Fuels, calorific efficiency

Make a rough estimate of the cost of steam per ton, produced from a packaged boiler. 10,000 kg per hour of steam are required at 15 bar. Natural gas will be used as the fuel, calorific value 39 MJ/m3. Take the boiler efficiency as 80 per cent. No condensate will be returned to the boiler. 

Calculation of the efficiency of the practical air-breathing fuel cell, based on the fuel calorific value, involves ignoring or not realising that combustion is irreversible, with destruction of the kinetic exergy of high-speed product molecules. 

The authors react Fe with a low-grade, acid, sub-bituminous coal (SBC) slurry to produce carbon dioxide and Fe. The latter ion can then be harnessed as the anode of the cell, with the oxidation reaction Fe to Fe coupled to the reduction of vanadium dioxide at the cathode of the fuel cell. The first primitive device is said to be 7% efficient, but that is based on the fuel calorific value, asserted to be irrational in this book and in (Barclay, 2002). An alternative catalyst is mooted to start the path to improvement. The fate of the used slurry is not discussed. It could be burnt or buried, depending on economics. 

The high hydrogen/carbon ratio of gas means that the quantity of water vapor in the products of combustion is greater than most other fossil fuels. The latent heat of this cannot be released in conventional appliances leading to a low net/gross ratio of calorific value of 90 per cent. (It is normal practice to quote gross CV in Europe net CV is often used. If net CV is quoted, efficiencies of over 190 per cent are possible.)... 

Condensing boilers are now available for both gas- and oil-fired plant, the advantage of these being that the flue gases are further cooled down to below 100°C so that the latent heat available in the flue gas water vapor is recovered. The condensate has to be removed and the boiler capital cost is higher than for conventional plant. However, the boiler plant efficiency is increased to the order of 90 per cent, based upon the fuel gross calorific value. Where the flue gas exit temperatures are in excess of 200° C a further economy can be obtained by the provision of a spray recuperator in the case of gas and flue gas economizers for oil and coal. 

Owing to the differing combustion characteristics and calorific values of the gaseous fuels which are commonly available [natural gas, liquefied petroleum (bottled) gas], slight variations in dimensions, including jet size and aeration controls, are necessary for maximum efficiency it is essential that, unless the burner is of the All Gases type which can be adjusted, the burner should be the one intended for the available gas supply. 

Fuels, especially coals, are ranked according to limits on fixed carbon, volatile matter, and calorific value. Typical gross heating values of some common fuels (based on approximately 80% fuel to steam efficiency) are listed here ... 

Isothermal chemistry in fuel cells. Barclay (2002) wrote a paper which is seminal to this book, and may be downloaded from the author s listed web site. The text and calculations of this paper are reiterated, and paraphrased, extensively in this introduction. Its equations are used in Appendix A. The paper, via an equilibrium diagram, draws attention to isothermal oxidation. The single equilibrium diagram brings out the fact that a fuel cell and an electrolyser which are the thermodynamic inverse of each other need, relative to existing devices, additional components (concentration cells and semi-permeable membranes), so as to operate at reversible equilibrium, and avoid irreversible diffusion as a gas transport mechanism. The equilibrium fuel cell then turns out to be much more efficient than a normal fuel cell. It has a greatly increased Nernst potential difference. In addition the basis of calculation of efficiency obviously cannot be the calorific value of the... 

Shakespeare s fairyland is mirrored in equilibrium thermodynamics all is simplicity and perfection For fuel cells, the gist of such a theory, tackled by Gardiner (1996), but challenged by Appleby (1994), is that the irreversible losses inherent in practical systems must be separated and evaluated. Then a comparison of practical with perfect, via a summation of the losses, leads to a calculated and understandable efficiency. The latter is an underlay to the economics, the final arbiter. The notion that the calorific value of the fuel, as distinct from its much larger chemical exergy, is a basis for performance calculations has been dismissed by Barclay (2002). In the foreword of this book, it is predicted that the novel ideas herein will get over, but rather slowly. But the ideas are not challenged. 

The DWSA installation can be divided into two main parts. The first part consists of an air preheater, fluidised bed reactor, solid fuel dosing vessel with on-line mass determination system and a hot gas cleaning section, containing a cyclone and a ceramic candle filter (Schumacher type). In the fluidised bed reactor the solid fuel is gasified with air to produce a low calorific value (LCV) gas that is cleaned of fly ash and unreacted solid carbonaceous material. Air and also additional nitrogen can be preheated and is introduced into the reactor by four nozzles just above the distributor plate. The reactor is electrically heated in order to maintain a constant temperature over bed as well as freeboard section. The solid fuel is fed into the bed section in the bottom part Just above the distributor by a screw feeder from beside. The hot gas cleaning section ensures a good gas-solid separation efficiency, with filter temperatures of about 500 C. 

From these measurements, combustion efficiency and potential heal recovery were calculated by comparison with the net calorific value (kJ/kg fuel as fired) of the fuel as described bellow ... 

Just as with other furnaces, fuel control is based on the boiler ten erature. This means that the thermal output momentarily used is measured, and on the basis of this thermal output and a supposed calorific value combustion efficiency, the required quantity of combustibles is calculated by an approximate method and finally set. On the basis of the temperature curve of Ae boiler, this calculated quantity of fuel is permanently adjusted. 

Pyrolysis of agricultural residue was experimentally assessed as a fuel production process for farm applications. A rotary kiln (3.4 m by 0.165 m I.D.) was used due to its ease of operation, commercial availability, low operating costs and ease of start-up and shutdown. Ground oat straw and corn stover at less than 10% moisture were pyrolysed in an indirectly fired continuous-flow rotary kiln located at the University of Sherbrooke. The principle products were char and gas, less than 1% of the feed mass was converted to tar. Calorific values were about 17 MJ/kg for the feed, 26 MJ/kg for the char, and 12 MJ/m3 for the gas. Calculations indicate that the thermal efficiency of a self-sustaining process would be around 65%. 

Because the function of a fuel is to produce heat, the calorific or heating value (ASTM D-240, IP 12) is one of the important fuel properties and a knowledge of this value is necessary in obtaining information regarding the combustion efficiency and performance of all types of oil-burning equipment. 

The calorific value (heat of combustion) is an important property, particularly for the petroleum products that are used for burning, heating, or similar usage. Knowledge of this value is essential when considering the thermal efficiency of equipment for producing either power or heat. Heat of combustion per unit of mass of coke is a critical property of coke intended for use as a fuel. 

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