‘FutureGen’ project - to design, build and operate a nearly emission-free coal-based electricity and hydrogen:

‘FutureGen’ project - to design, build and operate a nearly emission-free coal-based electricity and hydrogen:

The clean coal technology field is moving very rapidly in the direction of coal gasification with a second stage so as to produce a concentrated and pressurised carbon dioxide stream followed by its separation and geological storage. At present the high cost of carbon capture and storage renders the option uneconomic. But a lot of work is being done by many of the research institutes, to improve the economic viability of this system.

More recently department of energy (DOE) of Federal Govt. of the USA has announced ‘FutureGen’ project to design, build and operate a nearly emission-free coal-based electricity and hydrogen production plant. It will use cutting-edge technologies to generate electricity while capturing and permanently storing carbon dioxide deep beneath the earth. The integration of these technologies is what makes FutureGen unique. Researchers and industry have made great progress advancing technologies for coal gasification, electricity generation, emissions control, carbon dioxide capture and storage, and hydrogen production. But these technologies have yet to be put together and tested at a single plant - an essential step for technical and commercial viability.

Therefore, the FutureGen initiative would have comprised a coal gasification plant with additional water-shift reactor, to produce hydrogen and carbon dioxide. About one million tones of CO₂ would then be separated by membrane technology and sequestered geologically. The hydrogen would have been be burned in a power generating plant and in fuel cells. The project was designed to validate the technical feasibility and economic viability of near-zero emission coal-based generation. Construction of FutureGen was due to start in 2009, for operation in 2012.

Coal gasification processes –

(a) In conventional plants coal, often pulverised, is burned with excess air (to give complete combustion), resulting in very dilute carbon dioxide at the rate of 800 to 1200 g/kWh.

(b) Gasification converts the coal to burnable gas with the maximum amount of potential energy from the coal being in the gas.

(c) In Integrated Gasification Combined Cycle (IGCC) the first gasification step is pyrolysis, from 400°C up, where the coal in the absence of oxygen rapidly gives carbon-rich char and hydrogen-rich volatiles.

(d) In the second step the char is gasified from 700°C up to yield gas, leaving ash. With oxygen feed, the gas is not diluted with nitrogen.

(e) The key reactions today are C + O₂ to CO, and the water gas reaction: C + H₂O (steam) to CO & H₂ - syngas, which reaction is endothermic.

(f) In gasification, including that using oxygen, the O₂ supply is much less than required for full combustion, so as to yield CO and H₂.

(g) The hydrogen has a heat value of 121 MJ/kg - about five times that of the coal, so it is a very energy-dense fuel.

(h) However, the air separation plant to produce oxygen consumes up to 20% of the gross power of the whole IGCC plant system.

(i) This syngas can then be burned in a gas turbine, the exhaust gas from which can then be used to raise steam for a steam turbine, hence the "combined cycle" in IGCC.

(j) To achieve a much fuller clean coal technology in the future, the water-shift reaction will become a key part of the process so that:

(i) C + O₂ gives CO, and

(ii) C + H₂O gives CO & H₂, then the

(iii) CO + H₂O gives CO₂ & H₂ (the water-shift reaction).

(k) The products are then concentrated CO₂ which can be captured, and hydrogen. (There is also some hydrogen from the coal pyrolysis), which is the final fuel for the gas turbine.

(k) Overall thermal efficiency for oxygen-blown coal gasification, including carbon dioxide capture and sequestration, is about 73%.

(l) Using the hydrogen in a gas turbine for electricity generation is efficient, so the overall system has long-term potential to achieve an efficiency of up to 60%.

Environment-friendly Hydrogen Fuel Cell, its challenges and its efficiency:

Environment-friendly Hydrogen Fuel Cell, its challenges and its efficiency:

Fuel cell is an electrochemical energy conversion device, wherein the electricity is directly produced by chemical reaction of fuel and an oxidizer. A fuel cell does not require recharging. The supply of fuel and oxidizer are to be continued as long as it is in operation. A fuel cell essentially consists of an anode—to which fuel such as hydrogen, ammonia etc., is supplied—and a cathode—to which an oxidant, commonly air or oxygen, is supplied. The two electrodes of a fuel cell are separated by a membrane of ionic conductor electrolyte.

A fuel cell is very similar to a battery in that it makes use of the energy stored in chemical substances. However, unlike the battery, in the fuel cell both the high-energy reactants and the low-energy products are not stored inside the cell. The reactants are supplied from outside the cell continuously and the products are removed from it once formed.

Probably the best-known type of fuel cell is hydrogen fuel cell that used in spacecraft. These cells react hydrogen and oxygen, forming water as they do so. The energy transferred from the hydrogen and oxygen in this process is collected as electrical energy rather than as heat. Fuel cells are compact and clean; the water produced by the fuel cells on the space laboratory used for drinking and washing purposes.

The hydrogen fuel cell is structured like a sandwich. At its core is a thin plastic foil - the proton exchange membrane (PEM), which is coated on both sides with a thin catalyst layer, preferably platinum (Pt), and a gas/permeable electrode out of a graphite paper. In the outer layers, gas channels have been milled into the two so-called bipolar flow field plates (FFP). Hydrogen flows through the channels on one side, while oxygen through those on the other side. Upon contact of the hydrogen with the Pt-catalyst, it causes the H-atom to ionize (to decompose into a proton H⁺ and an electron, e^-). The positively charged protons permeate through the PEM membrane whereas the negative electrons do not. As a result of this diffusion process a voltage difference between the two electrodes out of graphite paper ensues. Attaching electrodes to the oppositely situated PME layers and connecting them via an external electrical load, the electrical gradient causing the electrons to flow through it, drive this load (e.g. DC motor). While the electrons are externally routed to the other side of the PME membrane, they join the oxygen, giving it a negative charge and finally merge with the protons that migrated directly through the foil. As a result of this electrochemical reaction, pure water (H2O) and a small amount of heat are formed.

Hydrogen and fuel cells have the potential to solve several major challenges facing the world today, i.e., increasing dependence on petroleum, poor air quality, greenhouse gas emissions and global warming. Many of the research programs are working the ways to accelerate the development and successful market introduction of these new exciting technologies.

Now the key challenges for commercialization of fuel cell and hydrogen infrastructure technologies include

(a) Fuel Cell Cost and Durability,

(b) Hydrogen Storage,

(c) Hydrogen Production and Delivery.

(d) Public Acceptance.

The efficiency of a fuel is dependent on the amount of power drawn from it. Drawing more power means drawing more current, which increases the losses in the fuel cell. As a general rule, the more power (current) drawn, the lower the efficiency. Most losses manifest themselves as a voltage drop in the cell, so the efficiency of a cell is almost proportional to its voltage. For this reason, it is common to show graphs of voltage versus current (so-called polarization curves) for fuel cells. A typical cell running at 0.7 V has an efficiency of about 50%, meaning that 50% of the energy content of the hydrogen is converted into electrical energy; the remaining 50% will be converted into heat. (Depending on the fuel cell system design, some fuel might leave the system unreacted, constituting an additional loss.)

For a hydrogen cell operating at standard conditions with no reactant leaks, the efficiency is equal to the cell voltage divided by 1.48 V, based on the enthalpy, or heating value, of the reaction. For the same cell, the second law efficiency is equal to cell voltage divided by 1.23 V. (This voltage varies with fuel used, and quality and temperature of the cell.) The difference between these number represents the difference between the reaction's enthalpy and Gibbs free energy. This difference always appears as heat, along with any losses in electrical conversion efficiency.

Fuel cells are not constrained by the maximum Carnot cycle efficiency as combustion engines are, because they do not operate with a thermal cycle. At times, this is misrepresented when fuel cells are stated to be exempt from the laws of thermodynamics. Instead, it can be described that the "limitations imposed by the second law of thermodynamics on the operation of fuel cells are much less severe than the limitations imposed on conventional energy conversion systems". Consequently, they can have very high efficiencies in converting chemical energy to electrical energy, especially when they are operated at low power density, and using pure hydrogen and oxygen as reactants.

In practice, for a fuel cell operated on air (rather than bottled oxygen), losses due to the air supply system must also be taken into account. This refers to the pressurization of the air and adding moisture to it. This reduces the efficiency significantly and brings it near to the efficiency of a compression ignition engine. Furthermore fuel cells have lower efficiencies at higher loads.

It is also important to take losses due to production, transportation, and storage into account. Fuel cell vehicles running on compressed hydrogen may have a power-plant-to-wheel efficiency of 22% if the hydrogen is stored as high-pressure gas, and 17% if it is stored as liquid hydrogen. Moreover, Fuel cells cannot store energy like a battery, but in some applications, such as stand-alone power plants based on discontinuous sources such as solar or wind power, they are combined with electrolyzers and storage systems to form an energy storage system. The overall efficiency (electricity to hydrogen and back to electricity) of such plants is between 30 and 50%, depending on conditions. While a much cheaper lead-acid battery might return about 90%, the electrolyzer/fuel cell system can store indefinite quantities of hydrogen, and is therefore better suited for long-term storage.