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thermochemical technologies > combustion > Flashcards

combustion Flashcards

1
Q

what is the chemistry of combustion?

A

CHxOy (biomass) + o2 (21% of air) +H2O = ch4 + co +co2 +H2 +H2O (unreacted steam) + char + tar

2C+O2 = 2CO (partial oxidation)

C+O2=CO2 (complete oxidation reaction)

C+2H2 = CH4 (hydrogasification reaction)

CO+H2O = CO2 + H2 (water gas shift reaction)

CH4+ H2o = co +3h2 (steam reforming reaction)

C+CO2 = 2co (Boudourd reaction)

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2
Q

learn these units

A 
Energy input: waste (MJ/kg)
Energy loss (MJ/kg)
- residual carbon 
- heat loss due to temperature difference of residue 
- heat loss due to radiation

Net energy output: Energy input - energy loss

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3
Q

what is the estimated solid waste management costs by disposal method, world markets 2012?

A 
WTE incineration 
- above 50%
lower middle income: 40-100$
upper middle income: 60-150$
high income: 65-150$
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4
Q

advantages of incineration

A 
reduction of volume (85-95%)
reduction of mass (70-85% the mass leaves through the stack)
Reduction of hygienic risks
energy recovery 
can be located close to the waste source
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5
Q

disadvantages of incineration

A

costly
air pollution
destroys resourcs that otherwise could be recovered

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6
Q

development of waste incineration plants

A

1st generation- 1950-1965 (no heat recovery, no flue gas cleaning)

  • major objectives: reduction of volume of waste and maximum burning
  • development of furnaces waste burning (mainly grates)
  • usually lack of heat utilization
  • lack of flue gas cleaning

2nd generation (1960-1975) (primary flue gas cleaning with heat recovery)

  • dedusting of flue gas
  • utilization of waste heat (heat utilization boilers)

3rd generation (1975-1990) (further flue gas cleaning, improvement of energy recovery)

  • reduction of gaseous pollutant emissions (mainly sulfur, chlorine and fluor compounds)
  • reductions of heavy metals
  • problems with safety storage of solid residues
  • improvement of waste heat utilization

4th generation (1990) further flue gas cleaning, safe application/ disposal of ash) to safety storage:

  • cement blocks
  • vitrification
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7
Q

what are some examples of incineration plants?

A

The spittelau incineration plant in Vienna, Austria

the marchwood silver dome Southampton UK
WtE facility in Brescia, Italy

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8
Q

No matter how good the design and public consultation to public image of incineration in the UK is poor!

A

an example in the poolbeg incineration- south London

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9
Q

what is public opposition based on?

A
  • potential human health and environmental risks from emissions and ash
  • whether recycling is being supported to the maximum extent possible
  • lack of early public involvement in decision making
  • facility reliability and the quality of operator training
  • impacts of property values and traffic patterns
  • contentions that sites are sometimes selected to avoid middle and higher income neighbours that have sufficient resources to fight such development.
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10
Q

environment agency 2003 statement

A

the weight of evidence from studies so far indicates that present day practice for managing solid municipal waste has, at most, a minor effect on human health and the environment, particularly, when compared to other everyday activities.

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11
Q

incineration in western Europe today

A
  • in mid 2013, approximately 520 WtE plants were operational in Europe
  • 95million tonnes of MSW and commercial waste per year
  • EfW supplied: electricity for 8million households and heat for 15.3million households
  • over the past five years, the European wte capacity grew by an annual treatment capacity of 19million tonnes (24%)
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12
Q

what are the combustion and incinerator stypes

A

mass burning of heterogenous waste

  • waste incinerators (moving grate system)
  • rotary kilns (e.g. cement production)

burning of homogenized waste

  • fluidized bed incinerators
  • co-combustion in power plants/ cement kilns
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13
Q

what is pretreatment of MSW prior to combustion

A

incineration of unprocessed waste is convenient and simple, but can lead to problems

separate:
- recyclables (plastics, glass, metal)
- hazardous materials (pesticides, cleaners)
- non-combustibles (concrete, stone)

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14
Q

advantages of RDF

A
  • simplifies handling of feed into incinerator
  • ensures a more homogenous feedstock
  • can include step to reduce water content, thereby improving heating value
  • can incorporate recovery of recyclables into process
  • incineration options increase (use circulating fluidized bed incinerators)
  • it is claimed that less pollutant production from fluidized bed incinerators
  • more costly than mass burn incinerators
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15
Q

RDF combustion

A

several variants

  • simpliest involve separating marketable material before combustion
  • shredding
  • removal of recyclables
  • possible sterilization, pelletisation, other
  • shredding enables feeding MSW in quasi-fluidized bed state
  • mechanical-biological treatment is usually used for RDF
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16
Q

what does a typical incineration plant include

A 
bunker and feeding system 
combustion system 
energy recovery 
air pollution control 
combustion ash handling
  • learn diagram for exam
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17
Q

what is waste storage and feeding

A

must maintain enough refuse to run incinerator 24hours/day
minimum 3-4 day storage
refuse loaded into a pit or onto a tipping floor

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18
Q

what is the capacity of a waste bunker delivery area?

A

3-4 days to ensure constant feed of waste over weekends and holiday periods

the bunker control room itself is kept under negative pressure. the air is fed into the furnace

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19
Q

describe the furnace and combustion

A

waste enters through the throat and a hydraulic ram pushes the material onto the grate

ash is removed over a water lock at the other end of the grate

primary combustion air is supplied from below, this air also has a cooling effect on the grate, although there may also be water cooling

typically can handle 35 tonnes of waste per hour

20
Q

what is the grate system?

A
  • grate is divided into a number of individually djustable sections for better control of incineration
  • grate length is determined by burnout criteria
  • grate width determines the capacity
  • good mixing
  • good burnout
  • distribution of primary air
21
Q

grate and furnace

A

during combustion on the grate, the waste must be kept moving

as the waste moves down the grate it heats up releasing flammable gas

at the end of the grate, the waste is burnt out with an organic carbon content <3%

15-25wt% pf the waste feed leaves the plant as bottom ash

22
Q

air consumption for combustion

A

primary air
in the first stage, waste is fed into the primary, or lower, combustion chamber, which is operated with less than the stoichiometric amount of air required for combustion. Combustion air enters the primary chamber from beneath the incinerator hearth (below the burning bed of waste). the air is called primary of underfire air.

secondary air:
in the second stage, excess air is added to the volatile gases formed in the primary chambers to complete combustion. secondary chamber temperatures \re higher than primary chamber

  • total air consumption: Approx, 4,500 m3/t of waste
23
Q

boiler and energy recovery

A

transfer of energy in the waste to water

  • three types of boilers:
  • hot-water (for district heating, 110-160C)
  • low pressure steam (for industry, 120-250C, 20 bar)
  • high pressure steam (for electricity generation, 400C, 40 bar)
24
Q

CHP/ cogeneration

A

integrates the production of usable heat and power (electricity), in one single, highly efficient process

by generating heat and power simultaneously, CHP can reduce carbon emissions by up to 30% compared to the separate means of conventional generation via a boiler and power station

25
Q

energy recovery from flue gas through boiler

A

electricity only: up to 40% recovery, surplus heat is cooled

CHP: 25% electricity + 60-65% heat up to 90%

Flue gas condensation: extra 10% (low district heating return temperature required)

26
Q

boiler efficiency

A

boilers have a primary efficiency of 85%

they are however, operated at lower steam conditions than thoses in power plants
(typically 400C and 40bar) (control corrosion)

as a result the power efficiency is only around 22-25%

after taking into account in-house power consumption export rarely exceeds 21%

27
Q

balance between power and heat

A

uk plants tend not to use heat

in Sweden, plants are designed only for heat recovery and achieve efficiencies of 70%

*Germany 
france
Denmark 
Sweden 
Switzerland

leaders

28
Q

UK example: Sheffield

A

The EfW facility generates electricity to 22,000 homes and 60MW of heat which is supplied to 140 buildings which include Sheffield city hall, an international sports centre and sheffiel theatres. 12,000 tonnes of co2 is saved.

29
Q

other examples please

A

Sheffield
225,000tpa MSW throughput recovery 17MWe (electrical) and 39 MWth (thermal)

Nottingham
160,000tpa MSW throughput recovery 14.4MWE and 44.2MWth

Slough Heat and power
110,000tpa throughput recovering 104MWE and 22MWth.

30
Q

Emission standards

A

Most stringent of all industrial combustion processes

meeting the standard is one of the most important and expensive parts of the process

in japan ranges between on plant throughput, stack height and other factors

31
Q

what are the pollutants in flue gas

A

Air pollution control

  • fly ash (particulates)
  • NOx
  • SOx
  • organics (dioxins, furans) (e.g. PCB/ PCDD/ PCDF)
  • metals - Hg, Cd and Pb, Sb, Cr, Co, Cu, Mn, Hg, Ni, Ti, V

HCL
HF

32
Q

fly and bottom ash

A

bottom: ash produced in furnance, which is discahrges from the moving grate
fly: the ash collection from air pollution system (scrubber, precipitator, baghouse) and boiler, which is from original MSW and contain condensable particles (metals) and organics (PCDD/F) like to sorb to particulates

in Europe, most facilities separately manage the bottom ash and fly ash streams

MSW fly ash is classified as hazardous waste, because of its leaching behaviour related to heavy metals, soluble salts, and sometimes to traces of chlorinated organic compounds

33
Q

standards for emissions from EfW plants

A

EU units mgm3

o2- 11
dust- 10
HCL- 10
HF- 1
SO2- 50
NOx - 200
CO- 50
Hg- 0.05
Cd- 0.05
34
Q

selection of APC system

A 
usually remove pollutants in the following order:
- fly ash 
acid gases
specific contaminants (Hg or PCDD/ F) 
NOx
35
Q

fly ash removal

A

fly ash particles carry most metals condences on their surface. they also contain soot, dioxins, PAHs etc.

removing fly ash generally takes care of heavymetal emissions (except Hg)

four main types of equipment: 
- cyclones
- ESP
fabric or bag house filters
venturi scrubbers
36
Q

function of a cyclone

A

particles collide with the wall and settle down into the hopper - not very good for fine particles

37
Q

ESP

A

an electric field charges the particles which are then attracted to the cathode plates. work well at particles sizes between 0.01 and 100um

38
Q

Fabric filter or bag house

A

flue gases pass through long thin bags

cake forms on the bag surface

bags are then cleaned by pulses of high pressure air

39
Q

removal of acid gases; wet scrubbing

A

Common in central Europe
depend on absorption of gaseous components in the scrubbing liquid
efficiency depends on surface area of liquid droplets

usually 2 stages: acid scrubbing (venturi or spray) followed by alkaline (or neutral) scrubbing (packed tower)

Flue gas temperature reduced

the wet scrubber
NaOH or Ca(OH)2
- very high removal efficiencies for halogen hydrides (HF, HCL, and HBr), for mercury and for SO2.

  • raw gas pollutant concentrations are easily reduced well below the emission standards
    disadvantage: produce waste water which requires treatment
40
Q

removal of acid gases; dry scrubbing

A

dry and semi-dry scrubbing processes are simple and cheaper than wet methods

they are the preferred solution in many plantd across the world

most common adsorbents are limestones, CaCO3, calcium oxide, Cao, of line

Ca(OH)2 is directly injected into the gas duct or into a spray dryer downstream of the boiler. this can be done in dry form (dry process) or as a slurry (semi dry process)

dry (semi-dry) scrubber
- acid gases absorbed in droplets and react to form salts
Ca(OH)2 + SO2 = CASO4
Ca(OH)2 +2HCl = CaCl2

dropets dry leaving a salt residue
limited by mass transfer of gases into the droplemt

*disadvantage: produce several times of APC residues compared with wet scrubbing

41
Q

Mercuary removal

A

injection of activiated carbon followed by particulate removal

42
Q

Dioxin and furan removal (PCDD/Fs)

A

carbon injection may be done before or after removal of fly ash, depending on the technology used:

  • before: then the carbon is removed in filters together with fly ash
  • after: the carbon is removed in a dedicated filter and may be fed into the furnance for destruction of dioxins
  • dioxin emissions have fallen dramatically as a result
43
Q

NOx removal

A

Two types of NOx removal exists:
Selective non catalytic reduction performed in the furnace (900-1000C) by injection of ammonia or urea;
NH3 +NO +O2 = N2 +H2O
CO(NH2)2 + NO + O2 = N2+CO2+H2O

Selective catalytic reduction
reduction is done in a dedicated tower with a V2 O5 catalyst
NH3+NO+O2=N2+H2O (160-300C)

44
Q

describe monitoring

A

all incinerators are regulated by the EA under the IPPC regulations which is a holistic approach based on the impacts of releases to air, water and land

monitoring of emission from energy to waste plants Is carriers out by the operators subject to independent checking by the EA. Monitoring data is placed on public registers which are open for inspection

  • air emissions from incinerators have dropped between 1996-1990
45
Q

UK- then and now

A

before 1996 the UK operated 40 mass burn incinerators

only 5 had energy recovery

by the 31st November 1996, all but 5 of these incinerators had closed

they could not meet the requirements of the council directives on the prevention and reduction of air pollution from MSW incineration plants

health risks were known and as a result the required standard has been tightened consideranly

work on the next generation of efw plants started and therefore has since between a graudual toughening of standards

46
Q

extra

A

on the 12 july 2000, a new directive in incineration was agreed and implemented over that decade by a stages approach

in 2010 the directive on industrial emissions (IPPC 2010)) was implemented and consolidated a number of previous directives. This was phased into UK law by 2014

thermochemical technologies (7 decks)

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