Lecture Objectives:

• Finish boilers and furnaces

• Start with thermal storage systems

Thermal NOx

- Oxidation of atmospheric N2 at high temperatures

- Formation of thermal NOx is at higher temperature

Fuel NOx- Oxidation of nitrogen compounds contained in the fuel

Formation of CO- Incomplete Combustion

- Dissociation of CO2 at high temperature

NOON 222

2221 NOONO

221

2 OCOCO

Formation of NOx and CO in Combustion

Coal-fired power plant filtersHigher the temperature of combustion mean more NOx

•Chemistry for NOx reduction:

• Large boilers typically use chemistry that produce N2 and H2O. Example is addition of Ammonia (NH3)

• Use of catalysts for NOx reduction:

•Often in combination with NH3

Coal-fired power plant filters• For Particulate Maters

• Electrostatic precipitator

• Filter bags

• Scrubber for SO2 (to prevent formation of Sulfuric acid – H2SO4)• Grinded Limestone in water (slurry) sprayed into the gas fluid

stream SO2 + Limestone slurry → Gypsum (used for wallboards)

What happened with other waist ?

Bottom ashcaptured at the electrostatic precipitators

Fly ashcaptured at the electrostatic precipitators

~55 % Ash dump

~45 % reused - substitute for cement - soil and waste stabilization - asphalt - bricks - …..

http://www.acaa-usa.org/Portals/9/Files/PDFs/revisedFINAL2012CCPSurveyReport.pdf

http://www.epa.gov/radiation/tenorm/coalandcoalash.html

Oil or Gas based boilers

Gas circulate through tubes water is in-between

Water tube boiler

Furnaces

For homes

Roof tops and induct heaters

Fuel combustion - Stoichiometry

• Boiler efficiency as a function of excessive air• Stoichiometry

• Chemistry of reactants, products and energy in chemical reactions– A stoichiometric ratio of a reagent is the optimum amount or ratio where,

assuming that the reaction proceeds to completion:

Stoichiometric combustion

+Q combustion

Depends on the fuel: - 5 to10% for natural gas - ~ 40 for coal

Air Pollutants from Combustion

Air-Fuel Ratio

- Rich mixture

- more fuel than necessary

(AF) mixture < (AF)stoich

- Lean mixture

- more air than necessary

(AF) mixture > (AF)stoich

Most combustion systems

operate under lean conditions!

However, lean mixture

results in Nox products!

Stoichiometric air/fuel ratio for selected gases

Energy densities of fuels

Higher heating value (HHV) vs. Lower heating value (HHV)

• HHV is the heat of combustion of the fuel when the water product is at liquid state (water vapor from the product are condensed)

• LHV is the heat of combustion of the fuel when the combustion product contain water vapor

For methane ~10% difference!

Condensing vs. noncondensing boilers

Example is for a small residential gas powered boiler - wall mount

fan coils, or baseboard hearts

Condensing vs. noncondensing boilers

~86%(depends on fuel)

Boiler Efficiency Definitions

• ASHRAE Standard 90.1-2004 describes the minimum acceptable ratings for new boilers

• Combustion Efficiency % = ((Fuel Input – Stack Losses) / Fuel Input) x 100

• Thermal Efficiency % = (Output / Input) x 100

• Annual or Seasonal Efficiency

…..

Boiler and Furnace Efficiency Definitions

Example (for large coal based boilers)

Typical boiler and furnace efficiency (based on the higher heating value)

• Condensing boilers

manufacturers claim up to 98% (be careful with this number; check for which conditions)

• Older conventional boilers 70%-80%

• Typical new models around 90%

• New gas furnaces are in the rage of 80-90%

• These numbers are for well maintained and tuned boilers & furnaces. Also, Seasonal Efficiency can be significantly smaller!

Thermal storage for adjustment production to consumption

We need a thermal storage somewhere in this system !

Thermal storage

• Store heat• Many issues (∆T, pressure, losses,…. )

• Store cooling energy• Chilled water

• For cooling condenser

• For use in AHU (cooling coils)

• Ice storage • Compact but…

• Other materials (PCMs) that change phase the temperature we need in cooling coils

• Many advantages, but disadvantages too!

On-Peak and Off-Peak Periods

This profile depends on the type of building(s) !

Chilled water tank

Use of stored cooling energy

Store Use

Which one is better ?

Depends on what you want to achieve:-Peak electric power reduction-Capacity reduction-…..

Downsizing the Chiller

• Lower utility costsLower on-peak electrical consumption(kWh)Lower on-peak electrical demand (kW)

• Smaller equipment sizeSmaller chillerSmaller electrical service (A)

• Reduced installed costMay qualify for utility rebates or other incentives

Sizing storage system (use Annual Cooling-Load Profile)

How often you need to use it?What are the cost-benefit curves ? What is the optimum size ?

Ice Storage Tank

Many issues !….As freezing progresses progress the ice becomes thicker and significantly impedes heat transfer

Open Ice Storage Tank

Also issues !…..

Fluid Flow Rate and Freeze ΔT

Impact on Chiller Efficiency