MEET the multi-fuel solution
Tokyo Institute of Technology
The competitive nature of the power generation industry could threaten the future of integrated gasification combined cycle technology (IGCC). But the Tokyo Institute of Technology is currently developing an IGCC technology, known as MEET, which not only addresses cost issues, but is also unique in its fuel flexibility.
Great strides have been made in improving the economics of coal and natural gas fired power plants in response to the global market demand for competitive power generation. With deregulation and privatization of the utility industry taking place all over the world, the priority for power generation technology selection is based on `bottom-line` economics.
Integrated gasification combined cycle (IGCC) technology offers an attractive alternative to conventional pulverized coal and natural gas power generation in terms of cycle efficiency and emission compliance. But while operating costs are low, the high capital cost of IGCC has threatened its competitiveness.
However, as a result of improvements in the turbine, gas clean up and other design improvements, the US Department of Energy (DOE) forecasts that by the year 2010, the operating cost of IGCC-based electricity power generation could be in the order of 3.7 à‚¢/kWh, which is cheaper than advanced coal fired power plant and also comparable to natural gas fired combined cycle units. A capital cost of around $1000/kW is also achievable.
An innovative IGCC technology, funded by the Japan Science and Technology Corporation, is under development to address this challenge. The uniqueness of the technology is its capability for universal application on any kind of solid fuel, from coal to biomass and waste. Called MEET (Multi-staged Enthalpy Extraction Technology), it is based on a simple concept of gasification using high temperature air at 1000à‚°C (see Figure 1). The syngas is cooled in a heat recovery boiler and cleaned up by conventional advanced gas cleaning technology.
A portion of the cleaned syngas is utilized for pre-heating the gasification air. The syngas is burned by means of high temperature air and the products are used for driving gas and steam turbines. The majority of the ash is converted to benign slag and collected from the gasifier in the form of unleachable products.
An efficient cycle
A cycle efficiency analysis of the MEET system has been carried out using coal and refuse-derived fuel (RDF). When the IGCC system is firing RDF with a composition shown in Table 1 and an ash flow temperature of 1150à‚°C, the gasification temperature is 1200à‚°C.
In the MEET process, RDF is fed to the high temperature pressurized gasifier. The product gas is cooled down to 400à‚°C and then in the next step to 100à‚°C by a gas-gas heat exchanger. A wet gas clean up system removes the contaminants. The syngas is compressed to a higher pressure for gas turbine combustion. A part of the clean gas is used for the air pre-heat. The power generation system is similar to a conventional coal IGCC, and a gas turbine of the 1300à‚°C class is employed. A thermal efficiency of 45 per cent has been calculated for the MEET process using RDF. When using US coal, a thermal efficiency of 46 per cent is possible.
Since most of the chlorides present in the syngas are water soluble, a wet type gas clean up system is expected to achieve a level of purity for the syngas needed to meet the required compliance levels. Other wastes – sewage sludge, corrugated board, textile, lumber and rubber – have been examined for the MEET system, where cycle efficiency from 38 per cent for sludge to 46 per cent using textile waste has been calculated.
Merits of MEET
The MEET-I system has significant advantages over current IGCC processes. These include:
à¢€¢ Use of high temperature air makes it possible to produce syngas with a high calorific value, the level of which can only be achieved in conventional IGCC processes by using oxygen. The syngas calorific value is then high enough to drive gas turbines.
à¢€¢ The use of high temperature air at 1000à‚°C makes it possible to obtain the same gasification temperature at low excess air, thereby achieving a decrease in the volume of the syngas produced. This significantly reduces the size and cost of the gas cleaning equipment.
à¢€¢ The system can use low-grade coal, municipal waste, biomass and other low-grade fuels.
à¢€¢ NOx formation is minimized by the use of high temperature air combustion with in-furnace recirculation of exhaust gas with a low oxygen content.
à¢€¢ Very low dioxin emission is expected due to the high temperature of gasification together with the high temperature air combustion of the syngas. The rapid cooling of the combustion gas by the heat storage honeycomb bed for high temperature air pre-heating also serves to reduce dioxin emission levels.
à¢€¢ Ash components are converted into a benign slag where the air toxins are converted into unleachable products.
à¢€¢ Clean energy is obtainable for areas where emission levels are highly stringent.
At Tokyo Institute of Technology, which is developing and managing the project, a MEET demonstration facility, known as MEET-I is being operated. This facility has a capacity of 200 kg of fuel per day. The main areas of development of the MEET system are the high temperature air blown gasifier, the high temperature air pre-heater and high temperature air combustion. A photograph of the facility is shown in Figure 2.
High temperature air blown gasifier: In a conventional air blown coal IGCC, compressed air at 300-400à‚°C is used, giving rise to a gas with a heating value of around 1000 kcal/Nm3. A low gasification temperature is required to obtain syngas with high calorific value, but the use of low temperatures is not suitable for the extraction of slag in molten conditions.
The use of high temperature air makes it possible to obtain syngas with a comparatively high calorific value, even from low-grade solid fuel.
Oxygen blown coal gasification produces syngas with a calorific value of about 2500 kcal/Nm3, but the air separation unit requires auxiliary power and poses additional costs. Thus the high temperature air based gasification gives rise to a higher net power efficiency than if oxygen is used.
A new gasifier concept which combines an entrained bed and fixed pebble bed has been developed and is included in the MEET-I system. The pebble bed is composed of high purity alumina pebbles with a diameter of 50-100 mm.
Volatile matter generated from solid fuels under a high temperature environment will be gasified rapidly in the entrained bed section. Then the pebble bed located below the entrained bed section has the following functions: high efficiency slag capturing; enhancement of complete gasification; and thermal stabilizer against the fluctuation of the thermal input of wastes.
A US bituminous coal with a heating value of 6500 kcal/kg has been successfully gasified in the 200 kg/day MEET-I gasifier using pre-heated air at 800à‚°C. A bed temperature of 1400à‚°C was recorded under atmospheric pressure. The syngas generated had a calorific value of 1000 kcal/Nm3 at a cold gas efficiency of 70 per cent. Complete gasification with carbon content in the molten slag of less than 0.1 per cent has been achieved under a gas residence time of less than 0.5 s.
Smooth flowing slag has been collected from the gasifier with a capture efficiency of 90 per cent. At the end of the test, the pebbles were found to be free from slag adherence and were easily discharged from the bed. A leachability test of RDF slag has met the Japanese regulation levels. Dioxin emission of 0.1ng-TEQ/Nm3 has been measured, which is well below the Japanese regulations for a small size furnace.
High temperature air pre-heater: This is the workhorse of the unique process. The operating principle is shown in Figure 3. It has a pair of heat storage beds, made of honeycombed ceramic material, through which the combustion gas and air pass alternatively, being switched every 15-30 s. The combustion gas at a temperature of 1200à‚°C passes through the bed transferring its enthalpy and as a result is cooled to approximately 150à‚°C. Air at an ambient temperature passes through the bed and is heated up to about 1000à‚°C.
Thus the pre-heated air is divided into two parts, one portion is passed to the gasifier, and the other part is used for the combustion of syngas to heat up the other honeycombed bed. The thermal balance between the beds is achieved by the supply of air at the ambient temperature to the combustion side of the bed. The uniqueness of the pre-heater concept is that the control is carried out in the low temperature sections only (delivered air, syngas and combustion gas exhaust), eliminating the need for a high temperature valving system. Thus the air pre-heater can easily be pressurized.
Extensive tests with the pre-heater have demonstrated the high level of pre-heat (1000à‚°C), minimal leakage and steady mass flow rate during switching on/off of each honeycomb.
High temperature air combustion boiler: This is based on the concept shown in Figure 4. Flue gas at about 1200à‚°C passes through a honeycombed bed, where it is cooled down to 150à‚°C as explained earlier. The thermal energy stored in the honeycombed bed is used for air pre-heating. A traditional boiler consists of radiative and convective heat transfer zones, where an air pre-heater and economizer are used to recover the flue gas enthalpy. In this concept, the boiler consists of a radiant zone only, where the adiabatic flame temperature is considerably high, due to the use of highly pre-heated air.
The radiative heat transfer dominates the heat transfer mode. With this feature, it is possible to design a boiler without the convective heat transfer section, and yet maintain the same thermal output. Elimination of the convective heat transfer region leads to a significant reduction of the overall boiler size and hence cost, even allowing for the higher cost of the boiler tube to withstand the higher temperature. The proof of concept has been demonstrated by firing with natural gas. The heat flux uniformity and the heat transfer rate were measured at various flow rates. These were found to correlate well with the calculated values.
In addition to the use of natural gas, low heating value fuels from 10 000 to 2800 kcal/Nm3 were fired, when stable combustion was achieved. Its capability of burning low heating value fuels makes it suitable for the combustion of byproduct gases from industrial processes or combustion of volatile organic compounds (VOCs).
Based on these significant achievements shown by the MEET-I facility, there are plans to install a second demonstration plant to be known as MEET-II. This facility will have a total capacity of 4 tonnes of solid fuel per day.
The MEET-II plant will be erected in Yokohama, Japan and is planned to be operational in April 2000. An artist`s rendering is shown in Figure 5. This plant will be open to the public internationally, so anyone with interest in the MEET system can join the demonstration operation of the MEET-II facility.
Figure 1. The uniqueness of MEET is its capability for the gasification of any kind of solid fuel
Figure 3. Concept of high temperature air pre-heater
Figure 4. Concept of the high temperature air combustion boiler
Figure 5. The MEET-II demonstration plant planned for Yokohama – an artist`s rendering