Environmental Safety

Saving Power: The Foundation Stone for Environmental Safety of Heat Power Stations

Vitaly A. Prisyazhniuk

Abstract

Heat (or thermal) power stations (HPS) and power plants are the major polluters of our environment, discharging into the atmosphere the basic product of carbon fuel combustion, CO2. It is this very gas that accounts for the greenhouse effect causing the global climate warm-up on our planet. A natural solution of the problem of reducing carbon dioxide discharge into the atmosphere lies in power saving, thus reducing the amount of the fuel burnt. This approach can be justified from any standpoint, both economically and ecologically. The ideal way of solving the problem would be to completely give up burning carbon-containing fuel, such as coal, petroleum products, and other power resources of organic nature. The article dwells upon various technological ways aimed at practically solving the problem: the Kalina cycle, magneto-hydrodynamic (MHD) resonance, and employing the heat of environment in the thermodynamic cycle of the HPS.

Introduction

Major polluters of main environment are heat power stations (HPS) and power plants. The international conference on ecology held in Kyoto in December 1997, in which 170 countries took part, proposed as a solution of the vital problem of preventing this planet's thermal death to reduce by 5.2 percent (in reference to the level of the year of 1990) the discharge of harmful gases and aerosols, and heat into the atmosphere. It should be added here that coal-fueled HPS, in addition to ashes and smoke, discharge into the environment a complete spectrum of hazardous chemical elements, including uranium and thorium. In the autumn of 1999 the next conference of the kind stated that no positive changes had taken place in the world's environmental situation during the preceding two years, and that the governments of the developed countries had not taken any effective steps to improve the situation. A similar conclusion was arrived at by the International Conference held in 2001.

It is common knowledge that the product of oxidizing carbon ¨C the principal component of any burning fuel of organic nature ¨C is carbon dioxide, or carbonic acid. It looks quite natural trying to solve the problem of reducing the discharge of carbonic acid by heat power stations and power plants through reducing the amount of carbon-containing fuel burnt while retaining or even increasing the amount of electric power produced. Since the ratio of the power produced to the amount of the fuel consumed is conventionally termed as the efficiency, an obvious way to solving the above problem lies through raising the efficiency of HPS and power plants. This approach can be justified from any standpoint, both economically and ecologically. It is especially urgent if we take into consideration the fact that in addition to carbonic acid the HPS discharge into the environment a great assortment of hazardous gases, aerosols, and heat.

Power-Producing Plants: The Major Dischargers Of Dust, Gases, And Heat Into The Environment

History of civilization and the progress in science and technology are closely associated with the growth of power consumption. A direct consequence of the developing heat power engineering based on combustion of carbon-containing fuel and of the growing amount of electric power produced is the increasing consumption of fuel-energy resources (FER).

Output of electric power in the world reached, in 1985, 9643000 §®Wh [1], having resulted in consumption of 751 million tons of coal, 615 million tons of oil, and 686 million tons of gas. It should be noted that annual production of electric power resulted in consumption, on the average, of 2 gigatons of conventional fuel [2].

For instance, the Soviet Union alone in 1990, at the beginning of its disintegration, produced 1726000 MWh of electric power. The structure of the output of electric power at the time looked as follows:

Power consumption is a measure of the evolution progress of human society, but in today’s environmental situation on the globe priority should be given not to the absolute amount of power consumption, but rather to its "quality", that is to the consumption of FER per head of population. Let us compare the levels of FER consumption in 1991 in some countries [3]:

While in 1981 all the fuel-fired plants in the USSR [3] discharged into the atmosphere (million tons): §³§°2 - 200; SO2 - 150; hydrocarbons - 50; aerosols - 250, the share of heat-and-power engineering in said polluting amounted to 50% for SO2, 30-35% for NOx, and 35% for aerosols. Besides, the heat-and-power engineering consumed up to 1% of the atmospheric oxygen, replacing it by CO2, which increased the greenhouse effect.

Taking into account the fact that from 1991 till 2002 the technology of electric power production in the FSU countries had not been improved, there is no reason to believe, that for the past 13 years the environmental characteristics of the operating fuel-fired plants had got improved. And this assertion holds true in relation to all the heat power stations on this planet, which resulted in working out and adapting the Kyoto protocol of 1997. To be more objective in this assertion we shall now turn to the data published by the GAO (General Accounting Office, USA) on June 20, 2002 [4].

Electric power stations of the USA that began operating before 1972 discharged in 2000 59% of the sulfur dioxide, 47% of the nitrogen oxides, and 42% of the carbon dioxide of the total discharge by the fuel-fired plants in 2000, while having produced only 42% of the total electric power.

Let us resort to simple calculations to show the correlation between gaseous discharge from the newer and the old power stations in reference to the unit of their actual capacity. To do so we shall use simple logic: if the older power stations produced 42% of the electric power obtained, the newer ones produced (100 - 42)% = 58%; if the older station discharged into the atmosphere 59% of the SO2, the newer ones discharged (100 - 59)% = 41%. And so on for each of the gaseous discharges. Then at 1% of the power produced by the older stations there fall 59/42 = 1,405% of the SO2, and so on. Let us list the resulting ¡°gaseous discharge ¨C power produced¡± ratios:

It is worth noting that the discharge of SO2 and NOx by the newer stations is lower than by the older ones. Discharge of CO2 is however, comparable, which can be explained by the vary nature of the system employed for producing heat energy. Since it is carbon-containing fuel that is burnt, the product of oxidizing the carbon turns out to be CO2. It can be stated that for the last 30 years there have not been found any radical ways of reducing the discharges of CO2 and heat ¨C the principal causes of the build-up of greenhouse effect on the planet and a global climate warm-up.

Low-potential thermal discharges from HPS by the middle of the 21st century, in comparison with the figures for 1983, will grow 20 to 30 times, proportionally to the growth of electric power production ]5]. Up to 15% of the overall thermal power of HPS is lost with the gases discharged, and up to 52% - with the water-cooling the condenser. Heat power stations, besides, consume 32 to 42 m3/s of water per one GWt of power produced. It is the heat losses that account for the low efficiency of power stations [6], which is as follows

Raising the efficiency of HPS, in combination with steps aimed at reducing solid and gaseous discharges through improving the technology of combustion process, design of furnaces, dust collectors, and gas traps, can make these stations look much more promising and make a real step towards complying with the requirements of the Kyoto protocol.

Raising Efficiency Of Power Plants: Real And Profitable Way To Improve Ecological Situation

Efficiency is interpreted as the ratio of the thermal power W produced by a power plant to the amount of the fuel Gf burnt to produce the power. ¦Ç = W/Gf (1)

Thus, efficiency of a power plant, for example a thermal power station, can be increased by two ways:

• by increasing production of electrical and thermal power (W) without changing the amount of consumed fuel (Gf) [7], the way it is realized at power-producing plants employing a combined cycle: a gas turbine with subsequent utilization of the heat of the discharge gases; or
• by producing a constant amount of power (W) while consuming reduced amount of fuel (Gf) .

Saving Fuel

Possibility of increasing power production at the existing HPS is limited to their installed capacity as represented by the complete set of the equipment installed. Taking into consideration the above limitations, we can modify the equation (1)

(Gf)35/(Gf)(35+i) = ¦Ç(35+i)/¦Ç35. (2)

The subscripts 35, (35+i) mean that, as a base for numerical examples, we take an efficiency of 35%. The equation (2) can be read as follows: a relative decrease in the fuel consumption per unit of the power produced by the station is proportional to a relative increase in its efficiency. So, an increase in the efficiency of an HPS from 35% up to 45% could result in a decrease in fuel consumption of 45/35 = 1.29 times, or by 29%. Taking into account the linear character of the relative decrease in fuel consumption as related to the efficiency, it may be argued that a 1% increase in efficiency within a range of efficiency values of 35% and over will result in a 2.9% decrease in fuel consumption.

Reducing Discharge Of Solids

The mass fraction of solids discharge Dsd, taking place at burning coal, as related to a unit of efficiency can be found from the equation

Dsd = 100*[1 ¨C G§àx/(Hr¡ÁGsd)]/¦Ç, (3)

where G§àx, Gsd represent the weight of the oxidable constituent of the coal, and the weight of the solid fuel, respectively; ¦Ç is the efficiency of the HPS; and Hr is the relative calorific value of the used fuel. If Hr = 1, then 100*[1 ¨C G§àx/Gsd] = Ash,%, which represents the ash content of the solid fuel. It should be mentioned that the calorific value of the equivalent fuel is taken to be 7000 kcal/kg, and the ash content of some coals can reach 50%.

The equation (3) is nonlinear relative to ¦Ç. And with the growth of ¦Ç the absolute value of Dsd is decreased. Let us estimate the decrease in solids discharge with the efficiency increased from 35% up to 45%. (Dsd)35/(Dsd)45 = 45/35 = 1.29. Thus, a 1% increase in efficiency results in reducing discharge of solids, on the average, by 2.9%. For the range of efficiency rise from 40% up to 50% that will make (Dsd)40/(Dsd)50 = 50/40 = 1.25, or 2.5 % for 1% of efficiency rise.

Reducing Gaseous Discharge From Heating Power Stations

As it is known, combustion of fuel is a process of oxidation, which takes place due to the oxygen of the air (23.2% by weight). The product of burning carbon fuel is carbonic acid. The simplest example of the reaction, that takes place at combustion of the oxidable part of coal, is given below

C + O2 ¡ú CO2 ¡ü (4)

It is this chemical reaction that is responsible for the greenhouse effect, threatening to turn the planet Earth into another planet Venus. Besides CO2, flue gases contain SO2 and NOx. The sulphur dioxide results from the sulphur compounds present in natural fuel, and the oxygen of the air. I mention these well-known facts to support once more the main thesis of this paper: the less fuel is burnt, the less harmful gases is discharged into the atmosphere. The nitrogen oxides result from the nitrogen of the air (75.5% by weight) and the high temperature in the boiler furnace, at which these oxides are synthesized. So, discharge of NOx can be drastically reduced just by lowering the temperature in the furnace.

As is clear from the equation (4) and the low of conservation of substance mass, weight of the gaseous discharge from power plants, especially from HPS, exceeds the weight of the carbon fuel burnt. To calculate the weight of carbon dioxide, discharged into the atmosphere when burning 1 ton of fuel, is possible from the equation

CO2, t = -7.784¡Á10-4 + 3.668¡Á Cfl, (5)

here Cfl is the weight of carbon contained in one ton of fuel.

The equation was derived by approximation of the appropriate data from stoichiometric calculations based on the assumption that the reaction (4) yield is 100%. Multiple correlation coefficient of the equation (5) is R2 = 1. There is one more assumption: coal ash content is taken to be 50%.

Let us look at a couple of examples. When burning one ton of coal with ash content of 50%, 1.833 tons of CO2 are formed. When burning one ton of natural gas (we assume that it is 100% methane), 2.75 tons of CO2 are formed. The relative weight of CO2 formed from one ton of carbon contained in the fuel and falling at 1% of efficiency of HPS is:

DCO2/Cfl = (-7.784¡Á10-4+3.668¡ÁCfl)/(Cfl¡Á¦Ç) = 3.668/¦Ç (6)

The number of [-7.784¡Á10-4/(Cfl¡Á¦Ç)] is neglected in view of its insignificant magnitude.