This article highlights that the European Commission is supporting a wide range of clean coal technology research and development initiatives, including those known as APAS (Activité de Promotion, d'Accompagnement et de Suivi) and Joule (after the 19th-century British physicist James Joule). APAS, a two-year multiple-partner program, was set up to evaluate gasification processes using biomass, sewage sludge, and other wastes as co-feedstocks with coal. The Joule 3 co-gasification initiative was designed to aid European industry to address the technical issues for fluidized bed co-gasification applications. The Joule 2 project for the enhancement of the efficiency of coal-fired power generation systems was undertaken by Siemens and the University of Essen in Germany, and Babcock and Wilcox Espanola in Spain. In the Joule 3 project on advanced cycle technologies, the University of Essen and four partners have investigated measures to reduce costs, enhance efficiency, and provide a basis for an advanced design. The studies also included co-gasification of coal and biomass in an entrained-flow gasifier suitable for IGCCs.
Europe will need new power generation plants within 20 years. More than 80 percent of the energy that is consumed in the European Union countries comes from fossil fuels. Coal accounts for over 40 percent of the power, and that comes almost entirely from conventional pulverized-fuel-fired boilers linked to a conventional steam cycle. Such systems have modest efficiencies and contribute to a large extent to the global emissions of nitrogen oxides, sulphur oxide, carbon dioxide, and particulate matter.
The European Commission is supporting a wide range of clean coal technology research and development initiatives, including those known as APAS (Activité de Promotion, d’Accompagnement et de Suivi) and Joule (after the 19th-century British physicist James Joule).
APAS, a two-year multiple-partner program, was set up to evaluate gasification processes using biomass, sewage sludge, and other wastes as co-feedstocks with coal. For example, Rheinbraun AG of Germany and the British Coal Corp. of the United Kingdom have examined the use of sewage sludge in combination with different types of coal. Rheinbraun AG studied the use of sewage sludge and loaded coke as co-feedstocks with dried brown coal in the high-temperature Winkler gasification process. Various tests were conducted in a demonstration plant that operates on 30 metric tons per hour of dried brown coal. The plant works continuously on an industrial scale and has full final gas treatment and waste water pretreatment stages.
During 11 individual test campaigns, a total of 504 metric tons of sewage sludge and 32 metric tons of loaded coke were co-gasified at feeding rates varying between 0.3 and 5 metric tons per hour. These tests took about 70 hours and were accompanied by a detailed analysis program to monitor such aspects as operability, conversion efficiency, syngas contaminants, solid residue characteristics, and emissions.
Emissions were well below the limits. For both sewage sludge and loaded coke, conversion efficiency and syngas yield were adequate. An increase in the benzene and naphthalene concentrations in the crude gas was noted. Thus, a commercial application would require additional gas treatment
An application was approved to operate the demonstration plant with a co-gasification rate of up to 15 metric tons per hour of waste materials. Wastes selected included dewatered sewage sludges, loaded rotary hearth furnace cokes, and processed packaging plastics. A number of plant modifications were made to accommodate these feedstocks. Recent trials have included the gasification of 800 metric tons of plastic wastes.
Rheinbraun concluded that co-gasification of sewage sludge or loaded coke with dried brown coal offered significant potential for disposing of these wastes without impairing plant efficiency and emissions. The commercial viability was demonstrated by an assessment study that included major aspects such as feed rate, total investment, and methanol price in order to establish the criteria for the use of sewage sludge in the high-temperature Winkler gasification process.
In a complementary study at its Coal Research Establishment, British Coal Corp. examined the use of sewage sludge as a partial feedstock with hard coal.
Preliminary testing with coal and pelletized sludge on an atmospheric fluidized bed gasifier rig was followed by more extensive trials on a pressurized unit. This unit had a thermal input of 2 MW and comprised a spouted bed gasifier, cyclone, hot gas filtration unit, and fuel gas combustor.
Test programs involved adding sewage sludge up to 25 percent (dry weight basis), increasing the peak bed temperature from 980 to 1,000 and 1,020°C, and reducing limestone addition from a Ca:S of 2:1 to 1.5:1 and 1:1.
The feeding and handling properties of the dried pelletized sewage sludge selected for study compared favorably to those of crushed coal. Co-firing sewage sludge with coal for extended periods of time, and with sewage sludge additions of up to 25 percent (dry basis), did not adversely affect the gasifier operability or process performance, provided that the input ratio of carbon in the fuel to oxygen in the fluidizing air remained constant. The fuel gas calorific value was typically 4.2 megajoules per cubic meter (wet, net, purge-free basis) and fuel conversion efficiency 78 percent (dry ash-free, mass basis). The sulfur retention efficiency attained during co-gasification, with limestone addition, was high, typically 92 percent. This efficiency was attributable partly to the sulfur retention properties of sewage sludge.
Sustained operation without the agglomeration of ash was attained for all test conditions, including operation at a bed temperature of 1,015°C, co-firing with 10 percent sewage sludge. There was no evidence of an increase in the elutriation of fines or the formation of tars as the cofiring ratio of sewage sludge and coal was increased.
Compared to coal, sewage sludge has higher levels of the more volatile heavy metals, and there were concerns that they could harm downstream components. However, most of the trace elements were partitioned into the solid stream at the hot gas filtration stage.
British Coal Studies
To support the technical work, British Coal carried out various techno-economic studies. Two biomass feed stocks—sewage sludge and straw—and two process technologies—oxygen-blown integrated gasification combined cycle (IGCC) and an air-blown gasification combined cycle (ABGC)—were selected. A wet feed IGCC process was used for sewage sludge and a dry feed process for straw. Plant sizes of 350 to 500 MW of electricity were dictated by the size of the large gas turbines used in most commercial power plants. Biomass feed rates within a range of 0-25 percent of the coal feed were modeled, based on an analysis of the likely availabilities of straw and sewage sludge within a reasonable radius of a plant.
Plant performances were predicted by CRE Group Ltd. (formerly part of British Coal) using the Arachne process flowsheet computer modeling package (an in-house package available for contract consultancy applications). Adding 25 percent straw to an IGCC plant was predicted to reduce the low heat value efficiency by 1.4 percentage points if lock hoppers were used. But it should be possible to virtually eliminate this penalty if an advanced feeding system could be developed. Even using lock hoppers, there should be no efficiency penalty from feeding straw in the ABGC, provided the gasifier bed temperature does not have to be reduced substantially for the low melting characteristics of straw ash.
Feeding 25 percent sewage sludge to an ABGC plant would increase the low heat value efficiency by 1.5 percentage points, but reduce the high heat value efficiency by 1.9 percentage points. If cold gas cleaning was required for removal of ammonia and heavy metals, the LHV efficiency would increase by 0.7 percent instead. Adding 25 percent sewage sludge to an IGCC plant would have very little effect on the LHV efficiency.
The Joule 3 co-gasification initiative was designed to aid European industry to address the technical issues for fluidized bed co-gasification applications.
Part of the Joule project, a program to develop and design coal-biomass systems components, was undertaken by VTT Energy and Carbona of Finland, Schumacher of Germany, British Coal, Technical University of Delft in the Netherlands, and Nuovo Pignone of Italy. The work investigated the effect of mixed feedstock properties on co-gasification processes, and resulted in improved hot gas filtration operations, increased overall carbon conversion, and reduced emissions. It was also confirmed that, with modifications, it was possible to fire turbines on the gas generated.
A separate part of the program, coal-biomass environmental studies, undertaken by CRE Group and Imperial College of the United Kingdom, and TPS Termiska Processer AB and Kungl Tekniska Hogskolan of Sweden, concentrated on the use of laboratory-scale experimental techniques to study the influences of several fuels on gasification behavior. The studies found that when coal and biomass or wastes were co-gasified, the overall level of tars generated was lower than for coal alone; the concentration of hydrocarbons in the range of C1 to C7 was increased, and product gas yields increased and char levels decreased, with co-gasification chars being significantly more reactive. In addition, the heightened char reactivity resulted in increased conversion of NO and NH3 to N2.
Technical and economic studies by the Energy Research Centre of Ulster found that in nearly all cases, the gasifiers could operate on a range of coals and generate gas of sufficient calorific value to be combusted in a gas turbine.
The strategic studies concluded that in markets where natural gas is available, new coal plants will be unable to compete directly until the gas price doubles. For coal- fired plants, unless credit is given for lower levels of emissions, pulverized fuel and pressurized fluidized bed combustion technologies will remain the least expensive options.
The Joule 2 project for the enhancement of the efficiency of coal-fired power generation systems was undertaken by Siemens and the University of Essen in Germany, and Babcock and Wilcox Espanola in Spain. It used the oxygen-blown Puertollano IGCC power plant in Spain, with good-quality coal feedstock as the base case. The project considered ways in which plant performance could be improved, with particular emphasis on efficiency and environmental impact.
Replacing the conventional wet gas cleaning stage with a dry, high-temperature system increased the plant efficiency of the base case by 0.8 percentage point to 49 percent. Increasing the clean gas temperature before the gas turbine combustion chamber from 350°C to 510°C enhanced the net efficiency by another 0.9 percentage point.
Further studies examined the effect of increasing the inlet temperature of the gas turbine. The gas turbine was modeled as a unit using the Aspen Plus power plant process flowsheet modeling package, suitably tailored, available commercially from AspenTech of Cambridge, Mass. It was assumed that the inlet temperature was 1,190°C, the compressor ratio was 17.0, air compressor polytropic efficiency was 91.5 percent, and the turbine isentropic efficiency was 89.5 percent.
A Gain in Spain
THE BENCHMARK IN EUROPE for IGCC is the 300 MWe combined-cycle power plant at Puertollano in Spain. The process uses an oxygen-blown Prenflo entrained- phase coal gasifier, followed by extensive coal gas cleaning stages and low N0X combustion in the gas turbine. Once fully operational, it is expected that the process will have a net efficiency of 45 percent.
A three-year demonstration phase began in 1997 with the first production of gas from coal occurring in December of the same year. Following this, an extensive assessment was carried out and a series of plant modifications made. Gas turbine operation on coal gas was achieved in March 1998. However, initial runs showed the need for a number of other modifications. These were carried out and trouble-free steady operation was achieved in October 1998. By the end of the year, some 58 gasification runs had been done, amounting to a total of 280 hours of operation. Ten gas turbine runs using syngas had been completed.
With increased fuel diversification in Europe, gas-fired power stations are currently the preferred option for new capacity. Nevertheless, coal will continue to have a role to play in power generation in the future. The technology of choice will not be the conventional pulverized fuel plant; rather, it will be either an advanced PF plant, with higher efficiency steam conditions and ultra-effective gas cleaning, or one of the new, advanced, clean coal technologies that will offer integral pollutant control plus optimized gas turbine and steam cycle systems.
The influence of increasing the inlet temperature from 1,150° to 1,400°C was investigated over a range of compressor pressure ratios. The studies indicated that raising the inlet temperature to 1,400°C would lead to IGCC net efficiencies (LHV) of 53.2 percent.
Studies confirmed that IGCC systems fired on a variety of fuels can realize increased efficiency, reduced emissions, and lower cost of electricity using proven technology within existing designs. Further developments in the fields of hot gas cleaning, gas turbine technology, and materials would have further positive effects.
In the Joule 3 project on advanced cycle technologies, the University of Essen and four partners have investigated measures to reduce costs, enhance efficiency, and provide a basis for an advanced design. The studies also included co-gasification of coal and biomass in an entrained-flow gasifier suitable for IGCCs.
The study concluded that, based on proven materials, components, and processes, in the near term, coal-fired IGCC technology is competitive with a modern pulverized coal steam power plant. It is expected that with the gas turbine inlet temperature operating at elevated temperature, IGCC net plant efficiency (LHV) would be approximately 51.5 percent, compared to a modern pulverized coal plant’s 45 percent. There are a number of other IGCC developments in hand that could ultimately increase efficiency to levels approximately 58 percent or more.
The study also investigated the use of coal/biomass combinations for IGCC applications. Findings confirmed that as much as 10 percent biomass in an oxygen-blown entrained flow gasifier was technically feasible. Net electrical efficiencies were lower as a consequence of the higher internal energy consumption required for biomass pre-treatment and process compressors. However, by using an optimized and integrated process, and by pressurization of the pyrolysis/gasification pre-treatment stage, the overall decrease could be limited to 0.5 percentage point LHV.