Syngas, the turbine driver of the future!


Into this largely synthetic world comes synthetic gas, syngas, as the latest answer to clean power generation.
Syngas consists primarily of carbon monoxide and hydrogen and has less than half the energy density of natural gas. It is combustible and often used as a fuel source or as an intermediate in the production of other chemicals. It contains small amounts of particulates and various other elements including sulphur which must be removed before use. This is not only to ensure a clean exhaust but to avoid damage to the gas turbine itself.
For use as a fuel, it is most often produced by gasification of carbon-based feedstocks such as coal or municipal waste. Gasification is a process that converts carbonaceous materials, such as coal, petroleum, petroleum coke or biomass, into carbon monoxide and hydrogen. It was originally developed in the 1800s to produce town gas for lighting and cooking. This was replaced by natural gas and electricity but the gasification process is still used for the production of synthetic chemicals and fuels. It could also be used to produce methane and hydrogen for fuel cells.
The breakdown of hydrocarbons into syngas is made by controlling the amount of oxygen present while heating the hydrocarbons to extreme temperatures, around 1600?C. This melts the inert material, which flows to the bottom of the gasification vessel where it is cooled into a glass-like non-leachable inert slag. This slag is used primarily as aggregate in road gravel or concrete applications.
Syngas is most efficiently used in a combined cycle power plant where the waste heat from the gas turbine is used to make steam to generate additional electricity via a steam turbine, increasing the overall efficiency of power generation. An IGCC plant, integrated gasification combined cycle, is a combined cycle plant that is supplemented by a front-end coal gasification plant for generation of syngas.
In a gasifier, the carbonaceous material undergoes three processes: pyrolysis, which releases volatiles and char, resulting in up to 70 per cent weight loss for coal; combustion, as the volatile products and some of the char reacts with oxygen to form carbon dioxide and carbon monoxide; and gasification, where the char reacts with carbon dioxide and steam to produce carbon monoxide and hydrogen.
Pyrolysis is a process of producing fuels by heating feedstock in an oxygen-deficient atmosphere to a very high temperature where the atoms vibrate apart at random positions, so the process is one of thermal decomposition, not combustion.
The resulting gas is called producer gas or syngas and may be more efficiently converted to energy such as electricity than would be possible by direct combustion of the original fuel. Also, corrosive ash elements such as chloride and potassium may be refined out by the gasification process, allowing high temperature combustion of the gas from otherwise problematic fuels.
Gasifier types
Several types of gasifier are currently available for commercial use: the up-draft gasifier consists of a fixed bed of carbonaceous fuel through which the gasification agent, steam and air or oxygen flows counter to the fuel flow; the down-draft version is similar but the gasification agent gas flows co-currently, or downwards, with the fuel; the fluid bed gasifier, where the fuel is fluidised in oxygen, or air, and steam; and entrained flow gasifiers, which remove the major part of the ash as a slag as the operating temperature is well above the ash fusion temperature.
In practice, an IGCC plant comprises an air separation unit (ASU), a gasification plant, a gas clean-up system and a combined cycle power plant. The ASU separates air into its component parts and sends the gasifier a stream of pure oxygen. After gasification, the syngas is then piped through environmental control processes where pollutants and particulates are removed. Then it can be cleanly burned in a turbine.
The integration of the gasification plant with a combined cycle power plant is the most efficient method currently available to convert solid fuel into electricity. An IGCC plant needs 10 to 20 per cent less fuel than a large-scale standard coal fired power plant and up to 35 per cent less than a small-scale industrial version. IGCC plants also use about 30 per cent less water than a coal fired power plant as gas turbines do not require cooling.
IGCC plants are considerably smaller in physical size and footprint than a standard coal fired power plant. The buildings are much smaller, with the outside facilities consisting of mostly vessels and pipes. The gas turbine exhaust stack is the tallest structure and, depending on the local terrain and dispersion modelling, is typically 100m tall, about half the size of a standard coal fired power plant.
The emissions from IGCC plants are below those of even the most advanced conventional coal power plants. Sulphur scrubbing is in excess of 98 per cent, with 99.9 per cent scrubbing levels reached in certain circumstances. Particulates from the combustion of syngas, both PM10 and PM2.5, are almost non-existent. The primary particulate emissions are from the handling of bulk materials and the movement of people on the plant site.
NOx emissions are also dramatically lower. In a standard coal plant, limits of 25ppm are common whereas a gasification power plant can meet limits of 15ppm without scrubbing and can be reduced to below 5ppm.
Virtually no metals or hazardous air products are emitted and instead are captured as inert slag or as small amounts of inert fly ash. Equipment vendors guarantee 90 per cent mercury scrubbing efficiency and in practice virtually 100 per cent is actually recovered. And future trends call for complete capture of CO2 from power plants with sequestration in underground caverns.
Commercial aspects
For companies wishing to move from natural gas to syngas, Foster Wheeler’s Global Power Group points out that the degree of integration between the gas turbine and the air separation unit is an important design aspect that can bring substantial benefits in performance efficiency and capital outlay. But the best degree of integration is strongly dependent on the characteristics of the selected gas turbine frame.
In the case of total integration, 100 per cent of the air required by the ASU is supplied by bleeding some of the air, generally not more than 15 per cent of total flow, from the discharge of the gas turbine compressor. Depending on the gas turbine frame this air is available at 10–15bar, so the air separation plant has to be a high-pressure type, delivering oxygen and nitrogen at 3–4bar.
Oxygen is recompressed and used in gasification, while nitrogen is recompressed and reinjected in the syngas to replenish the mass deficit caused by the air bleeding, and, at the same time, reduce N0x formation during combustion by lowering the flame peak temperature.
Alternatively the air separation plant can be stand-alone, or non-integrated. In this case a low-pressure air separation plant is needed, with its own air compressor delivering air to the process at the minimum pressure required to meet the energy demand of the unit. In this case, syngas humidification is generally preferred to nitrogen addition for N0x control, because of the large nitrogen compression energy consumption.
The company also points out that plant designed for natural gas needs significant changes when moving to syngas. Different burners have to be installed while the greater fuel flow of syngas can overload the turbine blades.
Foster Wheeler’s Global Power Group offers advanced, cost-effective and environmentally friendly energy solutions to customers around the world. It is expanding its manufacturing facility in China to meet increased demand for steam generators and replacement pressure parts at substantial savings.
Coal potential
In a pointer to the future, Siemens is acquiring a Swiss company, Sustec-Group, for its expertise in converting coal to electricity. This acquisition also brings a 50 per cent stake in a joint venture in China, a country heavily committed in the long term to coal as a fuel, supported by its vast coal reserves.
Siemens also plans to build a large-scale, 1000MW, coal gasification plant in Saxony. The syngas produced will be used for power generation and production of roughly 600 000 tons of methanol per year. It is scheduled to start commercial production about three years from now. The company says an attractive growth potential is now emerging for advanced coal gasification technology.
Currently, GE is one of the world’s leading providers of both gasification and power generation technologies. It provides long-term license agreements that include the gasification process design package, process and instrumentation. There are over 60 commercial sites worldwide using GE gasification technologies.
The company also provides software packages to improve plant efficiency. GateCycle predicts design and off-design performance of combined cycle plants, fossil boiler plants, cogeneration systems, combined heat-and-power plants, advanced gas turbine cycles and many other energy systems.
EfficiencyMap is a performance monitoring and optimisation system, designed for use with simple cycle gas turbines, combined cycles, conventional fossil steam plants and the steam cycle of nuclear plants. It helps plants operate more efficiently.
As the trend to coal continues, it closes a circle that opened with the world's discovery of the benefits of coal to manufacturing and the start of the Industrial Revolution. But this time these dark satanic mills will be much brighter.