Co-firing Biomass with Coal for Power Generation

Co-firing Biomass with Coalfor Power Generation Suthum Patumsawad Department of Mechanical Engineering King Mongkut’s Institute of Technology North Bangkok Thailand Fourth Biomass-Asia Workshop “Biomass: Sources of Renewable Bioenergy and Biomaterial” 20-22 November 2007, Grand BlueWave Hotel Shah Alam, Malaysia.

Presentation overview • Introduction • Co-firing: concept and technology • Drivers and barriers • Lessons learned • Conclusions

Power Generation Coal Used extensively to generate electricity and process heat for industrial applications. Poses significant world environmental problems: global warming (CO2) acid gases (NOx and SO2)

Power Generation Biomass: as a fuel source. Steadily increasing • Biomass fuels are CO2-neutral, hence reduce global warming effects. • The sulphur and nitrogen contents are often lower.

Biomass characteristics • Lower density • Higher moisture content, often up to 50% • Lower calorific value • Broader size distribution, unless pre-conditioned by screening, crushing or pelletising • The variability of the material as a fuel will be greater

Biomass characteristics • Such variations in fuel quality, compared to coal, may have a number of implications for plant applications that include process design and operation, and potentially, plant availability.

The features of a small (10 MWe) power plant • Large investment cost/MWe of electricity • Dependent on biomass availability • Technical issues have to be considered in design: erosion and corrosion, slagging and fouling of heating surfaces. • Lower plant efficiency than in large plant (scale effects)

Co-firing Definition: simultaneous combustion of different fuels in the same boiler. Objective: to achieve emission reductions. This is • not only accomplished by replacing fossil fuel with biomass, • but also as a result of the interaction of fuel reactants of different origin, e.g. biomass and coal.

Emission reduction

Attitude to co-firing • One regards coal as the problem (carbon dioxide). • The other attitude sees coal as the solution (more stable combustion characteristics). But both attitudes are environmentally sound.

Co-firing: merits • Some biomass fuels can be grown on redundant agricultural or set-aside land, improving local economics and creating jobs. • Increased plant flexibility in terms of fuels utilised. • Improved plant economics through the use of zero/low cost fuel feedstocks.

Co-firing: merits • Fuel feedstocks may be available locally, reducing transport costs. • Replacement of part of the coal feed can reduce dependence on imported fuels and help maintain strategic national reserves of coal. • Reduced emissions of main classes of pollutants through reduction in amount of coal burnt. This can occur through simple dilution or via synergistic reactions between biomass feedstocks and coal.

Co-firing: merits • Several types of combustion and gasification technology may be applicable to a particular combination of feedstocks. these may include pulverised fuel, bubbling fluidised bed combustion and circulating fluidised bed combustion.

Co-firing: demerits • Feedstock pre-preparation may be required. For instance, wood requires chipping, straw may require chopping up, etc. resulting in increased energy requirements. • Some biomass materials have low bulk density (e.g. straw), this resulting in the handling and storage of large quantities of materials. • Moisture content may be high, reducing overall plant efficiency. • Depending on the feedstock, the complexity of fuel feeding requirements may be increased; some materials can be co-fed using a single feed system whereas others require a separate, dedicated system.

Biomass co-firing (technology) • Direct co-combustion in coal fired power plant • Indirect co-combustion with pre-gasification • Indirect co-combustion in gas-fired power plant • Parallel co-combustion (steam side coupling)

Direct co-firing

Direct co-firing of biomass Two methods were developed: • Blending the biomass and coal in the fuel handling system and feeding blend to the boiler • Separate fuel handling and separate special burners for the biomass, and thus no impact to the conventional coal delivery system

Biomass co-firing via pre-gasification(Indirect)

Indirect co-firing for gas-fired boilers

Parallel co-combustion(steam-side coupling)

Drivers of co-firing biomass • Reduces the emissions of greenhouse gases and other pollutants • Co-firing in coal plants would strongly increase biomass use • Lowest capital cost option for increasing the use of biomass to produce electricity • Co-firing biomass and coal takes advantage of the high efficiencies obtainable in large coal-fired power plants • Improves combustion due to the biomass higher volatile content • Jobs creation

Technical barriers • Thermal behavior and efficiency • Fouling and corrosion of the boiler (alkalis, chlorine) • Environmental constraints - emissions

Lessons learned

The combustion penalties involved in co-firing less than 20 th% biomass with coal are relative slight, verging on the non-existent. efficiency

Slagging and fouling can be reduced with appropriate fuel blending.

Co-firing provides means for emissions reduction • Reducing NOx emissions • Biomass blending decreases SO2 emissions • Trace organic compounds • Particulates

Concluding remarks Co-firing represents a cost effective, short term option at a large scale Although biomass co-firing technologies can already be considered as proven, there is a continuous demand for equipment with: • Lower investment and operational cost • Increased fuel flexibility • Lower emissions • Increased reliability and efficiency

Thank you

Co-firing Biomass with Coal for Power Generation