A Fundamental Study of Biomass Oxy-fuel Combustion and Co-combustionTimipere S. FarrowProf. Colin Snape: Supervisor

Presentation overviewIntroductionCarbon capture technologiesDetailed oxy-fuel combustion processObjectives

Lab Scale Experimental techniquesThermo gravimetric Analysis (TGA)/Horizontal Tube Furnace (HTF)Drop Tube Furnace (DTF)

ResultsCombustion reactivity of Biomass Fuel under oxy-fuel and air combustionCo-firing sawdust and coal to identify the effect of biomass on coal char burnoutCo-firing in (DTF), the effect at higher temperature combustion

IntroductionThe presence of CO2 and other green house gas emissions in the atmosphere has become more problematic because of their negative environmental impact on climate

Stringent environmental laws on CO emissions from coal combustion. World energy consumption is predicted to rise to 44% and CO emissions to 39% in 2030 [1]

Increased interest in power generation industry towards technologies, which help to reduce CO2 emissions from fossil fuels combustion by means of CO capturing.

1. International energy outlook, 2009

Leading Techniques for CO Capture Biomass co-firing Presents a potential technique

Why Biomass and co-firing?Potential option for renewable based power generation

Unlike fossil fuels, biomass fuel is renewable and CO2-neutral in the sense that the CO2 it only releases recently fixed carbon when combusted thereby closing the carbon loop on a short time

Partial substitution of coal for combustion

In the UK, legislation is strong on CO reduction to meet Kyoto target and EUs target to reduce CO emissions by 20% by 2020.

Hence the combination of oxy-fuel combustion with biomass fuel become a CO2 sink for power plants

Oxy-fuel combustion Process for cleaner fossil fuel utilisationFundamental studies of oxy-fuel coal combustion have demonstrated that oxygen concentrations in the range 30-40% produced temperature profiles matching those of conventional air firing with lower NOx and SOx emissions.

Objectives To investigate the behaviour of biomass under oxy-fuel conditions in comparison to air fired condition in terms of:

Volatile yield

The associated nitrogen partitioning between char and volatiles in order to monitor NOx emissions.

Kinetic parameters which are useful for design of biomass oxy-fuel combustion system.

2. To investigate how biomass will affect coal char burnout during co-firing under oxy-fuel and air firing with particular emphasis on the catalytic effect of biomass-contained alkali and alkaline metals on coal char burnout

Schematic diagram of experimental Approach

Thermo gravimetric analyser (TGA)and horizontal tube furnace (HTF) heating rate of 150C/minTGA Heating rate is miles away from reality yet give fundamental combustion informationTGAHTF, replicates TGA char production

Drop Tube Furnace, High heating rate, short resident times (200-600ms) and 1600CHigh heating rate and high combustion temperatures, close to reality

What Effect does CO have on volatile yield?There is no particle size effect at both conditions except for the smallest particle size at 1100C

The impact of oxy-fuel firing is pronounced at 1100C due to volatile char gasification reaction but low at low temperatures due to Poor thermal conductivity

Why do we need to maximise Nitrogen (N) yield in the volatile phase? Char N contribute to NOx formation

Beneficial to oxy-fuel due to high transformation of N into the gaseous state at high temperature

TGA Combustion reactivity of biomass chars at 375CCO2 does not have effect on the combustion reactivity of the chars at low temperature hence the burnout is identical with air fired condition

Insignificant particle size effect is seen in during burnout in both conditions except for the smallest particle size.

Impact of low char combustion temperature on kinetic parametersVariation is less due to poor thermal conductivity effect of CO2 compared with that of N2

Benefits of Co-firing (TGA Analysis)

Improved burnout of blend but slightly more pronounced under oxy- fuel conditionStrong synergetic effect: an indication of interactions

SamplesNitrogen chars and Air combustion CO chars and 21%O/79% CO combustion1st order rate constants90% burnout time1st order rate constants90% burnout time(min)(min)(min)(min)sawdust char 700C0.49016.600.31147.85Kleinkopje (KK) HTF char 1000C0.073438.700.052648.00saw/KK char blend 50:50wt%)0.100222.150.108920.65Predicted sawKK char blend0.082925.600.072031.60

Moving close to Reality, does biomass char still affect coal char burnout?

Improved coal char combustion, effect of catalytic inorganic metals in biomass fuel

conclusions

High reactivity observed for CO at high temperature due to gasification reaction.

No particle size effect, can use bigger particle size for pulverised biomass fuel combustion systems

Biomass improved coal combustion. There is chemical interaction between the two fuels during co-combustion

Inorganic minerals in biomass catalysed coal char combustion

Completing PhD, what is left to be done

Devolatilisation of sawdust in DTF at different temperatures and different residence times

DTF char burnout

Co-firing at different temperatures and residence times at the two atmospheres

DTF burnout of blend chars

TGA burnout analysis of DTF chars (sawdust and blend chars) in air and oxy-fuel conditions

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