Enhanced Power and Heat Generation From Biomass and Municipal Waste

Introduction Generally, the electricity production from aThe combustion system and the fuels The gas turbine
waste-burning plant is quite low, due to the fact that itis equipped with a conventional combustor and 18 fuel
is tricky to burn waste. Waste is a fuel with a “wideinjectors for dual fuel operation. The present fuel is
composition” that includes not only substances thatgood diesel oil with high ash melting point (>900°C) to
require extensive exhaust gas cleanup but alsominimize risk of corrosion of the gas turbine hot parts.
corrosive elements that restrict the temperatures ofThe diesel oil is mixed with water in the fuel injectors
the steam tubes in the boiler. Thus the steam cycle willat a w/f ratio of 0.8 to reduce NOx from around 210
be quite small from a thermodynamic point of viewppmv to 40 ppmv. The addition of water with the fuel
with low inlet conditions (typical temperature ofincreases the turbine output by around 1 MWe, but also
210°C) on the back pressure steam turbine. Bythe stack losses by 3.7 MW. Natural gas will become
adding a gas turbine to the system, using the gasavailable at the plant within a few years, which will not
turbine exhaust steam generator as a superheater foronly reduce the cost for the gas turbine fuel, but also
the low temperature steam from the waste boiler, anprovide more operating hours and longer times
effective combined cycle power plant with a largerbetween overhauls. The present conventional
steam cycle is generated. With this solution, the overallcombustor and dual fuel injectors of the SGT-600 can
transformation of the fuel energy to electricity andbe used with natural gas, but they can also be directly
heat for district heating will be very good. The fuel forexchanged to a Dry Low Emissions (DLE) combustion
the gas turbine will, of course, be dependent onsystem, suitable for natural gas and mixtures of natural
availability. Normally, natural gas is preferable from thegas and biogas. With the DLE burners, the water
point of view of both economic and environmentalinjection can be deleted and the stack losses reduced.
considerations. The fuels Municipal waste consistsThe total efficiency will be increased to around 93%.
largely of recycled material, (gas, metals, newspapers,The NOx level at the gas turbine exhaust will come
plastic bottles and cans etc) but there is a residuedown from 40 ppm to 25 ppm. At the site there is a
consisting of food leftovers, scrap paper, polyethylenebiogas plant producing gas by fermentation of waste
etc. that has to be taken care of. Most of the waste isfrom farming. The gas is used mainly as fuel for
renewable biomass. Burning it in incineration plants, asbusses, cars and trains. In the future, this type of gas
an alternative to simple disposal, is becoming verycould be mixed with the natural gas to fuel the gas
common, especially for large and middle size towns,turbine. The steam turbine The admission data of the
but even in smaller communities with district heatinglow pressure steam turbine are 1.5 MPa/430°C.
systems. The combustion technology and exhaust gasDuring operation, the turbine is coupled directly to the
treatment have improved to make the rather difficultsame generator as the gas turbine, that is, at 3000
combustion of municipal waste environmentallyrpm, Producing 25 MWe electric power. The steam
possible even in small plants. The municipal energyturbine has a “district heating exhaust” which
system often includes the use of some biomass, inmeans that the steam flow is divided in two parts,
special boilers or added to the waste. Biomass in thecondensing at somewhat different pressure levels in
form of wood chips or pellets has become a commonorder to achieve the highest heat recovery. The
fuel for municipal heating in a places, but so far not forWaste Heat Recovery Unit (WHRU) The gas turbine
electricity production. From the combustion point ofexhaust gases are ducted to an unfired boiler
view, biomass and municipal waste are fairly similar inequipped with a superheater, boiler and economizer.
heating value. Both fuels have corrosive elements ofSome steam is raised in the WHRU, but the main flow
similar and different kinds, which prevent high surfaceis the saturated steam from the incineration boiler. The
temperatures in the boiler. The municipal wasteeconomizer is used to preheat the condensate. The
contains some polyethylene, metals etc. that willstack temperature for the oil-fired gas turbine is limited
produce chlorides, ammonia and metal vapors andto 135°C to prevent SO3 condensation and
salts. The wet compounds could prevent goodcorrosion. For a natural gas fired unit, the exhaust
combustion with dioxin as a result. The biomass in thetemperature could be lowered to 85°C, reducing the
form of wood chips also produces ammonia and alkalistack losses. The superheater is divided in two parts in
metal vapors and salts but has a more evenorder to position an ammonia-based Selective
composition and less moisture. The plants WasteCatalytic Reactor (SCR) for NOx reduction at a
incineration or biomass plants are normally just boilerssuitable temperature level. The combination of this
for heat production. The use of the district heatingSCR and the water injection in the gas turbine
system as a heat sink for electricity production willcombustor means that the NOx emission level is very
increase with increasing cost of electricity and with thelow, around 5-7 ppmv at 15% O2 on diesel oil. For a
expansion of CO2 trading. In the plants for electricitygas-fired unit with DLE burners in combination with the
production, steam turbine systems have been included.SCR, NOx-levels as low as 3 ppmv at 15% O2 would
However, due to the corrosiveness of the combustionbe achieved. Heat production only The plant can be
gases, the steam temperature has to be low, in orderoperated for heat production only. All the steam from
to avoid excessively high surface temperatures on thethe boiler is then directly condensed to produce district
boiler tubes. The plants are often built up in stagesheating water with an incoming temperature of 50°C
following the population development and theand an outgoing temperature of 90 to 115°C,
expansion of the district heating system. The stage todepending on the time of the year. The total heat
include electricity production is frequently late in theproduction from the direct condenser and the
process, which means that the steam cycle will noteconomizers in the incineration boiler is around 87MW,
always be optimal but adjusted to fit the existing plantwhich translates into a fuel utilization of 93%. Operation
as well as possible. The conclusion is that the electricitywith the combined cycle The steam from the
production with steam turbines has a rather low alphaincineration boiler is then going to the WHRU of the
ratio (= electricity/heat ratio) at around 0.2.Thegas turbine for superheating before expansion in the
combined cycle a way to substantially boost thesteam turbine. There are two modes of gas turbine
electric power and heat production from a wasteoperation due to a difference in production tax on
incineration or biomass plant is to include a combinedelectricity produced in a “district heating” mode
cycle. The low value steam from the incineration plantor in a “power generation” mode. The power
is then superheated in the waste heat recovery boilergeneration mode ( = gas turbine at full load) is only
of the gas turbine. The steam quality is then improvedprofitable when electricity prices are high, so even in
to suit a rather large back pressure steam turbine withwintertime the plant has most often been operated in
heat condenser. With such a cycle the performancethe district heating mode ( = 60-70% gas turbine
figures are very much improved with an alpha ratio ofpower), producing as much district heating as at the
0.52 and a total efficiency of around 90%. The gas“heat production only” mode. Conclusions ?
turbine fuel The fuel for the gas turbine could be theWaste incineration and biomass-fired plants will be
conventional ones, natural gas or diesel oil. Preferablyused in the future for electricity production. ? The
industrial waste gases (refinery gas, coke oven gasretrofitting of steam turbines to the plants is not very
etc) or, in the future, biomass-based fuels could beeffective due to the low steam data. The power to
used. In Sweden, one plant is fired with diesel oil andheat ratio will be low, around 0.2. ? A way to
another with liquid propane, gasified on site. Plantsubstantially improve the electricity production is to
description In a two incineration plants have beenretrofit with a combined cycle fired with current
retrofitted with combined cycles as described above.conditions on diesel oil or natural gas but in the future
In one of the plants the gas turbine has been closedon biogas. A power to heat ratio of >0.5 can be
down and sold off for two reasons: ? The municipalityreached. An optimized plant could reach a power to
did not expand as expected ? The fuel cost went upheat ratio of =0.6 and a total efficiency of 93%. ? In
more than expected. At the start of the project,the future the fuel to the gas turbine could be gasified
propane was a surplus product from the Norwegian oilbiomass, which would make such a plant fit the
and gas fields, but has later become quite expensive.strategy to operate mainly on renewable energy.