to its extremely variable composition; this makes it difficult to reach a standardization of advanced technologies, which can be very sensitive to the fuel used. On the other hand coal combustion produces structurally more pollutants than other fossil fuels since it contains mainly carbon (producing CO2) and sulfur (SOxis the resulting product) as reactive components but very little hydrogen (turning into H2O). We analyze perspectives of the particular field of CCT starting from analysis of the state of the art. The paper will focus on the emerging suite of technology options. In addition the current research and development (R&D) in the “clean coal field” is discussed. What makes coal “unclean” and what “clean coal technologies” claim?
Coal is a complex chemical latticework of carbon, hydrogen, and dozens of trace elements. When combusted, some of these elements, such as sulfur, nitrogen, and mercury, are converted to chemical forms that can create pollutants in air and water. Carbon, the main constituent, combines with oxygen during combustion to form carbon dioxide (CO2), which has been identified as a key contributor to global warming. Coal also contains sulfur, which bums to produce SO2. Moreover coal-fired stations emit tones of ash through their chimneys, 80% of which is particulates smaller than 10 μm (denoted as PM10), arsenic, hydrochloric acid, mercury, nickel, and lead. Moreover combustion produces NOx.
Public awareness and legislation have led to a policy of reduction of pollutant from coal-fired power generation, with regulations partially driven by international initiatives such as the Kyoto protocol. The local acceptance of new plants is generally based on choice of pollutant emission limits well below the existing legislation (e.g., in the USA, Japan, and Italy). The Italian environmental limits are represented today by 400/200/50 mg/nm3for SO2, NOxand particle matter, while the European Directive 2001/80/CE, which will be operative by the next years, reduces the limits to 200/200/30 mg/nm3for SO2, NOx, and particle matter. CCT are the basis for long-term acceptance of coal and is a flexible concept, which can be used by all countries. Three different stages to achieve “clean coal” are available:
(1) control and reduction of pollutants SO2, NOx, mercury, and PM (excluding CO2) without structural modification of cycles;
(2) advanced technologies (the efficiency pathway);
(3) long-term vision of CO2capture and storage.
Pollutant Emission Control
There are various technologies and processes that can be utilized throughout the coal fuel cycle to mitigate negative environmental impacts. The available technologies are:
(1) removal of the source of pollution (sulfur, nitrogen) from coal before it is burnt;
(2) avoiding the production of pollutants during combustion (in-fumace measures);
(3) removing pollutants from flue gases by “end-of-pipe” methods prior to emission.
NOxControl Options
Depending on the fuel used, combustion conditions, air ratio, and flame type in the burner, a considerable mass of nitrogen oxide might be produced during combustion processes. Three primary sources of NOxformation in combustion processes are documented:
(1) formation due to high-temperature combustion depending on the residence time of nitrogen at that temperature (thermal NOx);
(2) formation of fuel-bound nitrogen to NOxduring combustion (fuel NOx);
(3) formation due to the reaction of atmospheric nitrogen, N2, with radicals such as C, CH, and CH2fragments derived from fuel (prompt NOx).
One of the most common methods of post-treatment is the selective catalytic reduction (SCR) generally used when higher NOxreduction is required. The SCR achieves reductions of about 90% when it is applied at temperatures between 300℃ and 400℃.
SOxReduction
Coal contains significant amounts of sulfur. When burned, about 95% or even more of the sulfur is converted to sulfur dioxide (SO2). SO2can be removed from flue gases by a variety of methods (Table 16.1). SO2is an acid gas and thus the typical sorbents used to remove SO2from flue gases are alkaline. Post-combustion removal includes wet and dry flue gas desulfarator (FGD and DFGD, respectively) or spray dry scrubbing. FGD is the current state-of-the-art technology used for removing SO2from exhaust gases in power plants. Many “conventional” pulverized fuel (PF) stations (with low-NOXburners) have FGD fitted. For a typical coal-fired power station, FGD will remove 95% or more of SO2in flue gases. FGD utilizes a variety of slurry of sorbent materials to scrub gases in order to accomplish SO2 removal efficiencies approaching 99% (reduction in the treated flue gas). These reagents include limestone (Ca CO3), lime (Ca O), caustic soda (Na OH), and related variants to absorb and neutralize SO2in flue gases. The main control technologies with their potential reduction are as the following:
PM Controls (mainly post-combustion methods)
PM composition and emission levels are a complex function of coal properties, boiler firing configuration, operation, and pollution control equipments. In the combustion of solid fuel, dust and ashes, which are included in exhaust gases as small particulate, are produced. PM control is possible mainly with post-combustion methods, like electrofilters, cyclones, and ceramic filter, with quite good results.
Mercury Control
Mercury control R&D includes sorbents and oxidizing agents that can change gaseous mercury into solids, which can be captured. The oxidizing agents work inside wet flue gas scrubbers to capture mercury in sulfate byproducts. Hg capture with existing controls depends on coal and technology type, being more difficult to control Hg from low-rank coal-fired boilers. Sorbent injection is an emerging Hg control technology.
The New Concept of Coal Plant
Each component in the flue gas cleaning section is designed to remove a specific pollutant but, besides this, can also have a beneficial effect on other macro- and micro-pollutants, substantially increasing the global abatement performance. Good results can be obtained using various pollutant control technologies. But a different vision of coal plant as an energy system is emerging. generate significant quantities of solid byproducts such as fly ash or gypsum. The call for more stringent emission reductions through multi-pollutant regulations has the potential to alter the future use of coal byproducts and may make certain auxiliary product (limestone) or byproducts (gypsum) a problem, which needs to be considered.
Advanced Technological Options for Coal Conversion
Energy performances and pollutant emissions from electric-power-generating plants can be further reduced by improvement of the thermodynamic cycle of power generation. New requirements to limit environmental emissions impose a shift from steam-cycle-to the gas-cycle-based plants.Technologies of interest with possible variants are summarized. Those are mainly:
(1) advanced ultra-supercritical (USC) plants pressurized fuel combustion plants;
(2) fluidized bed combustion (FBC) incorporating also advanced supercritical steam cycle;
(3) integrated gasifier combined cycle (IGCC);
(4) externally fired combustion combined cycle (EFCC).
Ultra-Supercritical (USC) Plants
The use of USC parameters for steam represents for sure the evolution of pulverised-coal-fired power plants. In addition to advances in steam conditions, it incorporates several clean air technologies: new design of burners, new scheme of combustion in the boiler furnace, new design of steam superheaters, and gas cleaning systems. USC technology is well known; according to over 550 supercritical PCC are available all over the world for an amount of 300 GW (about 150 in the USA, over 100 in Japan and Russia, more than 30 in Germany). With the term“ultra-supercritical” the overcoming of limit conditions for steam at the level of 3×107Pa/600 ℃ to reach more advanced operating parameters towards an increase of pressure and turbine inlet temperature is evidenced. The currently available power plants based on supercritical steam boiler at 600℃ permit efficiencies of 45%-47%. The limits of this technology are today under discussion.
The analysis carried out by some researchers and producers indicated an agreement about the long-term objective of reaching a steam pressure level of 350 bar and maximum steam temperature of 700 ℃ with the use of advanced materials (AD700USC plants). The perspective is to achieve net efficiencies of 50% and more. Pressurized Fluidized Bed Combustion (PFBC)
FBC represents a straightforward evolution of circulating fluidized bed combustion (CFBC), which has gained great attention from the 1970s. Fluidization means that solid coal particles are supported and mixed with air that is injected into the system. Burning occurs at 760-930℃, well below the 1370℃ needed to generate nitrogen oxide pollutants. It permits basically the possibility of a strong reduction of SO2and NOxemissions with respect to pulverised-coal power plants. SO2is captured by limestone injection and CO2is controlled by sorbents. The resulting flue gas can be used in turbine.
FBC technologies are of various types. They include atmospheric pressure fluidized bed combustion (FBC), circulating fluidized bed combustion (CFBC) or pressurized fluidized bed combustion (PFBC), and pressurized circulating fluidized bed combustion (PCFBC).
From the thermodynamic point of view, the main benefit obtained from pressurized fluidized bed is the possibility of increasing plant efficiency by coupling a Rankine cycle with a gas turbine. The controlled combustion permits flexibility in the use of fuel (i.e., the use of low-quality coal). The resulting process is a hybrid cycle, but the steam turbine generates a high percentage of power (up to 80%). The currently available efficiency is lower than 40% and many problems during operation have been evidenced in various experimental facilities.
Integrated Gasifier Combined Cycle (IGCC)
For 20 years, IGCCs were considered the future of coal combustion. IGCC first turns coal into gas (mostly CO and H2). Then sulfur, ash, mercury, and other pollutants are removed and finally the clean gas is fed to the Central Power. IGCC allows benefit from gas turbine advances and permits simpler CO2control if required. Multiple gasification process technologies are available, like:
(1) entrained flow [Shell, GE (Texaco), Conoco-Phillips (Dow/Destec)];
(2) fixed bed (BGL, Lurgi, EPIC);
(3) fluidized bed (Southern Co, KRW).
These processes allow a large variety of plant configurations. Plants are operating successfully in Spain, Netherlands, and the USA. Among them a 253 Mwe IGCC power plant of Buggenum (the Netherlands), a 252 Mwe IGCC of Wabash River (Indiana, USA), a 250 Mwe IGCC of Polk County (Florida, USA), and a 318 Mwe IGCC of Puertollano (Spain) are of particular interest because they are based on coal as primary fuel. Efficiency is in the range of 35%-42%. The specific cost of commercial version of similar plants is estimated to be about 40%~60% higher than that of a conventional PCC plant. IGCC is basically the cleanest coal technology with inherently lower SOx, NOx, and PM, the lowest collateral solid wastes and wastewater, potential for the lowest cost removal of mercury, and the cheapest route to CO2separation. Notwithstanding some successful experiments, the low number of operating plants showed a lot of problems, mostly concerning the availability. Meanwhile the renewed interest in conventional PCC power plants made investments on IGCC quite-less attractive. But IGCC becomes a solution of interest for petrochemical industry. More than 120 plants were in operation in 2004. These facilities produce mostly chemicals (37%), gas (36%), or power (19%). In terms of feedstock, some of them are solid feedstock based (coal and petroleum coke), others are refinery high-sulfur heavy-oil based. Only a small number of them are based on coal.
For this reason IGCC technology holds great promise for the future due to the flexible feedstock, process options, and products, and opens new markets for coal (synfuels, chemicals, and fertilizers). It also provides the only feasible bridge from coal to hydrogen (directly converts coal to hydrogen). But in the meantime new barriers are growing against deployment of IGCC. The first is the power industry culture. While a conventional coal plant places a chemical plant at the back end, attempting to capture pollutants after combustion with much dilution, IGCC places the chemical plant in the front end of the power plant and it is basically a chemical plant. Power companies do not like chemical units. Moreover there are a lot of technical and financial risks and finally companies do not understand why they should build IGCC when it is possible to get a permit for a conventional coal plant.