52 MAKALE/ ARTICLE As applied to IGCC plant designs, Scott explains, it improves gasification waste heat recovery and facilitates pre-combustion carbon dioxide capture. Design studies of IGCC flow schemes incorporating gasifiers fitted with water quench coolers and desaturators have demonstrated thermal efficiencies of up to %42 (LHV). Tnat is beter than any other published figures lor water quench designs, be claims, and can be achieved w i thout having to increase capital costs. GEM concept Functionally, says Scott, you can seperate an IGCC planı into a "fuel generator'' which converts low cost feedstocks into a clean fuel and a "power generator'' which converts that fuel gas into electricity. Normally, the two sections are built up simultaneously when erecting a conventional IGCC planı. But there is nothing that says you cannot build an independent gasification unitto refuel an existing gas turbine installation. Jacobs has developed a design concept tor a stand-alone Gasification Enabling Modela (GEM) in which the gasification and gas treatment part of the IGCC planı is physically separated !rom the power island. Typically a GEM plant would contain a feedstock storage facility, handling and preparation units, air separation unit, gasifier, gas cooling and sulfur removal systems, and heat recovery units. it could alsa provide tor the removal of heavy metal contaminants and carbon dioxide. Operating flexibility A single GEM planı might fuel many combined cycle plants at multiple locations, Scott points out, or could be dedicated to a single combined cycle. Operationally it makes no difference it the GEM planı is located close to or at a distance !rom the combined cycle units serviced. Or whether it is owned by a power plant operator or separate firm that just selis fuel gas "over the fence". The concept has major advantages tor an existing combined cycle whose location makes it difficult to continue receiving low cost Natural gas. "These can now be refueled with syngas !rom coal", says Scott, "and in effect puts a ceiling on the price of natura! gas fuel." Recently performed studies show that overall coal-to-electricity efficiencies of %40 (LHV) and higher can be achieved lor a GEM located adjacent to the combined cycle and over %38 (LHV) lor a remotely located GEM. The lower efficiency is due to the facıthat the sensible heat !rom a romete GEM cannot be used in the high efficiency combined cycle and must be used instead in a steam cycle with lower efficiency. lncorporating a desaturator into the IGCC flow scheme increases the steam to dry gas ratio of the syngas leaving the water quench, says Scott, and facilitates the CO shift reaction. Under those circumstances, he adds, a desaturator in conjunction with a shift "upgrades all of low grade heat in the syngas stream leaving a gasifier fitted with a water quench." in effect, the gasification can be decoupled from power generation. Thereby "rationalizing the commercial relationship between the GEM and combined cycle to that of a gas contract." IGCC Economics Robert Jones and Narman Shilling of GE Energy Products in the U.S. addressed the Economic and technical viability of syngas combined cycle versus Natural gas plants given the projected Outlook tor fuels and environmental regulations. Key factors favoring IGCC are its fuel flexibility combined with superior environmental performance. However, solid fuel plants such as IGCC have Capital costs two time sor more that of Natural gas combined cycle plants. This Capital recovery burden can be more than offset by the advantage of low cost opportunity fuels, they point out, but those fuels (such as refinery residuals, suldges, petcoke, wastes and biomass) present difficulties in terms of environmental performance. These difficultes will only increase as regulatory and public sensitivity continues to demand environmental performance approaching parity with natural gas. Jones and Shilling note that currently the fuel choice decision has been complicated by the volatility and high level of Natura! gas prices. it is further complicated by the possibility and hope that gas prices will moderate within the lifetime of the plant. Sol id fuel factors The choice of solid fuel technology is made less deterministic by the uncertainty of regulatory changes, says GE, which have the potential to occur an Economic planı lifetime of 30 to 50 years. On the other hand, they see IGCC technology as providing the most 'headroom' lor future compliance since it is fundamentally easier and more economical to clean a lower volume fuel steam than the net products of combustion. A key factor to be considered is the fuel flexibility that can be leveraged after a solid fuel plants is constructed. in this regard, IGCC boasts a significant advantage relative to conventional combustion technology. This has been demonstrated in actual experience, both the Tapma Electric and Wabash IGCC plants, lor example, have been able to switch to lower cost coal and petcoke respectively. Another competitive power generation advantage is that, as the lower variable cost power producer, an IGCC planı would enjoy higher dispatch than a Natural gas fireci combined cycle planı. Cost offsets From a fuel flexibility and cost parity standpoint, Jones and Shilling observe, a $3 spread to petcoke at $0.50 per MMBtu is equivalent to a $3.50 natural gas price-which is not a realistic projection lor future gas price, even in the most optimistic of scenario planning. Also, they stress, this analysis does not incorporate the marketable value of IGCC byproducts to ofset gasification operating and maintenance costs. Further economic upside lor IGCC will be increasingly derived from the co-production of chemicals out of the basic syngas and the potential tor environmental credits that will further balance Capital costs associated with the gasification planı. An important consideration is the overall system capability of gasification combined cycle plants, and the gas turbine in particular, to maintain high reliabiulity and availability performance on these low grade fuels. in general, they note , significant progress has been made in the design development and application of gas turbines modified tor low-Btu syngas operation and integration with the gasification process. Design features IGCC gas turbines have dual fuel combustion systmes designed to operate on conventional fuels as well as syngas. A conventional non-hydrogen bearing fuel is used tor start up and shut down as well as enabling operation over the engine load range it load demand requirements cannot be satisfied by syngas production. Heavy frame gas turbines designed with can-annular combustion systems offer a variety of configurations tor single or multiple fuel nozzle arrangements to suit a multitude of syngas applications. Several gas turbine models including the Fr 6FA, 7FA provide large diameter combustor designs, says GE, that have been engineered tor IGCC operation. Their combustor designs are optimized to deal with the increased mass and volumetric flow of low calorific value fuels and allow tor head-end injection of diluent directly into the air stream lor emissions control. Kaynak: Gas Turbine World, Ağustos-Eylül 2004 1ENERJi & KOJENERASYON OÜNYASI ♦ "Kojenerasyon: Yüksek Verim, Temiz Çevre, Enerjide Yeniden Yapılanma"
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