Solid Wastes and Residues - American Chemical Society

11 A Thermal Process for Energy Recovery from

Downloaded by UNIV OF MASSACHUSETTS AMHERST on June 3, 2018 | https://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0076.ch011

Agricultural Residues RITCHIE D. MIKESELL, DONALD E. GARRETT, and DINH CO HOANG Garrett Energy Research and Engineering Co., Inc., 911 Bryant Place, Ojai, CA 93023

In May, 1976, a one-year exploratory contract, E(04-3)-1241, was l e t by the U. S. Energy Research and Development Administration to Garrett Energy Research and Engineering Co., Inc. to study the thermal decomposition of biomass materials in a bench scale pilot plant of the multiple hearth reactor type. In October, 1977, this contract was extended by DOE for 18 months for operation in a larger, process development unit (PDU) pilot plant. This unit will have a six ton/day, four-foot diameter, four hearth reactor. The program to date has consisted of vacuum drying, direct contact drying, pyrolysis, the char-steam reaction, and combustion experiments. The results of these studies are reported in the following paper. GERE Process The proprietary Garrett Energy Research and Engineering Company biomass process is based upon a multiple-hearth furnace. This type of equipment has been successfully employed for continuous high temperature processing in the chemical and metallurgical industries for over 100 years. There are several vertically stacked compartments with a common central shaft. Rabble teeth are mounted on arms attached to the central shaft whose slow rotation imparts a positive mechanical motion to the solid material on each hearth. Downcomers through which the solid drops onto the hearth below are located alternately near the inner and outer periphery. They may have star valves for good sealing, and the conditions on each hearth may be optimized for i t s desired function. Since the solid is spread in a thin layer with constant raking and tumbling action, the residence time is adjustable, and efficient solid-gas heat- and mass-transfer are achieved. The multiple-hearth furnace is well suited for processing materials that are moist, fibrous, sticky, or susceptible to ash fusion at high temperature. In the GERE process, Figure 1, the raw material may be first chopped and mechanically dewatered if necessary, and then conveyed 0-8412-0434-9/78/47-076-217$05.25/0 © 1978 American Chemical Society Jones and Radding; Solid Wastes and Residues ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

Jones and Radding; Solid Wastes and Residues ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

Figure 1.

Multiple Hearth Reactor

Water

I—wvW*-

—/ΛΛΛΧ

.Manure Feed

GERE multiple hearth pyrolysis process

Ash

F l u e Gas


» Medium BTU Gas P r o d u c t


11.

MIKESELL

E T

AL.

Energy Recovery from Agricultural Residues

219

to the top of the multiple-hearth reactor by means of a jacketed screw conveyor. The material is partially dried under vacuum by moist flue gas flowing countercurrently through the conveyor jacket and screw, and condensing inside. The conveyor has star valves at each end to maintain a good seal. Following the vacuum drying the remaining approximately half the water is removed by direct contact with hot flue gas on the f i r s t , or drying hearth. This water is then partially condensed in the vacuum screw feeder to provide multiple effect economy. Because biomass usually contains a great deal of moisture, i t is very important that this moisture be removed as e f f i c i e n t l y as possible. The biomass, after drying to the desired moisture content, then f a l l s to the pyrolysis hearth. Here i t is heated by hot char from the steam-char reaction, and also by the hot gases accompanying the char. The char is introduced through an external steam l i f t in which synthesis gas is formed as the char reacts with superheated steam. The mixture of this synthesis gas and the gaseous products of pyrolysis (H , FLO, CO, C0 , CH , CnHm, HpS, NHL, COS) are cooled and condensed. The condensed tar is recycled to the pyrolysis zone. Char from the pyrolysis hearth drops onto the combustion hearth where i t is partially burned to generate steam and process heat. The hot flue gas preheats the incoming air in an external heat exchanger, and then goes to the drying section. Some of the hot char is reacted with steam in the steam l i f t and then sent to the pyrolysis zone, while an ash-bleed stream drops to the cleanup burner where i t is completely burned. The ash then drops to the lowest hearth where i t is cooled by incoming air and discharged from the reactor. The gas leaving the condensate collector may either be u t i l ized directly or sent to a scrubber in which hLS, C0 and moisture are removed. This product gas may be piped for use at a nearby industrial plant, or blended into a natural gas pipeline. 2

2

4

2

Experimental Biomass Feed. The GERE pilot plant work is done in Hanford, California, an agricultural community, with steer manure as the f i r s t pilot plant feed. The moisture content of fresh manure is about 80%, although the moisture content of piled manure ranges from about 10 wt_% {]_) to about 50 wt % (2) depending on the climate and length of storage. A typical ultimate analysis of bovine manure is given by Schlesinger, Sanner, and Wolfson (3) in Table I. For the pilot plant work, the moisture content of the feed manure was adjusted to 50 wt %.


220

SOLID

Table I.

A N D RESIDUES

Bovine Manure, Typical Ultimate Analysis 41.2 wt % 5.7 33.3 2.3 0.3 17.2 100.0 wt %

C H 0 Ν S Ash Total dry Downloaded by UNIV OF MASSACHUSETTS AMHERST on June 3, 2018 | https://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0076.ch011

WASTES

Pilot Plant Equipment. During the f i r s t phase of this work a single-hearth reactor was used. Each process--direct contact dry ing, pyrolysis, water gas char gasification, and combustion—was studied in turn in this hearth. Figure 2 shows a schematic of the reactor; detailed description of the equipment is given in Refer ence (4). Laboratory Experimentation. Thermogravimetric analysis of manure pyrolysis showed that over the temperature range 25-700 C, the reaction is slightly exothermic and that the use of NaoCOj had l i t t l e i f any effect on the reaction rate. It was found that flue gas as hot as 180 C could be used to dry manure without burning i t . Manure can be decomposed (pyrolyzed) at a temperature as low as 250 C, though the product gas at this temperature is mostly C0 . If the tar produced in pyrolysis is i t s e l f pyrolyzed, more gas is produced. In particular, the hydrocarbon gas production is en hanced. Manure ash begins to fuse at about 950 C. Manure drying experiments were f i r s t conducted in a laborato ry oven. The data were correlated using the relationship 2

fj=

4.27-M-exp -

2474

(1)

According to Reid, Prausnitz, and Sherwood (5), the exponential term is associated with the moisture d i f f u s i v i t y . Hot a i r was passed through a small, well insulated, fixed bed of manure in order to dry i t . Since the drying rate ought to depend upon the superficial gas velocity to the 0.8 power, and on the moisture diffusivity to the 0.56 power, the data were correlated as Μ'ΰτ

27·

G 150.8 V

•exp -

1385

(2)

This relationship is consistent with Equation (1) and represents the laboratory data to within ± 20%. Using bed thicknesses com parable to those used in the pilot plant (about 3 cm) and super f i c i a l gas velocities close to those used in the pilot plant (about 150 cm/min), i t was found that the gas and solids temperatures very rapidly came together. This was taken to mean that the pro duct of the heat transfer coefficient and the heat transfer area,


MiKESELL

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Energy Recovery from Agricultural Residues


11.


221

222

SOLID

UA, for the bed is essentially i n f i n i t e . with

Moisture Balance


Equation (2) together (3)

( C ) . ^ - λ . ^ = (C ) .((T ) p

A N D RESIDUES

T Q = T

Solid Wastes and Residues - American Chemical Society

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