Energy and the Environment

Energy and the Environment Fall 2014 Instructor: Xiaodong Chu Email：chuxd@sdu.edu.cn Office Tel.: 81696127, 13573122659

Fossil-Fueled Power Plants: Introduction • Fossil-fueled electric power plants worldwide consume more than half of the annual supply of fossil fuel, of which most is in the form of coal • Fossil-fueled power plants are major contributors to the anthropogenic emission （人为排放）of CO2 and other pollutants, such as SO2, NOx, products of incomplete combustion, and particulate matter • Fossil-fueled power plants work on the principle of a heat or combustion engine, converting fossil fuel chemical energy first into mechanical energy, then into electrical energy • Most large-scale power plants use the Rankine steam cycle providing the base load for a regional grid in combination with nuclear plants • Peak loads are sometimes supplied by gas turbine plants that work on the Brayton cycle

Fossil-Fueled Power Plants: Introduction • The relatively low thermal efficiency of power plants is due to two factors • The first is a consequence of the second law of thermodynamics, whereby in a heat engine cycle, after performing useful work, the residual of the fuel heat needs to be rejected to a cold reservoir, usually a surface water or the atmosphere via a cooling tower • The second factor is due to parasitic heat losses（寄生热损失）through walls and pipes, frictional losses（摩擦损失）, and residual heat escaping（余热逃逸）with the flue gas （烟气）into the atmosphere

Fossil-Fueled Power Plants: Components • Major components of a fossil fueled power plant • Fuel storage and preparation （燃料存储与制备） • Burner（燃烧器） • Boiler（锅炉） • Steam turbine（汽轮机） • Gas turbine（燃气轮机） • Condenser（冷凝器，凝汽器） • Cooling tower（冷却塔） • Generator（发电机） • Emission control（排放控制）

Fossil-Fueled Power Plants: Components • Fuel storage and preparation • Coal-fired power plants • Coal is delivered to a power plant by rail or, in the case of coastal or riverine plants, by ship or barge（驳船） • A power plant usually has several weeks of coal supply on site and some power plants are situated right near coal mines, mine-mouth plants（坑口电厂） • When coal arrives, it is usually carried by a train consisting of wagons that are emptied by a rotary dump（旋转翻车机）, and the coal is carried by conveyors（输送机） to a stockpil（堆场）, or directly to the power plant • Coal is delivered to a plant already sized to meet the feed size of the pulverizing mill（磨煤机）, in the order of a few to ten centimeters per coal lump • The pulverizing mill reduces the coal lumps to particles smaller than 1 millimeter and the pulverized coal is stored in vertical silos（筒仓）whence it is blown pneumatically into theburners（燃烧器）

Fossil-Fueled Power Plants: Components • Fuel storage and preparation • Oil-fired power plants • Oil is stored in large tanks, to which oil is delivered either by pipeline, by railroad tankers, or by tanker ship or barge • Power plants like to have at least a 30-day supply of oil in their tanks • The oil is purchased from refineries in the form it is combusted in the burners, with specified sulfur, nitrogen, and ash content as well as other properties, such as viscosity（粘度）and vapor pressure（蒸气压）

Fossil-Fueled Power Plants: Components • Fuel storage and preparation • Natural gas-fired power plants • Gas is delivered to the power plant by pipeline at high pressure (compressed natural gas, CNG) • Some gas-fired power plants use liquefied natural gas (LNG) • The LNG is stored in refrigerated tanks（冷冻罐）until used

Fossil-Fueled Power Plants: Components • Burner • The role of the burner is to provide a thorough mixing of the fuel and air so that the fuel is completely burned • In the combustion chamber a pulverized coal particle（煤粉颗粒）or atomized oil droplet（雾化油滴）burns in a fraction of a second, leaving behind incombustible mineral matter that is called ash • In modern pulverized coal and atomized oil fired power plants, more than 90% of the mineral matter forms the so-called fly ash（飞灰）, which is blown out of the boiler by forced or natural draft and is later captured in particle collectors（除尘器） • About 10% of the mineral matter falls to the bottom of the boiler as bottom ash（底灰） • Coal burns relatively slowly, oil burns faster, and gas burns the fastest • For complete combustion, excess air is delivered—that is, more air than is required by a stoichiometric balance of fuel and the oxygen content of air

Fossil-Fueled Power Plants: Components • Burner • Pulverized coal burner

Fossil-Fueled Power Plants: Components • Boiler • Most modern boilers are of the water wall type, in which the boiler walls are almost entirely constructed of vertical tubes that either carry feed water into the boiler or carry steam out of the boiler • In a modern water wall boiler the furnace and the various compartments of the boiler are fully integrated

Fossil-Fueled Power Plants: Components water wall

Fossil-Fueled Power Plants: Components • Boiler • Water wall boiler（水冷壁锅炉）

Fossil-Fueled Power Plants: Components • Boiler • Water wall boiler • Water from the high pressure feed water heater（给水加热器）at a temperature of 230–260oC is further heated in the economizer（省煤器） section of the boiler to 315oC, then flows into the steam drum（汽包）, which is mounted on top of the boiler • In the steam drum liquid water is separated from the steam, and liquid water flows down the downcomer tubes（下降管）into the header（联箱）, where the hot pressurized water flows upward through the riser tubes（上升管）, where the actual boiling of water into steam occurs • The separated steam passes the superheater（过热器）, where its temperature is raised to 565 oC at a pressure of 24 Mpa, higher than the critical temperature (Tc= 374 oC) and pressure (pc = 22 MPa) of water • The supercritical steam（超临界蒸汽）drives the high-pressure turbine • The exhaust steam（排汽）from the high-pressure turbine flows through the reheater（再热器）, where the temperature is raised again to about 500 oC at a pressure of 3.7 Mpa, driving the low-pressure turbine

Fossil-Fueled Power Plants: Components • Boiler • Water wall boiler • Near the burners, heat is transferred from the combustion gases to the boiler tubes by radiation（辐射）and away from the burners, heat is transferred mainly by convection（对流） • Most of the radiative transfer of heat from all flames occurs in the nonvisible infrared portion of the spectrum

Fossil-Fueled Power Plants: Components steam drum

Fossil-Fueled Power Plants: Components • Steam turbine • The steam turbine is the most complex piece of machinery in the power plant • The steam turbine must face problems of the high pressure and temperature of the steam, the enormous centrifugal stresses（离心应力）on the shaft, and the fact that steam condenses to water while expanding in the turbine (creating a two phase fluid flow)

Fossil-Fueled Power Plants: Components • Steam turbine • Impulse turbine（冲动式汽轮机） • In an impulse turbine a jet of steam impinges on the blades of a turbine • Steam coming from the superheater, when expanded through a nozzle（喷嘴）will have a linear velocity of about 1650 m/s • To utilize the full kinetic energy of the steam, the blade velocity should be about 820 m/s, generating unsustainable centrifugal stresses in the rotor • Turbines usually employ compounding or staging, where two or more rows of moving blades (rotors) are separated by rows of stationary blades (stators) and each pair of stator and rotor blades is called a stage（级） • When the steam kinetic energy is divided among n stages, the linear blade velocity of the rotors will be 1/2n that of a single rotor

Fossil-Fueled Power Plants: Components • Steam turbine • Impulse turbine

Fossil-Fueled Power Plants: Components • Steam turbine • Impulse turbine • The force exerted on a rotor • The power generated by the blade • The maximum power

Fossil-Fueled Power Plants: Components • Steam turbine • Reaction turbine（反动式汽轮机） • A reaction turbine consists of rows of moving (rotor) and stationary (stator) blades • Within the converging blades the steam pressure, density and temperature decline while converting its enthalpy to kinetic energy • The steam pressure drops steadily through all rows of blades, moving and stationary, but the steam velocity oscillates, depending on location within the blade

Fossil-Fueled Power Plants: Components • Steam turbine • Reaction turbine

Fossil-Fueled Power Plants: Components • Steam turbine • Reaction turbine • The optimum blade velocity • The maximum power

Fossil-Fueled Power Plants: Components • Steam turbine • Impulse turbines are usually used for high-pressure steam, and reaction turbines are usually used for intermediate and low-pressure steam • In both the impulse and reaction turbines, efficiency losses are due to supersaturation, fluid friction, leakage, and heattransfer losses • The combined efficiency losses and auxiliary power amount to 10–20%; that is, turbines convert only 80–90% of the available steam enthalpy into mechanical energy that drives the generator

Energy and the Environment