Wood Gasification for Gas and Power Generation - ACS Symposium

29 Wood Gasification for Gas and Power Generation ARUN VERMA and G. A. WEISGERBER

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: August 29, 1980 | doi: 10.1021/bk-1980-0130.ch029

Research and Development Centre, Saskatchewan Power Corporation, 2025 Victoria Avenue, Regina, Saskatchewan S4P 0S1

A study (1) was initiated by Saskatchewan Power Corporation (SPC) in 1976 to look at alternatives for heat and electrical power production in isolated northern communities. The present method of electrical power production at these communities is from diesel engines and fuel oil is largely used for space heating. The cost of these fuels has been going up recently, as well as the cost of transportation to these communities. The aim of the above mentioned study was to zero in on the most practical method for providing heat and power from locally available energy sources. The energy source was identified as wood. The work identified and evaluated a number of technologies, their status and processes which could be used to go from wood to production of electricity or for space heating. The appropriate technology for space heating was found to be the direct use of wood in high efficiency wood furnaces and for electricity production it was concluded that wood gasification followed by combustion of this gas in an internal combustion engine was the practical alternative. Evaluation of high efficiency wood furnaces is being carried out under a separate project (2). For power production, a decision was taken to build a plant by scaling up the gasifier at B.C. Research Council (3, 4) which is said to be a fluidized bed gasification unit. A 4.22 GJ/h gasifier thus was fabricated with other auxilliaries. The Saskatchewan Forest Products plywood mill was chosen as the site for installation of the gasifier. The reason for that location was the easy availability of fuel as well as the ease with which the gasification technology could be transferred to the forest product industry in Saskatchewan for their own use of drying plywood. The plant was run for the first time in January 1979. Parallel to the above work, a detailed resource, social and environmental impact study (5) was initiated to see the effect of power production via wood gasification plants on Northern Communities in the province. 0-8412-0565-5/80/47-130-379$05.00/0 © 1980 American Chemical Society Jones and Radding; Thermal Conversion of Solid Wastes and Biomass ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

380

THERMAL CONVERSION OF SOLID WASTES AND BIOMASS

Gasification Reactions The term gasification as used here refers to the reaction of wood (wet) with air to yield a low heating value gas. The major components of the fuel gas produced are formed by various reactions between carbon and water in the fuel with oxygen in the air. The first step towards gasification of wood after it has been introduced into the gasifier, is drying (100-200 C), Wet wood + heat •> Dry Wood + H 0 (Steam) 2


The second step is the pyrolysis step C200-5Q0°C) which releases the pyroligneous acids, tar, etc. Dry Wood + heat -> Char + CO + C0 + H 0 2

2

+ CH + C H k

2

[+

+ Pyroligneous acids + tar The oxidation or partial oxidation of char occurs next (300°C upwards). Char + 0 + H 0 (Steam) -> CO + C0 + H + heat 2

2

2

2

The first two processes are driven by the heat given out by the third process. The detailed thermo-chemistry can be summarized as follows: C +0

2

C + C0

2

Ζ

C0 , Exothermic

(1)

Ζ

2C0, Endothermic, Boudouard Reaction

(2)

H 0 + CO Ζ 2

2

C0 + H , Endothermic, Water gas Shift 2

2

Reaction H0 + C Ζ 2

CO + H , Endothermic 2

(3) (4)

C + 2H Ζ CHi+j Exothermic (5) The first reaction is the only one which provides the thermal energy for carrying out the other endothermic reactions. Reaction 5 is favoured only at lower temperatures and high pressures. Because of this, most of the methane formed in the product gas in gasifiers operating under atmospheric pressures, is a result of the pyrolysis step. 2

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

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Wood Gasification for Gas Generation

381

Hudson Bay Plant Details Following are some of the details of the plant. details may be seen elsewhere (6).

Complete


Gasifier Design Criteria. The gasification plant (Figure 1) has incorporated considerable design improvements and modifications over the smaller unit although the gasifier output target of 4.22 GJ/h and the grate heat release rates determined from B.C. Research unit served as the basis for determining the gasifier internal diameter. An expanded freeboard model was decided upon to reduce gas velocities in order to lower particulate entrainment in the gas exit stream. Design parameters are listed below: Net reactor output Conversion efficiency Gas H.H. V. Waste Wood Moisture Content Fuel Consumption

4.22 72% 5.59 50% 568

GJ/h ~ MJ/m (wet basis) kg/h (as fired) J

Gasifier. The gasifier is approximately 4 m in height having an internal diameter of 1.2 m in the lower section and 1.8 m in the expanded freeboard section. The gasifier outer shell is composed of carbon steel. A layer of refractory forms the inner surface of the gasifier. The windbox region of the gasifier is bolted to the bottom section of the gasifier to facilitate repairs to the ash removal system. Distribution of the incoming air is accomplished with a pinhole grate supported above the windbox. The sectional grate has refractory lining to protect the steel from the 1000°C temperatures expected in the combustion zone immediately abovç. Access to the gasifier is through a manhole. The windbox zone is also supplied with a clean out door. The gasifier is equipped with 8 ports for thermocouple installation and 4 ports for pressure sensing. Two sets of ports at 180 degrees to each and capped with tempered glass are provided for the fuel height optical sensors. Wood Feeding System. The feed system consists of a shift drag conveyor, metering bin, air lock and screw conveyor. The live bottom shift bin has a capacity of 6 m . A grid covers the bin to prevent large sized particles from damaging the system. An inclined drag conveyor transports fuel from the shift bin to the metering bin. The metering bin has a capacity of 1.6 tp? and has a live bottom discharge. A hydraulic motor drives the drag chain conveyor. A hydraulic power unit equipped with an electrically driven pump, flow regulator valve, pressure relief valve, and solenoid valve control the flow and pressure of hydraulic fluid bin,

3




382


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383


to the hydraulic motor. When the solenoid valve which allows fluid to reach the hydraulic motor is closed the fluid circulates within the reservoir. The 12 position flow regulator valve directs the required flow of fluid to the hydraulic motor and bypasses the excess back into the reservoir. A gear motor drives the rotary air lock and screw conveyor. The air lock provides a seal between the screw conveyor below and the small fuel hopper above. The 15 cm diameter screw conveyor has heavy duty 0.62 cm thick flighting. It extends to the inner refractory wall of the gasifier. Air Feed System. A turbo pressure blower heat exchanger and burner compose the air feed system. The blower is designed to deliver 12 m^/min of air at 12 kPa guage pressure at 70°C. At 315°C this decreases to 6 kPa guage pressure. On the outlet of the blower is a manually controlled damper to regulate flow of air. Air is transported to the windbox area via a pipe equipped with an orifice plate. The air preheater is a double shell direct-fired heat exchanger designed to deliver air at a maximum of 315°C. It is 1.2 m in diameter and is 2.3 m in length. A 0.45 GJ/h oil burner supplies the heat. Ash Removal System. A rake, screw conveyor and ash box comprise the ash removal system. A rake mounted above the pinhole grate ploughs ash towards the centrally mounted ash discharge screw. A screw conveyor transports the ash via a gate equipped chute into the ash bin below. Flare System. Following the gasifier is the flare system which is composed of a cyclone, cyclone hopper, butterfly valve, flare and stack. The cyclone is 2 m tall with the diameter decreasing from 1.2 m at the raw gas entrance to 0.3 m at the particulate discharge point at the bottom. The 0.95 m^ cyclone hopper is castor mounted and equipped with a cleanout door. On top of the cyclone is located a cross-tee which is equipped with two manually operated, and one motorized butterfly valve. The manual valves are employed to direct the gas to a particular end use. The motorized valve controls the gas entering the flare. The flare chamber is refractory lined and has an adjustable louvre at the bottom to regulate combustion air. Ignition is provided with a 0.20 MJ/h oil burner. A totally enclosed hood captures the combustion products and directs them to the 0.30 m diameter stack which extends above the hood. Gas Sampling System. The gas sampling system consists of a knockout pot, filters, gas partitioner, integrator and pressure-vacuum pump.


384



The pump draws gas from the gasifier outlet through a knockout pot where moisture, tars and dust are removed. Between the knockout pot and the gas partitioner are a couple of filters which ensure the gas is clean and will not damage the analyzer. The gas is drawn through a Fisher gas partitioner and is exhausted outside. The integrator then provides the gas composition on a volume per cent basis. Wood Gas Cleaning System. The wood gas cleaning system consists of a cyclone, centrifugal tar extractor, drier and blower. In the cyclone hot raw gas enters tangentially with ash and carbon particles dropping to the bottom into a hopper and the gas exiting from the top. The hot gas is directed from the cross-tee to the mechanically driven tar extractor in which water is injected to cool the gas and condense the tars which are then separated from the gas by the centrifugal force. The blower draws the cool gas through the column filled with dry wood blocks which remove moisture. The water and tar mixture is taken to a separation tank where the water and tar are separated by gravity. The water is recirculated back to the tar extractor. Diesel Generator System. The cool clean gas can be combusted in a naturally aspirated 250 HP Deutz diesel engine (Model F12L413F) which has been modified for burning wood gas. This has been purchased from Imbert Co. in Germany. The diesel oil injection system has been modified to allow approximately 10 per cent of the normal flow to be injected. This amount of 011 is ignited in compression which in turn ignites the wood gas and air mixture. The wood gas is fed into the engine intake system, mixed with air and sucked into the engine. A Brown Boveri generator rated at 150 kW is frame mounted with the diesel engine. Three banks of 75 kW electric furnaces act as the loading units. Burner System. A modified 5.28 GJ/h burner is also on site to combust the low heating value gas. The burner consists of a 15 cm special wide range burner, pressure pilot, flame rod, combustion air blower with motor, micro ratio valve assembly and an emergency shut off valve. The burner size has been increased compared to a normal burner operating with natural gas. The pilot is operated on propane to ensure adequate ignition. Because of the low heating value of the gas, the air enters the normal gas inlet and the wood gas enters through the normal air inlet. Process Control. The control panel contains the start-stop locations and indicating lights. Temperature, pressure and flow parameters are also displayed. A bindicator on the metering bin and a timer control the wood feed addition to the metering bin. The bindicator shuts



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385

off the conveyor from the shift bin when the metering bin is full. A timer which can be varied from 0 to 30 minutes auto matically starts the shift bin conveyor after the selected time interval. The rate of addition of solid fuel is controlled from the metering bin. A hydraulic system drives the metering bin conveyor. When the metering bin conveyor is stationary the electrically driven pump on the hydraulic power unit recirculates the hydraulic fluid to the reservoir. To start the metering bin conveyor a solenoid valve is opened allowing hydraulic fluid to turn the hydraulic motor. The flow and pressure of fluid to the hydraulic motor and ultimately the conveyor speed is controlled by a 12 position regulator valve on the hydraulic power unit. A single motor drives both the screw conveyor and airlock. The motor is equipped with reversible switch gear. Both the solenoid valve on the hydraulic power unit and the screw conveyor motor are actuated by one start location on the control panel. Flow of combustion air to the gasifier is controlled by a manually adjustable damper at the outlet of the turbo blower. An orifice plate in the air line to the gasifier indicates the rate of air addition on the control panel. Hot combustion air for start-up of the gasifier is obtained by operating the oil burner on the air preheater. Ash removal is on an intermittent basis with time between removal and amount removed at a single time determined by operating experience. The gasifier is equipped with chromel-alumel Type Κ thermocouples which are tied into a multipoint temperature chart recorder housed in the control panel. Temperatures of the combustion air, fuel bed at various heights, exit gas and combustion products in the stack are monitored. Ports in the gasifier are connected to manometers to provide pressure data. Two sets of optical sensors provide fuel bed height determination by operating indicating lights on the control panel. Fuel bed height is maintained between the two sets of sensors. The operating pressure of the gasifier is controlled between 5 and 16.5 cm water column. The pilot oil burner on the flare can be operated manually or automatically with it starting and stopping as the flare valve opens and closes. Several safety features have been included in the control system. The exit gas thermocouple is connected to a variable temperature controller to maintain gasifier temperatures below safe operating limits. A pressure switch is connected to the outlet point of the gasifier which will shut the combustion fan down if the pressure exceeds 120 cm water column. Various pieces of equipment are also interlocked to protect the system.



386


Results and Analysis The Gasifier. Table 1 shows a typical analysis of wet spruce chips of nearly 58% moisture content. Data for the gasification of these is shown in Table 2. The gas compositions are only preliminary and do not represent gas quality when the gasifier is operated under optimized conditions. Even in these preliminary runs the gas heating value has been determined to be in the range 4.4 - 5.07 MJ/m3. The gasifier output was 4.27 GJ/h. Optimization of the gasifier operation is proceeding. Figure 2 shows the temperatures of the various points in the gasifier over a two hour period while gasifying wet spruce chips. The temperature in the combustion zone varied between 600 - 700°C with less than 1% oxygen showing in the exit gases. Other temperatures were considerably lower. Large differences between thermocouple points 1 and 5 indicate that the bed is not in a fluidized state. This was confirmed by further experimental tests carried out on a three dimensional cold model. It was observed that the chips tend to lock with one another and cannot be fluidized under the design flow conditions in the reactor. It was also observed that some channeling also occurs in the bed. Further work is presently under way to determine the fluidizing conditions for other types of waste wood fuels. The Burner. The burner to date has been run effectively with clean wood gas without any problems and it has been demonstrated without a doubt that the clean gas can be easily utilized in a modified burner or a burner designed to burn wood gas. The hot raw gas also burns effectively in the burner. The Engine. The diesel engine, by the time of writing, had run for eight hours total under part load, using the wood gas from this gasification plant. However, the same engine was run on wood gas from a down draft gasifier having similar quality gas. Data from this run is shown in Table 3. As may be seen, with 11% diesel and balance wood gas, the output of the engine drops by 20%. The rest of the engine performance is similar to that when operating with diesel fuel alone. Environment, Resource and Social Impact of Gasification. Here, the forest resources and the social - environmental consequences of power generation via wood gasification were evaluated on 15 northern communities in Saskatchewan which are serviced by diesel electric generators. A general assessment was undertaken for 14 communities and a detailed one for 'Pinehouse'. These communities are listed in Table 4. The table shows the size and the electrical load at these communities. The general assumptions of this study were that forest


29.

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Table I Typical Analysis of Spruce Wood Chips 1.

Moisture (as received), %

2.

Proximate Analysis (dry basis) Ash, % Volatile Matter, % Fixed Carbon, % Heating Value

3.

Ultimate Analysis Carbon, % Hydrogen, % Nitrogen, %

44.11 5.59 0.08

4.

Ash Analysis, % A1 0 Si0 CaO MgO Na 0 K0 P,0 Fe 0

0.21 1-34 39.72 5.56 17.27 11-59 5.78 0.01

2

3

2

2

2

5

2

3

57.74 0.79 79.40 19.82 20,099 J/g


387


4.54 4.36 4.60 4.98

4.62 4.43 4.68 5.07

17.39 16.49 19.03 22.7

1.712 1.661 1.91 2.05

55.69 56.79 55.74 54.3

0.99 1.37

0.61

9,.93 9..84

11.42

9.62

At 14.73 psia & 60°F.

.991

0.56

11..74

11.77

0.507

1.065

0.604

11..97

11.55

4.77 4.86

18.5

1.86

53.54

0.908

0.605

12..37

12.21

4.46

4.53

55.35

0.898

1.63

17.3

CO 17.91

0.525

12..05

4

CH 1.95

54.79

12.24

2

N

4.62

2

H.H.U, MJ/m3 Sat. Dry 4.69

0 0.880

0.501

11..89

6

12.08

2

CH

ι

2

co

Composition of gas produced from 57.74% Moisture Spruce Chips

Table II


AND

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VERMA



390


Table

III

Performance Data of Deutz Engine (F12L 413F) Unmodified engine performance with 100% diesel fuel

Ambient Air Temp. °C rpm Total kW Horse Power

15 1,800 196.5 267

Modified engine performance with 11% diesel, balance wood gas

15 1,800 156 212.4



Camsell Portage Fond du Lac Stony Rapids Black Lake Wollaston Kinoosao Southend Deschambault Sturgeon Landing Stanley Mission Pinehouse Patuanak Di11 on Michel Brabant

Community

100 500 500 500 600 100 350 425 125 500 600 500 550 100 125

Total Capacity (kW) 30 100 110 90 120 20 80 50 25 100 100 100 no 15 25

Average Load (kW)

TOTAL

45 150 180 130 200 35 130 95 45 175 175 175 185 25 45

Peak Load (kW)

2,731,764

75,009 272,760 270,487 264,577 268,214 75,009 183,972 184,113 102,740 233,033 206,566 225,695 220,936 73,645 75,009

Diesel Fuel Consumption (litres) 1977 24 92 95 94 92 20 87 77 40 121 82 111 80 14 17

No. of Electrical Connections 1978

Table IV Communities in Northern Saskatchewan Using Diesel for Power Generation


5 6 2 6 5 4 7 6 5 9 7 6 5 8 3

No. of Persons per Connection



392

resources would have to be harvested on a sustained yield basis and use of waste wood could be made in the gasification plant. Wood reserve data was found adequate for 9 of the 15 communities. The remaining 6 communities which are located north of 57 degrees latitude did not have sufficiently detailed forest volume information to allow an accurate assessment of its wood reserves. Nevertheless, the findings indicate that forest resources are adequate to supply a small wood gasification plant for at least 50 years in each community. Such a time frame work for logging ensures a renewable timber resource even without an active reforestation program although it is desirable. One of the major findings was that waste wood could provide an adequate supply of fuel for a few communities. Examples of wood waste could be found at local sawmills, right of way clearing for roads, and wood damaged by forest fires. The social consequences of wood gasification on a community are positive. Though the scale of the gasification project is small, it may require local people to operate the gasifier and to harvest an adequate supply of wood. With the use of local labour, the construction of the project in the community would also have mainly positive implications for the community. The environmental consequences based on emission data could not be completed as the data is being collected but the effect of gasification on vegetation, wildlife and aquatic l i f e , etc. was estimated to be minimal. Overall, the project would offer permanent local employment and ensure a greater electrical power capacity. Earned income would be increased and spin-off benefits could result in the establishment of small businesses. The utilization of waste wood is another positive element associated with this proposed project. As a result, the gasification project is likely to be viewed positively in northern communities. v

Conclusions i)

ii) iii) iv)

The scale up version of the B.C. Research gasification unit did not perform as a fluidized bed gasifier when operating on wet spruce chips. It essentially operated in a fixed updraft flow mode, The fuel gas produced from wood can be easily combusted in a modified burner or a burner built to combust this type of gas. The naturally aspirated modified diesel engines can be run successfully with wood gas while using 10% diesel fuel for ignition purposes. The drop in output is 20%. Power generation via wood gasification in Northern Communities has a minimal impact on the environment and resources while it has a net positive social impact.


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Wood Gasification for Gas Generation 393


Abstract A 4.22 GJ/h waste wood gasification plant is being evaluated at Hudson Bay, Saskatchewan, Canada. The demonstration scale gasifier is a scale up of a smaller unit at the B.C. Research Council, Vancouver. The gasifier is designed to convert minus 1 inch wood waste with less than 50% moisture into a low heating value gas of approximately 4.5 - 5.6 MJ/m3. In operation wood waste is metered out from a live bottom storage hopper into the gasifier via a feed screw. Air is fed into the windbox under the pinhole grate. Hot raw gas exits from the expanded freeboard gasifier at the top. The gas is cleaned in a 'Crossley' type cleaning train consisting of a cyclone, centrifugal extractor and a drier. The clean gas is fed to a modified diesel engine or a burner. The aim of the work is to evaluate the gasification plant for production of fuel gas suitable for use in a burner and a modified engine for electrical power production. Plant details and preliminary performance data are discussed in the paper, along with environment, social and resource impact of wood gasification on northern communities in the province. References 1.

Verma, Α., Stobbs, R.A., "Wood for Power generation in isolated Northern Communities." Saskatchewan Power Corporation report, 1977.

2.

Verma, Α., Weisgerber, Gordon, "Thermochemical Conversion and other biomass related work at Saskatchewan Power Corporation", Proceedings of "Energy from Biomass seminar", Ottawa, March 1979.

3.

"Engineering Feasibility Study of the British Columbia Research Hog Fuel Gasification system", H.A. Simons (International) Ltd., May 1978.

4.

Liu, M.S., Serenius, R., "Fluidized Bed Solids Waste Gasifier", Forest Products Journal 26a, 56-59, 1976.

5.

"The Social, Environmental and Resource Impact of Wood gasification on Isolated Northern Communities, Part I"; March 1979, Report by Saskatchewan Power Corporation for the Federal Government of Canada under contract. DSS File No. 07SB.KL016-8-0060.

6.

"Evaluation of the Saskatchewan Power Corporation Wood gasifier at Hudson Bay, Saskatchewan, 1978-79". May 1979, Report by Saskatchewan Corporation for the Federal Government of Canada under contract. DSS File No. 07SB.KL016-8-0061.


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CONVERSION

O F SOLID

WASTES

A N D BIOMASS

Acknowledgement: This project is a joint project of Saskatchewan Power Corporation, Saskatchewan Forest Products Corporation and the Federal Government of Canada. November 16, 1979.


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Wood Gasification for Gas and Power Generation - ACS Symposium

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