Process and control technologies can reduce dependence on fossil fuels

Oct. 3, 2017
Converting previously coal-fired power plants to biomass not only creates jobs, it's also less expensive than building new ones.

Former Vice President Al Gore likes to point out that in one hour, the sun provides as much energy as mankind uses in a year, or that our total energy consumption can be met by 1/4th of our planet's wind energy. I, too, believe that new technologies will be needed to stop global warming and prevent such consequences as climate-migration, but that will take time.

Today, we need not wait for the arrival of new technologies; we can use technologies that already exist. For example, we can convert coal-burning power plants to burning biomass. Biomass is a general term, referring to all organic matter that originates from plant life. Biomass is a "renewable" fuel because the CO2 released by its combustion would be released anyway by putrefaction if it was just allowed to rot. The only difference is that, when burning biomass, we recover its energy content.

This process is renewable because growing new plants will take up the same amount of CO2 as was released by burning the previous crop. If a forest is managed in a sustainable manner, the process of harvesting and regrowing can be continued indefinitely.

The use of biomass fuel in power plants started earlier in the United States than elsewhere (Figure 1), and today it's increasing worldwide. Most of these plants produce both electricity and heat (cogeneration), and the generated steam or hot water is often used for district heating. Some of the plants also produce gas or liquid fuels, such as ethanol.

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(One might note that the Nobel laurate George Oláh recently invented a methanol production process that removes CO2 from flue gas by injecting with H2, producing methanol (CO2 + 3H2 + CH3OH + H2O). While as yet only one such plant is in operation in Iceland, I believe this process could be applied to more fossil power plants to reduce global warming.)

Larger biomass-fueled plants exist in Copenhagen, Denmark (530 MW), Port Talbots, England (350 MW), Alholmens, Finland (240 MW), Teesside, U.K. (295 MW), Polaniec, Poland (205 MW), Vaasa, Finland (140 MW), etc. They use a variety of fuels, including agricultural byproducts such as baled straw, woodchips, wood pulp (from paper plants), rubbish and others.

The total annual primary energy use of mankind is equivalent to the energy content of about 14,000 million tons of oil (mtoe). According to the International Energy Agency's ( 2016 report (Figure 2), this energy is obtained from the following sources: oil (31.3%), coal/peat/shale (28.6%), natural gas (21.2%), biofuels and waste (10.3%), nuclear power (4.8%), hydro power (2.4%), and solar and wind (1.4%).

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Global electricity consumption is about 15% of the above total, and is equivalent to the energy content of 2,100 mtoe/yr or 24,500 TWh/yr. The fuels that generate this electricity break down as: coal/peat (40.8%), natural gas (21.6%), hydro power (16.4%), nuclear power (10.6%), renewables such as biomass, solar and wind (6.3%), and oil (4.3%).

From the above numbers, we can see that biofuels already represent a significant portion of our total energy consumption (twice that of nuclear), but coal is still drastically greater. This is unfortunate, because coal is the worst fuel in terms of pollution and CO2 emissions—there's no such thing as "clean coal"—while burning biomass doesn't increase the net CO2 release into the atmosphere. In addition, converting previously coal-fired power plants to biomass not only creates jobs, it's also much less expensive than building new plants.

Circulating fluidized-bed boilers

There are two reasons why fluidized-bed combustors (FBC) are good for burning biomass. The first is that they can handle a wide variety of solid fuels, including ones that are difficult to burn in other boilers. The second is the ability of FBCs to achieve low emission of both nitric oxides and sulfur dioxide. Figure 3 illustrates a typical circulating fluidized-bed (CFB) boiler.

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The controls of CFB boilers for burning biomass are similar to coal-burners, but being newer, manual controls are slowly being replaced with automatic ones. The key pollution controls are directed at reducing the emissions of sulfur dioxide (SO2), nitrogen oxides (NOX) and carbon dioxide (CO2), mercury, the volume of voltatile organic compounds (VOC), particulates, stack opacity, and other greenhouse gas pollutants.

Electrostatic precipitators are commonly used to remove particulates, and scrubbers are commonly used to remove both particulates and SO2.

Reduction of NOX emissions is a major goal because of their role in forming ground-level ozone and acid rain. The NOX analyzer designs used in these control systems (XT in Figure 4) include infrared (IR), ultraviolet (UV), chemiluminescent, colorimetric, electrochemical and chromatographic. The features of these devices are covered in Chapter 1.44 of the Analyzer volume in the 5th edition of my Instrument Engineer’s Handbook.

NOX formation depends on the nitrogen content of the fuel and the temperature of the flame because at high flame temperatures, more dissociated nitrogen combines with the oxygen present in the excess air. Therefore, the best way to minimize NOX formation is to reduce flame temperature, reduce excess oxygen, and/or to burn low-nitrogen-containing fuels. Once the NOX is formed, selective catalytic reduction (SCR) can be applied by injecting mixtures of ammonia and air or ammonia and steam into the exhaust gas, which then passes through a catalyst and triggers the NOX reduction reaction:

4NO + 4NH3 + O2 --> 4N2 + 6H2O

The SCR typically consists of an NH3 injection flow control system, an ammonia injection grid, and NOX monitoring equipment both at its inlet and outlet. Therefore, precise control of ammonia injection is critical. One such control configuration is shown in Figure 4, and is explained in detail in Chapter 8.27 of the Control volume in the 4th edition of my handbook.

Similar to NOX, removal of sulfur oxide (SO2) is also a major concern due to the relatively high sulfur content of most biomass fuels. Wet SO2 scrubbing is the most widely used method of flue gas desulfurization (FGD). The most popular sorbent is limestone, which is favored because of its wide availability and relatively low cost. The overall chemical reaction that occurs with limestone or lime sorbents is:

 SO2 + CaCO3 = CaSO3 + CO2

The controls used for removing SO2 are similar to those used for NOx control (Figure 4) and are also explained in detail in the Control volume of the 4th edition of my handbook.

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Combined cycle cogeneration

Biomass-based power plants often serve the dual functions of electricity generation and heat production (usually steam) for both industrial plants and district heating. For these plants, it's important that the biomass fuel be available year-around, which requires either large storage facilities or fuels like woodchips (or garbage), which are available year-around. This means that seasonal energy users, like the heat demand of sugar mills or district heating, are not as desirable as paper mills or tire production plants that operate year-around. The steam users should be near the power plant (100-5,000 ft, 300-1,500 m) to reduce steam transportation and heat losses. Smaller, combined-cycle power plants (up to 60-100 MW) are particularly suitable for cogeneration because they can be located very close to the thermal user or even within the fence of the industrial plant.

A load-following cogeneration control system is shown in Figure 5. It follows the demand for steam in such a way that when the demand of the steam users is below the supply, it sends the excess to the turbine, and when it is above, it supplements it from the turbine.

The steam supply pipe has two PICs in series and a fail closed (FC) control valve (PV-2) between them. The setpoint of PIC-1 is higher than that of PIC-2, which provides the driving force for the steam to pass through the valve. The direct acting (DA) PIC-1 controls the excess steam to the turbine by throttling the control valve (PV-1), while the reverse acting (RA) PIC-2 controls the steam supplement from the turbine by throttling the FC control valve (PV-3). PV-2, between the two controllers, is controlled by the lower of the two controller output signals, whose selection is made by the low signal selector (PY-2).

All three control valves are fail closed (FC), but their throttling ranges differ: PV-1 and PV-3 throttle between 50% and 100%, so they're closed when their control signal drops below 50%, while PV-2 throttles between 0% and 50% and therefore is fully open when its control signal rises to 50%.

When there is no demand for steam, the pressure in the pipe rises, and the output signal of the direct acting PIC-1 rises to its maximum (100%), fully opening PV-1, which sends all the steam to the turbine. At this time, the low signal selector (PY-2) blocks the PIC-1 output, and selects the output of PIC-2 because that output signal is lower (0%), which closes PV-3.

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When some demand for steam starts to develop, the same configuration remains in operation, but as the pipeline pressure drops, PIC-1 starts to close the previously fully open PV-1, while also reducing its signal to PY-2 (down from 100% toward 50%). At the same time, as the pipeline pressure starts dropping, the PIC-2 output signal starts rising. As long as it’s below 50%, PV-2 remains closed and the PIC-2 output continues to control PV-3. So, at this point, the PIC-1 output is dropping, but is still above 50%, while the PIC-2 output is rising, but is still below 50%. This condition is maintained until the demand for steam starts to exceed the supply.

At that point, the steam pressure in the supply pipe starts to drop, which signals that the steam supply to the users needs to be supplemented. As the pressure drops, the PIC-1 output signal also drops, and when it has dropped to 50%, it fully closes PV-1. At the same time, as the pipeline pressure drops, the output signal of PIC-2 rises, and when it has reached 50%, it starts to open PV-2.

At this point, the low signal selector (PY-2) switches the control signal to PV-3 and instead of PIC-2, now PIC-1 starts to control it. This state of operation remains in effect as long as demand exceeds supply, and is reversed back to the previous configuration when the demand drops below the supply. This automatic "load following" increases the efficiency of operation by 10% or more.

About the Author

Béla Lipták | Columnist and Control Consultant

Béla Lipták is an automation and safety consultant and editor of the Instrument and Automation Engineers’ Handbook (IAEH).

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