Energy, fuels, and chemicals 3193 



estimated at 12 percent with 40 percent excess air and 500°F stack gases. After 

 the gases have passed through the steam-generating heat exchangers, heat can be 

 salvaged from the gases by passing them through a second set of heat exchangers 

 called economizers. Economizers can be used to preheat boiler feed water. 

 Some of the stack gases can also be used to predry incoming fuel and to preheat 

 incoming combustion air. 



The overall conversion efficiency for conventional steam generation of elec- 

 tricity from wood is about 25 percent (Benemann 1978; Love and Overend 

 1978). This estimate includes boiler efficiency, steam cycle efficiency, and 

 auxiliary power requirements, all of which can vary. The amount of pine-site 

 hardwood required to generate a kilowatt-hour (Kwhr) can be calculated as 

 follows. For green pine-site hardwood of 42 percent moisture content (wet 

 basis), the available heat is 4,540 Btu/lb (table 26-5). As 1 Kwhr = 3,412 Btu, 



3,412 Btu 



1 Kwhr = = 3 lb green pine-site hardwood 



4,540 Btu/lb (0.25) 



The amount of wood required to fuel electrical generating plants, assuming 25 

 percent conversion efficiency, is as follows: (1 megawatt, Mw = 1,000 

 kilowatts): 



Electrical generating Amount of green pine-site 



plant size hardwood required for fuel 



5 Mw 7.5 ton/hr 



25 Mw 37.5 ton/hr 



250 Mw 375 ton/hr 



500 Mw 750 ton/hr 



Co-generation of steam and electricity from a boiler plant is becoming an 

 accepted practice in the forest products industry (Pingrey and Waggoner 1978). 

 It is not a new process, but interest in it has been renewed in recent years. In the 

 1920's, co-generation was common in New England and on the West Coast as it 

 was the only way to obtain electricity for mill operations. As electricity from 

 utilities became available at low cost, most of the electrical generation systems 

 were shut down. 



Cogeneration of steam and electricity is economically most advantageous in 

 plants having a large demand for process steam and also a need (or market) for 

 electrical energy. One method of cogeneration finding favor utilizes a back- 

 pressure type of turbo-generator. Steam is passed through the turbo generator to 

 produce electricity, and the exhaust provides process steam for the mill. Since 

 there is no cooling water or cooling tower, the turbo-generator takes only the 

 amount of energy out of the steam necessary to generate electricity sufficient for 

 the load, and the remaining energy is in the process steam. Exhaust gage 

 pressures commonly vary between 25 and 200 psi depending on the usage. 

 Under ideal conditions where demand for electricity and process steam are in the 

 proper balance, no condenser is needed on the turbine and overall efficiency of 

 the system may approach or exceed 60 percent. This is a significant improve- 

 ment from the 25 percent overall efficiency commonly observed in plants pro- 

 ducing only electricity. 



