PRELIMINARY SUBSTANTIATION OF ADVANCED DIRECTIONS OF MODERNIZATION OF COAL-FIRED POWER PLANTS

The article considers issues related to the modernization of coal-fired power technologies. To increase the efficiency of energy resources, operational reliability, loss reduction and environmental safety the possibilities of trigeneration are considered with application of fuel cells, hydrogen technologies and RE-components that are expedient to use for additional electric energy production. The possibilities of innovative energy components to integrate them into traditional power generation systems are shown, technological schemes are given.

Improving the energy efficiency of an electric generating facility, using CHPP as an example, can be achieved by increasing the specific supply of electric power through consistent optimization of plant equipment.
The basic principles of evaluating the effectiveness of projects are as follows: 1) the organization is considered as a profit center, and 2) the account of the increase in cash flow from the implementation of the investment project is calculated as the difference of values with and without the implementation of the project. This is necessary to assess the effectiveness of commercial investment projects in the development of business plans, feasibility studies and other documents that require evaluation of the effectiveness of commercial investment projects, which lead to economic effect: to obtain additional income, to reduce costs, to avoid costs or to avoid reducing income. The peculiarity of formation of industrial energy resources in Kazakhstan is that more than 80% of electric energy is generated by thermal power plants. Thermal energy is also produced by industrial heat generating plants of medium and small capacity. In the technological process they use mostly high-ash power coals from the Ekibastuz deposit. These features, combined with the inevitably obsolete power equipment and technologies, mainly of the last century, create an urgent need for modernization of energy facilities in order to significantly reduce energy losses and to bring specific energy consumption in the foreseeable future to the levels of Proposed for consideration of potential transfer the mentioned technologies of cogeneration and trigeneration allow in preliminary estimates to achieve the following results: reduction of energy costs up to 70%; the efficiency of transformation and use of energy may reach up to 93% at place of production of electricity using natural gas; net payback period of the project may usually be from 2 to 5 years; additional savings of 10% to 20% due to heat recovery from existing flows of exhaust gases. Low-temperature fuel cell systems are air-or liquid-cooled. In addition, using a coolant, it is easier to transfer thermal energy in the cogeneration process, both for space heating and water heating.
In high-temperature fuel cell systems such as MCFC and SOFC, the thermal energy of the exhaust gases can be used to preheat the incoming process air. In addition, electricity can be additionally generated by gas-fired microturbines forming a hybrid energy system.
Cold is obtained in the absorption machine, which uses the heat produced in the cogeneration plant. In this case, the heat used in the production of cold can be considered useful under the following conditions. In the production of refrigeration for air conditioning, (5÷7)°C is beneficial: a) all the heat used in simple-action machines, when this heat has a temperature below 120°C; b) all the heat used in double-action machines, when their temperature is below 180°C. In industrial refrigeration production, for cooling down to -50°C, all the heat consumed in absorption machines when the temperature is below 180°C is useful.
Rationale for the possibilities of innovative trigeneration based on PEMFC fuel cells. The development of fuel cell technology creates great opportunities in the field of power generation. In this system the residual heat allows to have hot water with a temperature that normally ranges between 80°C. This temperature is sufficient to run absorption cooling cycles. The fuel cell cooling system showed electrical efficiency results of 42.27% and thermal efficiency of 44.21%. Thus, the total combined fuel efficiency is 86.48%. Figure 1 shows for example a configuration with SOFC technology considering electrical and thermal energy production. This SOFC scheme by Siemens-Westinghouse has shown an electrical efficiency of 43,3%, thermal efficiency: in heating -43,7%, in cooling -52,6%, in hot water production -46,7%. The efficiency results in three operating modes are up to 87.95%, 95.9% and 90%.
Thus, the characteristics in trigeneration processes are considered, which makes it possible to achieve a high level of overall efficiency in the use of coal-fired CHPP fuel through the use of residual heat.
In relation to industrial processes requiring heat, they are classified according to the temperature level of the heat required: low temperature processes, below 100C; medium temperature processes, 100 to 300C; high temperature processes, 300 to 700C.  Advantages: simple design and reliability, no moving parts, lightweight, compact, modular power ramp-up principle; compact system; efficiency similar to alkaline, but with higher current density; very fast response time; high operating pressure; high potential for lower investment costs by implementing the modular principle.
The proton exchange membrane, as an example for hydrogen gas production, has the following parameters. Working pressure potentially up to 300 bar, working temperature -60...80C with the prospect of 130... 180C; flexibility of technological modes is high, the minimum possible working load -0%; reactivity -from stopped state to full load -not more than 10 sec, for the best results -about 1 sec; "black start" time -not more than 10 minutes; hydrogen gas purity -99.9%; system commercial efficiency -77%, potential -84% at 1.0A/cm2; maximum stack size 308 electrolyze both H2O water and CO2 carbon dioxide to produce a mixture of hydrogen H2 and carbon monoxide CO, called "syngas," which can be further processed into synthetic fuel. Another advantage of SOECs is that SOECs can be used both as electrolysers and as fuel cells, creating new technological opportunities to store electricity efficiently and inexpensively in both directions.
In the last decade, there has been renewed interest in electrolyzers using RES technologies to generate electricity.

Solid oxide electrolyzer cells [SOEC]
, high temperature -800°C Energy conversion efficiency approaching 100% at stack level and 90% at system level at current densities up to 1A/cm2. Specific energy-to-hydrogen conversion efficiencies can be as high as 100% at the system level.
The priority is to further reduce investment costs per unit of installed capacity.
It is assumed that modern electrolysers use surplus electricity from low-cost RES, which implies a relatively limited number of operating hours per year. At low utilization rates, investment costs become a more important factor than efficiency.
There are two main opportunities to reduce the capital costs of electrolysers: i) lower production costs per unit cell area and ii) higher current densities.
The effect of electricity price and capital investment depends on the utilization or annual load factor of the electrolyzer plant. If the electrolyzer is only used less than 20% of the time, the most important component becomes the investment cost.
When utilization rates range from 20% to 90%, both capital costs and efficiency have the most significant impact on project economics. Fuel cells and "combustion turbines" can be used for extended use of hydrogen technology as a tool to reduce the consumption of primary fuel at coal-fired thermal power plants like CHP plants. In doing so, thermal energy losses can be recycled in two ways: (1) for heating purposes, as part of the application in the combined heat and power production -CHP plants. It should be noted, however, that heat is very difficult to transport efficiently over long distances, so micro-CHP systems for decentralized applications are a very important part of modern fuel cell systems. The energy efficiency of hydrogen-fueled CHP systems should be as high as 75%; and (2) converted to electricity in a combined cycle power plant to improve electrical SCIENTIFIC COLLECTION «INTERCONF» | № 60 309 efficiency in continuous operation. High-temperature waste heat combined with a large-scale power system is needed to offset the increase in capital costs, limiting the application to gas turbines or high-temperature fuel cells. Fuel cells and combustion turbines have areas of best use and do not compete directly for the same application. Namely: a) Fuel cells give priority to reliability, autonomy and low maintenance requirements in operation. A typical example would be standby power supply systems, uninterruptible power supply systems, in which the most important factor is the reliability and continuity of energy supply. The value of specific capital costs in this case is not critical; and b) Hydrogen H2 turbines will be stationary and capacity of at least 10 MW, due to the reduction of specific costs by increasing the unit capacity. This is positive compared to high-modulus fuel cells.
Fuel cells are grouped into low-temperature and high-temperature categories.
Low-temperature fuel cells. The most promising is the proton exchange membrane [PEM]. The PEM fuel cell has always been the most produced type of fuel cell. It is also suitable for stationary applications and is a popular choice for grid management services because of its reactivity. High-temperature fuel cells are generally more efficient (up to 50%) and well-suited for stationary megawatt-scale cogeneration plants. They are commercially available with a decent lifetime (unlike hightemperature electrolysers), but remain quite expensive to produce. Solid oxide fuel cells are especially promising because they can easily be converted to electrolysers and can be operated using H2, syngas, methane, or methanol.
Gas turbines can also be used to burn hydrogen as a fuel gas. Flexible gas turbines can handle an undifferentiated mixture of H2 and CO with hydrogen content up to 70% by weight. They are commercialized for coal gasification power plants.
Stored hydrogen can be used to generate heat or electricity in a combined heat and power plant using fuel cells or gas turbines.
Hydrogen, which is essentially fuel gas, can also be used in combustion turbines. When hydrogen and oxygen are burned, water and heat are produced. The heated steam is fed into the turbine to produce mechanical energy, which in turn is converted into electricity by the generator. According to estimates of turbine manufacturers, mixing 1-5% in volume will not require any design changes.   hydrogen is 56% with a projection of up to 65%. Heat recirculation is necessary to improve the energy efficiency of stationary fuel cells and turbine H2 units.

Conversion of energy into electricity in a steam turbine. Combining a fuel
cell or combustion turbine with a steam turbine creates a combined cycle power plant. To drive a steam turbine with sufficient efficiency, these plants require higher quality thermal waste with temperatures above 500°C. They must also be large systems, since scaling up the steam turbine capacity provides important economic benefits and efficiencies. The combined energy cycle and conventional CHP technologies can be used together. In addition, there are currently no significant technical barriers to the joint application of these technologies to hydrogen fuel cells or turbines. The fuel cell is only a small part of the plant, both in terms of cost and size. In the last few years, stationary equipment for standby power supply systems has been increasingly used, including for the own needs of coal-fired thermal power plants.
Hydrogen fuel cells for own needs of coal-fired thermal power plants. The fuel cell system that can be used for a stationary power system, for example, coalfired CHPP can be represented as a scheme in Figure 5. Comparative advantages: the highest efficiency among fuel cells; suitable for CHP and combined cycle power; no noble metal catalyst; fuel flexibility; good fuel impurity tolerance; simple system; easy reversibility into electrolyzer. SOEFC fuel accepts not only H2, but also any light hydrocarbons (e.g. methane, propane), CO or methanol as a feedstock. Due to their technical characteristics SOFCs are mainly suitable for large-scale stationary plants or for decentralized CHP plants.
Gas and hydrogen turbines for modernization of coal-fired thermal generation. Gas turbines can be a good alternative to fuel cells for large-scale stationary applications. Minor technical modifications will be required to operate gas turbines using methane or hydrogen, or mixtures such as hydrogen-enriched natural gas [HENG] (CH4+H2) or syngas (H2+CO). Flexible-fuel turbines are already commercially available for use in coal-fired gasification power plants, where the hydrogen content of the mass can vary up to 50%.
Energy production from fossil fuels with carbon dioxide capture. Carbon capture and storage has recently gained increasing interest in the thermal energy industry, mainly as an option to reduce CO2 emissions. If CO2 is captured in the flue gases during combustion and does not enter the atmosphere, it will not contribute to the greenhouse effect. An alternative is to replace the combustion air in the power generation process with pure O2 mixed with recirculated flue gases.
This produces a flue gas consisting only of CO2 and H2O, and the cooling and condensation is sufficient to produce virtually pure CO2. This can provide almost 100% CO2 capture.