Solid Waste Gasification and Energy Utilization - American Chemical

22 Solid Waste Gasification and Energy Utilization Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: August 29, 1980 | doi: 10.1021/bk-1980-0130.ch022

FRANKLIN G. RINKER Midland-Ross Corporation, Thermal Systems Technical Center, P.O. Box 985, Toledo, OH 43696

Until recently, solid waste has been considered only in the context of a disposal problem. With the threat of possible fuel curtailments and subsequent price increases, the energy content of solid waste is being recognized as a viable energy resource. The U. S. Bureau of Mines has forecast that the gross energy consumption will double by the year 2000 to approximately 193.4 quadrillion BTU's. Of this, approximately 41.2% will be consumed by industrial users. In many cases, these same industrial users are generating waste materials containing a significant amount of recoverable energy. Gasification of hydrocarbon bearing materials is not new. Gasification of coal and wood was practiced from the mid-19th century until the advent of large natural gas distribution systems. The gasification of solid wastes can be thought of as an extension of this existing technology, using updated materials, process control, and end use combustion techniques. In my discussion, I will attempt to answer the following questions concerning solid waste gasification: 1. What is gasification? 2. Why gasify solid waste? 3. How is gasification accomplished? 4. What is the present industrial application? 5. What is the future of solid waste gasification? What is Gasification? Technically speaking, gasification is the controlled thermal decomposition of organic material, producing a gaseous fuel and an inert solid ash residue. This controlled thermal processing can be accomplished through two types of reactions: 1. Pyrolysis of the solid waste is generally a low temperature endothermic reaction yielding a fuel rich offgas and solid residue containing the inert gas and fixed carbon fractions of the original material. 2. Complete gasification consists of the pyrolyzer reaction and an additional exothermic reaction of the fixed car0-8412-0565-5/80/47-130-291$05.00/0 © 1980 American Chemical Society In Thermal Conversion of Solid Wastes and Biomass; Jones, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

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THERMAL CONVERSION OF SOLID WASTES AND

BIOMASS

Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: August 29, 1980 | doi: 10.1021/bk-1980-0130.ch022

bon with oxygen. The r e s u l t a n t products of t h i s process a r e a f u e l r i c h gas and a s o l i d i n e r t ash r e s i d u e . P y r o l y s i s has the advantage of not r e q u i r i n g an o x i d i z i n g agent such as a i r ; thus the offgas has a high heating value and, i n some cases, l i q u i d hydrocarbon f r a c t i o n s can be separated and stored. The process i s normally c a r r i e d out under r e l a t i v e l y low temperatures (800-1200°F), s i m p l i f y i n g equipment design and mat e r i a l handling requirements. The main disadvantage of p y r o l y s i s i s that a s i g n i f i c a n t p o r t i o n of the chemical energy contained i n the o r i g i n a l m a t e r i a l e x i t s the p y r o l y s i s process as s o l i d char. The u t i l i z a t i o n of t h i s char f o r end use heating a p p l i c a t i o n s i n v o l v e s s p e c i a l combustion and handling equipment. I t i s , theref o r e , u s u a l l y l a n d f i l l e d as char with a r e s u l t i n g l o s s i n i t s h e a t i n g value. Why

G a s i f y S o l i d Waste?

S o l i d waste g a s i f i c a t i o n o f f e r s the f o l l o w i n g b e n e f i t s over conventional d i s p o s a l means: 1. G a s i f i c a t i o n provides a c a p t i v e energy source i n a time of questionable energy a v a i l a b i l i t y and c o s t . 2. G a s i f i c a t i o n reduces the quantity of waste m a t e r i a l to be l a n d f i l l e d and the a s s o c i a t e d c o s t s . 3. The i n e r t ash m a t e r i a l discharged from the process i s o f t e n more compatible with l a n d i l l r e s t r i c t i o n s than the o r i g i n a l waste. 4. The f u e l gas can be used i n conventional end use systems, such as steam b o i l e r s , process and l i q u i d a i r h e a t e r s , and l a r g e i n d u s t r i a l furnaces. 5. The low process v e l o c i t i e s inherent i n g a s i f i c a t i o n m i n i ~ mize the need f o r offgas p a r t i c u l a t e c l e a n i n g devices r e q u i r e d f o r a i r q u a l i t y assurance. From a f u e l gas generation standpoint, the complete g a s i f i c a t i o n of s o l i d waste i s d e s i r a b l e . P r e v i o u s l y , the high temperatures r e s u l t i n g from the o x i d a t i o n r e a c t i o n of the f i x e d carbon contained i n the o r i g i n a l m a t e r i a l l i m i t e d the a p p l i c a t i o n of gasi f i c a t i o n to w e l l d e f i n e d feedstocks, such as c o a l , where the operating parameters were somewhat w e l l d e f i n e d . We have found that by using steam as an o x i d i z i n g agent i n the g a s i f i c a t i o n process, these r e a c t i o n temperatures can be c o n t r o l l e d , thus widening the a p p l i c a t i o n of g a s i f i c a t i o n to the s o l i d waste area. How

i s G a s i f i c a t i o n Accomplished?

G a s i f i c a t i o n of s o l i d waste can be accomplished i n a v a r i e t y of furnace designs. At Midland-Ross, we have s e l e c t e d the v e r t i c a l s h a f t furnace p r i m a r i l y because of i t s uncomplicated m a t e r i a l transport m e t h o d — g r a v i t y — a n d i t s e f f i c i e n t counterflow of mat e r i a l and process gas. This furnace i s shown on Figure #1.

In Thermal Conversion of Solid Wastes and Biomass; Jones, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


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Solid Waste Gasification and Energy Use

FUEL GAS TO PROCESS

Figure 1.

Solid waste gasifier


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294

THERMAL CONVERSION OF SOLID WASTES AND BIOMASS

M a t e r i a l enters the top of the furnace through a charging hopper and metering mechanism and f a l l s through to the top of the m a t e r i a l bed. I t then moves down slowly through the four zones of r e a c t i o n and i s discharged f o r d i s p o s a l at the bottom as ash. At the same time, a i r and steam are admitted at the bottom of the shaft i n a manner designed to promote uniform d i s t r i ^ b u t i o n of these gases throughout the c r o s s - s e c t i o n of the s h a f t furnace. These gases then t r a v e l upward through the ash c o o l i n g zone, c o o l i n g the ash and p i c k i n g up super heat as they t r a v e l . As the a i r and steam enter the char r e a c t i o n zone, two d i s t i n c t and d i f f e r e n t chemical r e a c t i o n s occur. The oxygen contained i n the a i r r e a c t s with the carbon l i b e r a t i n g heat and forming carbon monoxide and carbon d i o x i d e . Concurrently, the steam r e a c t s with the carbon, g i v i n g o f f carbon monoxide and hydrogen. This water gas s h i f t r e a c t i o n i s endothermic and absorbs heat. By c a r e f u l l y c o n t r o l l i n g the balance of these two r e a c t i o n s , the temperature of the char r e a c t i o n zone can be c o n t r o l l e d producing optimum r e a c t i o n . The determination of t h i s balance i s best accomplished by p i l o t s c a l e t e s t i n g on the a c t u a l s o l i d waste to be g a s i f i e d . A f t e r passing through the char r e a c t i o n zone, the steam and a i r have been converted to a high temperature gas stream c o n s i s t i n g of mainly n i t r o g e n , hydrogen, carbon d i o x i d e , and carbon monoxide. This h i g h temperature gas stream then passes through the v o l a t i l i z a t i o n zone where i t t r a n s f e r s t h i s heat to the waste m a t e r i a l and causes a p y r o l y t i c r e a c t i o n , r e l e a s i n g the v o l a t i l e hydrocarbons contained i n the waste m a t e r i a l . These hydrocarbons c o n s i s t mainly of methane and ethane with h e a v i e r f r a c t i o n s o c c u r r i n g at lower r e a c t i o n temperatures. The f i n a l r e a c t i o n i s the forced d r y i n g of the incoming waste m a t e r i a l through the t r a n s f e r of heat from the hot v o l a t i l e gases to the waste by convection. The now f u e l r i c h gas then e x i t s the bed and passes out of the g a s i f i e r f o r end use a p p l i c a tion. What i s the Present I n d u s t r i a l A p p l i c a t i o n of G a s i f i c a t i o n ? The g a s i f i c a t i o n of i n d u s t r i a l s o l i d wastes i s not u n i v e r s a l in application. The f o l l o w i n g p o i n t s must be considered p r i o r to a p p l i c a t i o n : M a t e r i a l C o n s i d e r a t i o n s . The m a t e r i a l should conform to c e r t a i n c r i t e r i a f o r the s u c c e s s f u l a p p l i c a t i o n of g a s i f i c a t i o n . F i r s t , i t should have a r e l a t i v e l y low moisture content i n the range of 0-50%. The a d d i t i o n of water to the incoming m a t e r i a l causes a decrease i n o v e r a l l process e f f i c i e n c y as the excess moisture must be c a r r i e d through the process and discharged from the process at elevated temperatures. Second, the m a t e r i a l should not melt or s o f t e n when exposed to g a s i f i c a t i o n temperatures. Since these temperatures range


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from 1200-2200°F most p l a s t i c wastes would be e l i m i n a t e d f o r consideration. However, even these m a t e r i a l s can be g a s i f i e d using a p y r o l y t i c process and a d i f f e r e n t furnace design. T h i r d , the m a t e r i a l should have a r e l a t i v e l y high h e a t i n g value somewhere i n the order of 5000-20,000 BTU/lb as r e c e i v e d . Fourth, the m a t e r i a l should be able to be s i z e d f o r proper flow through the s h a f t furnace. S i z i n g s i m p l i f i e s continuous charging of the m a t e r i a l while s t i l l maintaining an atmosphere seal. I d e a l p a r t i c l e s i z e s range from 1/2" to 6" mean dimension. End Use Considerations. The main requirement f o r s u c c e s s f u l end use of energy generated by g a s i f i c a t i o n i s a s u f f i c i e n t and continuous c a p a c i t y to u t i l i z e a l l of the energy produced. The most common end use i s the production o f process steam where the g a s i f i e r f u e l gas i s burned i n the conventional water tube b o i l e r and the steam produced i s f e d to the main plant steamline, r e l i e v i n g the load on f o s s i l f u e l f i r e d b o i l e r s . Other end uses are process a i r heaters f o r l a r g e product d r y e r s , i n d u s t r i a l furnace f i r i n g , and e l e c t r i c a l generation b o i l e r s . Economic Considerations. Economics, of course, w i l l j u s t i f y the technique o f s o l i d waste d i s p o s a l and f u e l gas generation. Some important v a r i a b l e s to be considered i n the economic balance are as f o l l o w s : 1. The q u a n t i t y of s o l i d waste generation should provide a s u f f i c i e n t load f o r maximum u t i l i z a t i o n o f the s i z e g a s i f i e r selected. 2. S o l i d waste generation should be c o n s i s t e n t or s u f f i c i e n t storage c a p a c i t y should be i n s t a l l e d to provide cons i s t e n t l o a d i n g of the g a s i f i c a t i o n system. 3. The present and f u t u r e f u e l cost and a v a i l a b i l i t y must be balanced against the c a p i t a l and operating c o s t . 4. The present and f u t u r e s o l i d waste l a n d f i l l cost and r e s t r i c t i o n s must be considered. Taking the above c o n s i d e r a t i o n s i n t o account along with- the cost of c a p i t a l and r e t u r n on investment goals, the economic j u s t i f i c a t i o n of s o l i d waste g a s i f i c a t i o n can be evaluated. An example of the proper a p p l i c a t i o n of s o l i d waste g a s i f i c a t i o n i s a p r o j e c t Midland-Ross i s p r e s e n t l y c o n s t r u c t i n g f o r a major t o bacco manufacturer. The s o l i d waste c o n s i s t s of c i g a r e t t e papers and f i l t e r s , packaging paper, cardboard, used tobacco c o n t a i n e r s , and transport p a l l e t s . The average heating value of t h i s m a t e r i a l i s 7500 BTU/lb as r e c e i v e d , with an average moisture content of 10%. The waste i s p r e s e n t l y baled and t r a n s p o r t ed to a l a n d f i l l . A c o n s i s t e n t end use heat requirement e x i s t e d i n the form of process steam generation f o r the manufacturing operation. The g a s i f i c a t i o n system s e l e c t e d f o r t h i s a p p l i c a t i o n cons i s t e d of two (2) v e r t i c a l s h a f t g a s i f i e r s , each capable of g a s i -




296

f y i n g 5000 l b s / h r of s o l i d waste, one conventional water tube b o i l e r r a t e d a t 50,000 l b s of steam per hour a t 175 p s i g and equipped with a Surface r i c h fume burner u n i t designed to combust the hot f u e l r i c h gases produced by g a s i f i c a t i o n ; and f i n a l l y , a Surface Rich Fume I n c i n e r a t o r , used as a standby fume d i s p o s a l means i n the event of a b o i l e r shutdown. R e f e r r i n g to Figure #2, the g a s i f i c a t i o n system w i l l be charged with 10,000 l b s / h r of s o l i d waste with the waste flow being e q u a l l y d i s t r i b u t e d to each g a s i f i e r . The steam and a i r mixture w i l l be proportioned to the waste flow to each g a s i f i e r . The r a t i o of t h i s a i r to steam w i l l a l s o be adjusted to provide proper g a s i f i c a t i o n temperatures. The ash discharge r a t e i s adj u s t e d to maintain a constant bed l e v e l at a l l times. The gas generated from the g a s i f i e r w i l l amount to approximately 25,000 l b s / h r with a gross heat content of 68 MM BTU/hr. This gas i s then mixed with an appropriate amount of a i r i n the r i c h fume burner to f u l l y combust the fumes a t an excess a i r l e v e l of 10%. The p r o p o r t i o n of a i r and f u e l i s c o n t r o l l e d to maint a i n t h i s 10% excess a i r r a t e under v a r i o u s waste loadings thus a c h i e v i n g a high b o i l e r e f f i c i e n c y . A l l process v a r i a b l e s are monitored at a c e n t r a l system c o n t r o l panel operated by one man. On weekends, or o f f s h i f t s , when the waste generation i s h a l t e d , the system i s put i n a banked c o n d i t i o n and operating temperatures are maintained u n t i l resumption of waste flow, thus m i n i mizing r e s t a r t time and maintaining system p r o d u c t i v i t y . The operating economics of g a s i f i c a t i o n as compared to a l a n d f i l l o p e r a t i o n break down as f o l l o w s : Gasification Operating annual c o s t s (4000 h r s / y r ) : Manpower $110,000 Fuel 13,800 Power 10,000 Maintenance 60,000 Ash l a n d f i l l 25,000 T o t a l Operating Costs $218,800 Income Steam production @ $4/1000 l b s steam Net Income

$800,000 $581,200

Landfill T o t a l annual cost

$300,000

Net savings through g a s i f i c a t i o n

$881,200

This i s shown g r a p h i c a l l y as a f u n c t i o n of operating hours



STEAM

REACTION A.R

COMBUSTION

loci

m

RICH FUME INCINERATION (STAND BY ONLY)

OXYGEN CONTROL

AIR FLOW CONTROL

Figure 2. Solid waste gasification system

10,000 #/HR

WASTE FLOW




298

1600

Figure 3.

Income generation curve


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per year on F i g u r e 3. As seen i n t h i s f i g u r e , the energy u t i l i z a t i o n provides the bulk of operating savings, although the l a n d f i l l volume r e d u c t i o n of 30:1 and weight r e d u c t i o n of 15:1 prov i d e a p p r e c i a b l e savings.


What i s the Future of S o l i d Waste G a s i f i c a t i o n ? During the next decade, a t Midland-Ross we expect s o l i d waste g a s i f i c a t i o n to grow t e c h n i c a l l y i n two areas: waste product a p p l i c a t i o n and f u e l product end use. In our p i l o t s c a l e g a s i f i e r we have demonstrated the s u c c e s s f u l g a s i f i c a t i o n of many indust r i a l waste m a t e r i a l s , from rubber t i r e s to f o r e s t r y waste to waste paper to pharmaceutical sewage sludge. Each a p p l i c a t i o n has i t s unique o p e r a t i n g parameters to d e f i n e . As more a p p l i c a t i o n s a r e i n v e s t i g a t e d , t h i s parameter development w i l l become less tedious. F u e l product end use i s p r e s e n t l y r e s t r i c t e d to "same time" energy users such as steam b o i l e r s , process a i r and l i q u i d heate r s , and l a r g e furnaces. Midland-Ross, through i t s Thermal Systems T e c h n i c a l Center, i s p r e s e n t l y i n v e s t i g a t i n g and developing processes to s t o r e t h i s energy i n a r e a d i l y t r a n s p o r t a b l e and usable form. This would be e i t h e r as a concentrated h i g h BTU gas or an o i l - l i k e product. This development, when i t comes, w i l l enable i n d u s t r y to generate i t s own f u e l and s t o r e i t f o r end use as economic or a v a i l a b i l i t y f a c t o r s d i c t a t e . In summary, s o l i d waste g a s i f i c a t i o n i s a here and now technology f o r i n d u s t r y ' s p r o f i t a b l e use. Using a process developed over a century ago and a p p l y i n g today's method and m a t e r i a l s , we are able to r e l i e v e the burden placed on our l a n d f i l l s and produce a d d i t i o n a l income through an environmentally acceptable process. RECEIVED November 16, 1979.


Solid Waste Gasification and Energy Utilization - American Chemical

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