GASIFIER DESIGN

Gasifier Design Capacity

The first point in the gasifier design is to identify the application and the size/capacity of the gasifier in term of the producer gas requirement and the fuel to be gasified. The main applications of the gasifier considered are as under:

1. Use of Producer Gas as supplementary fuel in diesel engines. In an unaltered diesel engine (compression ignition), the producer gas can only be used as a supplementary fuel replacing about 70% diesel and the engine can not be operated on producer gas alone.
2. Use of producer gas as fuel in SI engines. The engine can be operated on producer gas alone, dual fueling is not necessary.
3. Thermal application of producer gas. This includes the combustion of producer gas in an appropriate burner and use of sensible energy of gas for hot water, steam and hot air production.

Material and energy balance over the above systems will allow to determine the producer gas requirement for the engine and the fuel/wood requirement for the gasifier.

Material and Energy Balance

1. Engine application

During the computations on material and energy balance on different systems, the following assumptions based on the reported information, are made:

Cold gas efficiency of the gasifier	70%
Hot gas efficiency of the gasifier	80%
LCV of producer gas	4.5-5 MJ/Nm3
Engine generator efficiency in dual fuel mode	22%
Engine generator efficiency (SI engine)	18%
Producer gas burner efficiency	90%
Diesel replacement	70%
Lower heating value of fuel	15-18 MJ/kg
Nitrogen in producer gas	52% (Vol)
Nitrogen content of fuel wood	0.3% (Wt)

During all calculations air is assumed to have a volumetric/mole composition of 79% Nitrogen and 21% Oxygen. All the quantities of air and gaseous products are at normal conditions of temperature and pressure i.e. 0 ^oC or 273 ^oK and 1 atm pressure. The calculations are on the basis of one-hour operation.

1. Gasifier Diesel Engine Alternator System

A material and energy balance was carried out over a 10 kW capacity gasifier and engine (diesel engine operating in dual fuel mode) system.

The gasifier system comprises of a gasifier reactor, gas-cleaning unit consisting of a water scrubber, a dry filter and a fabric filter and diesel engine-alternator. The material and energy flow diagram for the system as whole is shown in Figure 1. The system was divided into two sub systems; the diesel engine-alternator constituting the first sub system whereas the gasifier and the gas cleaning unit as the second sub system as shown in the figure.

By writing an energy and material balance over the first sub system, the diesel and producer gas requirement was calculated as follows.

Out put energy from the sub system 1 = 10 kW

Energy contribution of PG to engine = (10x3.6x0.7)/0.22 = 114.54 MJ

Amount of PG required = 114.54/4.5 or 25.45 Nm³

Now, consider the second sub-system, comprising of gasifier reactor and gas cleaning unit. In the output energy of the second sub system, only the chemical energy of producer gas (useful energy as far as the engine operation is concerned) is considered. Fuel wood and air used in the gasification process were calculated from an energy balance over the second sub system.

Input energy to the gasifier = 114.54/0.7 or 163 MJ

Quantity of fuel wood used = 163/15 or 10.9 kg (dry weight basis)

Figure 1. Gasifier Diesel Engine Alternator System

Air requirement for the gasifier is determined by a nitrogen balance over the second sub system. Following relation represents the nitrogen balance over the process. F_a in the equation below is flow rate of air into the gasifier.

The air flow/consumption rate comes out to be 16.75 Nm³ h^-1. Thus the gasifier should be capable of generating 25 Nm³ of gas per hour using about 11 kg dry fuel wood in an hour.

b) Energy and Material Balance over Gasifier-SI Engine System

Le the capacity of the system is 10 kW. The gasifier and the SI engine alternator system is shown in Figure 2. The system was further divided into two sub systems; the SI engine-alternator as the first sub system and the gasifier and the gas cleaning unit as the second sub system as shown in the figure. Writing an energy and material balance over the first sub system, the producer gas requirement can be calculated as under.

Output energy from the sub system 1. = 10 kW

Input energy to engine = (10x3.6)/0.18 = 200 MJ

Amount of PG required = 200/4.5 or 44.4 Nm³

Figure 2. Gasifier SI Engine Alternator System

Now, consider the second sub-system, and writing energy balance equation to evaluate the fuel wood requirement:

Input energy to the gasifier = 200/0.7 or 285 MJ

Quantity of fuel wood used = 285/15 or 19 kg h^-1 (dry weight basis)

Air requirement for the gasifier is determined by a nitrogen balance over the second sub system. Following relation represents the nitrogen balance over the process. F_a in the equation is flow rate of air into the gasifier.

The air flow/consumption rate comes out to be 28.65 Nm³ h^-1. Thus the gasifier should be capable of generating 44.4 Nm³ of gas using about 19-kg fuel wood in an hour.

2. Thermal Application

Now consider gasifier-producer gas burner system capable of generating gas having sensible energy equivalent to say 100 MJ. Le the whole system be divided into two sub systems as in case of engine applications. The first sub system consisting of producer gas burner and the second sub system comprising of gasifier as shown in Figure 3. The out put energy from the sub-system 1, is in the form of sensible energy of flue gases.

Figure 3. Gasifier producer gas burner system

Writing energy and material balance over the sub system 1.

Output energy from the sub system 1. = 100 MJ

Input energy to engine = 100/0.9 = 112 MJ

Amount of PG required = 112/4.5 or 24.7 Nm³

Now, consider the second sub-system, and writing energy balance equation:

Input energy to the gasifier = 112/0.8 or 138 MJ

Quantity of fuel wood used = 138/15 or 9.25 kg h^-1 (dry weight basis)

Thus the gasifier should be capable of generating 25 Nm³ of gas per hour using about 9.25 kg fuel wood in an hour.

Turn Down Ratio

The turn down ratio varies from 3 to 18 in down draft gasifiers with throat, depending on the insulation of the hearth of the gasifier. It is defined by the following relation. For vehiel requirement a turn down ratio of 8:1 is essentially required

DESIGN OF DOWN DRAFT GASIFIER WITH THROAT

Once the size/capacity of the gasifier in terms of capability of the gasifier to produce certain amount of producer gas, and a decision is made on the type of the gasifier to used depending on the fuel available, the critical dimensions of the gasifier can be worked out using the techniques given below.

We are considering the design of Imbert type or down draft gasifier with throat in the present exercise. In a down draft gasifier with throat the gas is forced to pass through a constriction or a narrow part called throat of the gasifier. Due to the reduction in the area, the gas velocity increases and a temperature rises and become uniform in this zone. Since most of the combustion takes place around or just above the throat, the oxidation zone is formed at the throat. The high and uniform temperatures help in better quality of producer gas from the gasifier in terms of higher carbon monoxide and hydrogen and also help in thermal cracking and combustion of tar and the resultant gas has low tar and high calorific value. With the introduction of throat the gasifier becomes highly fuel specific and only wood pieces of specific size can be used for gasification.

Hearth Load

It is volume of gas produced in m³ at NTP per unit cross-sectional area at the constriction/ throat (cm²) per hour. The maximum value of the hearth load is about 0.9 Nm³/cm²-h. It is also referred as specific gasification rate (SGR) in the literature.

Superficial Gas Velocity

It is the velocity of gas at the throat of the gasifier in the gasification zone considering the throat to be empty. The superficial velocity corresponding to a hearth load of 0.9 Nm³/cm²-h is 2.5 m/s. The actual gas velocity at the throat of the gasifier is much more compared to this value due to presence of char/wood particle and high temperature in the oxidation zone of the gasifier.

Throat Diameter

Once the size of the gasifier is known, the hearth load value can be used to calculate the throat diameter. Corresponding to an electrical output of 10 kW using SI engine:

Gas production rate: 44 Nm³/h

Cross-sectional area of the throat = 44/0.9 cm² = 48.9 cm²

Throat diameter = 79 mm say 80 mm

Superficial gas velocity at the throat = 2.5 m/s

Air Tuyers

Air enters into the gasifier through a number of nozzles arranged along the periphery of the gasifier in the oxidation zone. The diameter of the air tuyers taken together is generally small compared to the flow duct or pipe and the air velocity becomes very high as air enters the gasifier which helps the flame to penetrate in to the fuel bed as shown in Figure 4. This ensures the proper and uniform temperature distribution in the oxidation zone of the gasifier. Thus oxidation and pyrolysis of the fuel is completed before the material enters the reduction zone. The higher gas velocity results in higher temperatures in the biomass bed leading to reduced tar content and higher carbon monoxide content in the producer gas.

Figure 4 Air entering the gasifier through the air tuyers

For the capacity of gasifier reactor under reference, the recommended number of air tuyers in Table 1 is 5. The air tuyers diameter can be calculated in the following way.

Corresponding to a gas production rate of 44 Nm³/h and the throat diameter of 80 mm. The number of air tuyers is 5 and (Am*100)/Ah is 6.4 (Reference Table 1). From this the air tuyers diameter can be computed as under.

(Am*100)/Ah = 6.4 with Ah equal to 48.9 cm² Am can be calculated

Am = 2.95 cm²

Cross-sectional area of one tuyer = 2.95/5 Þ 0.59 cm²

Diameter of the air tuyer (dm) = 0.87 cm

Other dimensions of the hearth

Once the hearth or throat diameter is calculated, the other dimensions of the hearth such as the distance between the throat and the air tuyers, reactor diameter at the air tuyers and other dimensions as shown in the diagram below can be calculated from the tables. The corresponding values for the gasifier under reference are given below.

h 95 mm

dr 268 mm

dr` 150 mm

H 256 mm

R 100 mm

Figure 5 Dimensions of the hearth of the gasifier

Table 1. Design parameters for the Imbert type gasifier (No. 1)

dr/dh	dh	dr	dr`	H	H	R	A	(Am*100)/Ah
268/60	60	268	150	80	256	100	5	7.8
268/80	80	268	176	95	256	100	5	6.4
268/100	100	268	202	100	256	100	5	5.5
268/120	120	268	216	110	256	100	5	5.0
300/100	100	300	208	100	275	115	5	5.5
300/115	115	300	228	105	275	115	5	5.0
300/130	130	300	248	110	275	115	5	4.6
300/150	150	300	258	120	275	115	5	4.4
400/130	130	400	258	110	370	155	7	4.6
400/150	135	400	258	120	370	155	7	4.5
400/175	175	400	308	130	370	155	7	4.2
400/200	200	400	318	145	370	155	7	3.9

Fuel Hopper

In batch operation of the gasifier the fuel holding capacity determines the maximum operating hours of the gasifier once the fuel is charged to the gasifier. A reasonable continuous operating period for a batch is 6 to 8 hours, thus the fuel holding hopper should hold fuel for 6-8 hours. Knowing the bulk density of the fuel to be used in the gasifier, the volume can be determined.

Table 2. Design parameters for the Imbert type gasifier (No. 2)

dr/dh	h/dh	Range of gas output (Nm3/h) Max Min	Wood Consumption (kg/h)
268/60	1.33	30 4	14
268/80	1.19	44 5	21
268/100	1.00	63 8	30
268/120	0.92	90 12	42
300/100	1.00	77 10	36
300/115	0.92	95 12	45
300/130	0.85	115 15	55
300/150	0.80	140 18	67
400/130	0.85	120 17	57
400/150	0.80	150 21	71
400/175	0.74	190 26	90
400/200	0.73	230 33	110

For 6 hours operation the fuel required 19*6 = 114 or say 120 kg

Bulk density of the wood pieces 350 kg/m³

Fuel hopper volume 120/350 = 0.34 m³

Gasifier diameter is generally 2 to 3 times the dr (let us take it 2.5 times). Accordingly the gasifier reactor diameter comes out to be 26.8*2.5 = 67 cm

If h is the height above the air tuyers

0.34 = P/4 (0.67)²*h Þ h is 1 m

Thus the height is 1m.

Considering 30% extra the actual height above the air tuyers is 1.3m

Total height of the gasifier = 1.3 +0.256 +0.1 = 1.656 m or say 1.75 m. Here the ash holding space i.e. R may be increased to 0.194 m to facilitate the installation of ash removing gate.

Note: Use simplified design as Explained in the Class
A 330 MJ/h capacity gasifier along with producer gas burner system designed at PAU is shown in the diagram below.

Figure 6 Line diagram of a down draft gasifier burner system

DESIGN OF A DOWN DRAFT THROATLESS GASIFIER

Description of the gasifier unit

Throatless gasifier system is a double walled constant diameter reactor containment tube unit. The inner shell is reactor having grate at the bottom. The outer shell acts a containment tube. The top of the gasifier remains open at the time of operation. The gasifier is a special gasifier designed for gasification of agricultural crop residues such as rice husk, groundnut shell etc. Rice husk and air enter the gasifier from the top whereas the producer gas exit is at the bottom. To start this gasifier, a small quantity of husk is burned just above the grate followed by a small charge of rice husk with gas sucked though the system by means of a hand blower. When fir spreads well in whole cross-section of the reactor, the reactor is filled with husk upto top and the gasifier is connected to gas application unit. In this gasifier ignition is started over the grate and the reactor is filled with rice husk. During the gasifier operation, the oxidation/fire zone keeps on moving up and so does the reduction zone and char accumulation. The oxidation zone is formed at the interface of the rice husk and char, where oxygen comes in contact with the husk at high temperature. Reduction zone is formed just below the oxidation zone. The pyrolysis reactions take place above the oxidation zone. A line diagram of the gasifier is shown in Figure 7.

Figure 7 Throatless down draft rice husk gasifier

Depending upon the requirement, the size of the gasifier in terms of the fuel requirement and gas production can be determined by making an energy balance over the system as discussed earlier. During this analysis the following assumptions may be used.

Cold Gas efficiency of the gasifier	65%
Hot Gas efficiency of the gasifier	75%
LCV of producer gas	4.5 MJ/Nm3
Engine generator efficiency in dual fuel mode	22%
Diesel replacement	70%
Lower heating value of fuel	14.35 MJ/kg
Nitrogen in producer gas	52.5%
Nitrogen content of fuel wood	0.3%
Equivalence ratio	0.36
Calorific value of diesel	37 MJ/l

Once the size/capacity of the gasifier in terms of capability of the gasifier to produce certain amount of producer gas, and a decision is made on the type of the gasifier to used depending on the fuel available, the critical dimensions of the gasifier can be worked out using the techniques given below.

Specific Gasification Rate:

Specific gasification rate is the quantity of dry rice husk consumed per unit time and cross-sectional area of the gasifier (kg/h-m²). SGR was determined using the following relation.

A study on the determination of reactor scaling parameters, performance of four gasifier reactor with SGR as a variable parameter was estimated. The SGR and gasification efficiency values for the four reactors is in Figure 8. In all cases the best gasification efficiency correspond to a SGR of about 190 kg/h-m².

This value of SGR has been selected and used as a design parameter for the rice husk operated throatless gasifier. For a 10 kW capacity rice husk gasifier-diesel engine-alternator, the energy balance indicates a that the hourly rice husk requirement will be about 12.25 kg. The producer gas requirement comes out to be 25.5 Nm³/h

Specific gasification rate kg/h-m²

Figure 8 Effect of SGR on gasification efficiency in different capacity gasifiers

Cross-sectional area = (12.25/190) m²

Diameter = 0.3 m

Continuous gas supply to the engine can be ensured with the installation of two gasifier reactors in parallel to a single gas conditioning unit.

Diameter of containment tube

Containment tube checks the quantity of solid particulate in the producer gas and also prevents excessive heat loss from the reactor to the atmosphere. The recommended superficial gas velocity in the containment tube is 0.15 m/s against the superficial gas velocity of about 6 m/s in the gasifier reactor. Using the following relation the diameter of the containment tube can be determined. In the equation d₁ and d₂ are the outer reactor diameter and inner diameter of the containment tube respectively.

Substituting the values of producer gas flow rate (25.5 Nm³/h) and outer diameter of reactor as 30.6 cm, the inner diameter of the containment tube comes out to be 36 cm. It is assumed here that the gasifier reactor is made of 3 mm thick MS sheet. The length of the containment tube was kept about 10 cm more than the reactor. This was to allow the producer gas after its exit through the grate to enter the containment tube.

Grate

The grate in the gasifier reactor supports the fuel bed and at the same time allows the producer gas to flow through it. Grate of the gasifier was made from a 15 ASTM stainless steal wire mesh. Wire mesh was fitted on a circular frame/ring of 6 mm thick MS rod and was supported on three legs. The grate was positioned at a distance of 5 cm above the bottom end of the gasifier reactor. The grate was a removable one, which could be taken out for cleaning. The gasifier reactor was rested on an MS angle frame on two hinges provided on the outer surface of the containment tube. The reactor could be tilted along the hinges for ash removal.