Power Plant Description:

Michigan City Generating Station is located on Lake Michigan in northwestern Indiana, approximately 30 miles from Gary.  Michigan City is a one-boiler facility, owned and operated by Northern Indiana Public Service Company (NIPSCO), now NiSource. 

The #12 boiler is a 469 MWe (net) cyclone coal boiler manufactured in 1974 by Babcock & Wilcox with a steaming capacity of 3,250,000 lb/hr. of 3500 psig/1000oFsuperheat/1000oF reheat.  The station typically burns a blend of 60% Black Thunder (Powder River Basin - PRB) and 40% Shoshone coal.  At times the boiler burns a blend of 60% Caballo Rojo (PRB) and 40% Shoshone coal.

Project Description/Objectives:

This test program involved burning over 1,000 tons of urban wood waste and kiln dried sawdust in a blend with the main fuel supply.

The objective was to determine the fuel handling needs and the boiler combustion effects of cofiring with urban wood waste in order to utilize cofiring with wood as a means to generate renewable power and reduce CO₂ emissions at the station.

The impacts on long term storage of the fuel was assessed as well as the effects of cofiring on the boiler capacity, the stability of boiler operations, the effects on boiler efficiency, the temperature profile of the unit, and the formation of airborne emissions (NOx, SO2, and CO2).

Project Design:

The overall system design, developed by Foster Wheeler, was to receive kiln dried sawdust and urban wood waste from outside sources.  The urban wood waste (including broken pallets and other clean wood materials) was pre-chipped offsite to minus 1/2”.  Both fuels were then delivered to the station by walking floor trailer.  The wood waste was then screened with a shaker screen.  The oversized material was rescreened and added to the wood fuel used in the blend.

The sized wood fuel was then blended with Shoshone coal on a one-to-one volumetric ratio and fed into the plant normally with the main coal.  This yielded a 20% wood and 80% coal blend by mass.

Project History/Status:

The wood waste was received, sized and blended with Shoshone coal in May, 1997.

The blended pile was stored until the combustion tests were conducted in September, 1997 which consisted of three baseline tests and six cofiring tests carried out over a five day period.

Results/Conclusions:

The fuel receiving, processing and storage of the blend over an extended period of time, demonstrated the fact that wood waste could be stored outdoors for extended periods of time and successfully handled without spontaneous combustion or other problems.  

Utilizing a shaker screen for initial screening of the wood is both labor and equipment intensive.  Also, the screen was not efficient.  Much of the minus 1/2” wood wound up in the reject pile and had to be rescreened.  This required a front end loader and a back hoe and operators to rehandle the wood.

The combustion tests were complicated by the difficulty in getting enough PRB coal at the time of the tests.  Three baseline tests were run with a blend of Caballo Rojo and Shoshone coal and six cofiring tests were run with a blend of wood waste, Shoshone coal, and Black Thunder coal (10/40/50% by mass).  The variables during the test were load, fuel blend, and excess O2 at the economizer exit. 

Baseline conditions on the boiler required a reduction in full load conditions to 425 MWe.  “Full load” was determined by economizer exit gas temperature.  Minimum load of 300 MWe was determined by slag tapping characteristics.  Excess O2 was varied from 2.2% to 2.9% and the fuel blend was either baseline fuel or 10% wood by mass blended with coal.

Cofiring 10% by mass (6% by heat) caused a 0.65% loss in boiler efficiency.  The reduced bulk density and lower Btu of the wood also resulted in a significant increase in feeder speeds.  If the boiler had been operating at true full load of 469 MWe, it would have been constrained to about 425 MWe when cofiring wood due to feeder capacity.  Boiler stability was not affected.  Unburned carbon in the flyash, excess O₂ requirement, air heater in-leakage, and air heater exit temperatures were not impacted.

Cofiring did not reduce flame temperature, economizer exit temperature, or air heater exit temperatures but it did reduce the furnace exit gas temperature due to early ignition of the wood in the cyclone barrels rather than in the primary furnace due to the high volatility of the wood.

NOx was reduced approximately 10%.  SO2 and CO2 were reduced by nature of substitution for coal.  The CO2 reductions during the test was about 1,400 tons.  This equates to an estimated annual reduction in CO2 of more than 187,000 tons.

The cofiring tests were very successful but the possibility of plant derating at high loads and the costs of procuring and handling the wood is higher than the costs of burning coal.  The station is not currently cofiring with wood at this time.

Project Partners:

­          Michigan City Station Personnel

­          NIPSCO Corporate R&D

­          EPRI

Contacts:

­          Patty Hus, NIPSCO, 219-647-5263, pjhus@nipsco.com

­          Michigan City Generating Station, Wabash Street @ the Lake, Michigan City, IN 46360,  219-873-7210