**3.2. The demo furnace burning soya straw bales**

**Figure 3.** Bale storage and feeding system with push piston

60 Sustainable Energy - Recent Studies

**Figure 4.** Thermal scheme of distribution facilities

with volume of 5 m3

In order to obtain to plant work at nominal power, heat accumulator (thermal reservoir,

what the current needs for heating buildings are, boiler always works with the nominal power. The transitional periods (spring, autumn), for example, the need for heating usually amount to 20-40% rated power boiler, which would mean a much lower level of utility plant. Thermal scheme of distribution facilities is shown in Figure 4. From it can be seen fol‐ lowing thermal circles: a) Hot water from the boiler goes directly into a building that is heat‐

) has been installed (Figure 2). In this way it is ensured that no matter

In order to assess the combustion quality and to obtain data for the design of a soya strawfired hot water boiler, a demo furnace with thermal power of 1 MW has been designed and built [10, 18, 19]. The appearance of the furnace, with the thermocouple probes, the primary air fan and channel, and the fuel feeding channel is shown in Figure 6. This furnace has been adopted for cylindrical bales, with 1.2-1.5 m in diameter which were available at that time. The cross is clearly visible on Figure 7 where the scheme of the experimental demonstration unit for burning large rolled soy straw bales was presented. There can be also clearly distin‐ guish three characteristics combustion zones in the cigar burner: drying zone (6), zone of de‐ volatilization (5) and zone of char burning (13).

**Figure 5.** The boiler control system and cyclone-type particle precipitator

The proximate analysis of soya straw used in testing is given in Table 1. The sum of five tests was done. A summary of main test parameters is given in Table 2. During all tests, three gas temperatures in the combustion zone were measured, with shielded type K thermocouple probes. Gas sampling was done with a probe placed near the furnace exit. Gas samples were continuously analyzed with two analyzers, collected every 5 seconds and stored on-line.

In test 2, along with temperatures, gas composition was continuously measured. Less air was supplied as tertiary than in test 1. In the initial, start-up period (Figure 9), gas samples were taken directly from the combustion zone, and very high levels of CO in the flue gases were noted. After the choking of the gas sampling probe and its cleaning, and also in all fol‐ lowing tests, gas samples were taken only from the top of the furnace. As the temperature in this period increased to approximately 1000°C, bale feeding was slowed down, and this cor‐ responds to the temperature downfall (min. 50-75). Soon after that, stable conditions were

**Test 1 2 3 4 5**

Amount of straw [kg] 134,6 280 327,97 458,3 554,9 Primary air [m3/h] 1548 1548 1548 1350 1404\*\* Secondary air [m3/h] - - 234 418,25 228,14\*\* Tertiary air [m3/h] 504 252 108 259,2 259,2\*\*

[kW]+ 485,2 529,3 556,5 551,7 455,7

Test duration [min] 47 89 99 140 205

\* - The air excess coefficient in Test 4 was calculated for the period shown in the diagrams (Figures 13 and 14) \*\* - The air flow rates refer only to the period shown in the diagrams (Figures 15 and 16), since in test 5 variable speed drives were used for changing the speed of the fans. The air excess coefficient λ was calculated for the same

High level of CO concentration at the furnace top in test 2 urged the introduction of a small amount (approximately 10% of total air) of secondary air in the combustion zone, which would cool down the movable cross at the same time. It was also noted that tertiary air flow rate should be decreased, and therefore secondary air was introduced to the detriment of tertiary air. This change in design was examined in test 3, with two bales placed in the feeding channel.

The supply of the secondary air through the cross provided excellent conditions for combus‐ tion (Figure 11) – the concentration of CO was equal to zero for most of the time during the test. The air distribution (82% primary air, 12% secondary, 6% tertiary) was found to be well suited for maintaining steady conditions inside the furnace. On the other hand, the stability of the thermal output was found to depend largely on the active length of the bale immersed

1 2 2 3 3

Development of the Technology for Combustion of Large Bales Using Local Biomass

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63

not measured 4,71 2,61 4,12\* 2,92\*\*

obtained (Figures 9 and 10), primarily by adjusting bale feeding.

Conditions: + - The thermal power was calculated over the entire test period

Number of bales in the

Calculated thermal power

Average air excess coefficient

feeding tube

λ [-]

period

**Table 2.** Test parameters

into the furnace.

It should be noted that secondary air supply through the movable cross was not present in the first version of the demo furnace, which was examined in tests 1 and 2. The results from these tests stressed the need to introduce secondary air in the combustion zone, at the bale forehead, and the furnace with secondary air supply through the cross was examined in tests 3, 4 and 5.

**Figure 6.** The appearance of the demo facility for burning large rolled straw bales

Test 1 was conducted with one bale of straw placed in the feeding channel. Only tempera‐ ture measurements were done, and the results showed that the temperature in the combus‐ tion zone, in steady conditions, was quite stable (730-830o C, Figure 8) for a reasonable period of time (40 minutes). It was noted that the amount of tertiary air did not contribute much to overall combustion conditions, and that in fact this air over-cooled the flue gases in the combustion zone.


**Table 1.** The proximate analysis of soya straw used in the tests

In test 2, along with temperatures, gas composition was continuously measured. Less air was supplied as tertiary than in test 1. In the initial, start-up period (Figure 9), gas samples were taken directly from the combustion zone, and very high levels of CO in the flue gases were noted. After the choking of the gas sampling probe and its cleaning, and also in all fol‐ lowing tests, gas samples were taken only from the top of the furnace. As the temperature in this period increased to approximately 1000°C, bale feeding was slowed down, and this cor‐ responds to the temperature downfall (min. 50-75). Soon after that, stable conditions were obtained (Figures 9 and 10), primarily by adjusting bale feeding.


Conditions: + - The thermal power was calculated over the entire test period

\* - The air excess coefficient in Test 4 was calculated for the period shown in the diagrams (Figures 13 and 14) \*\* - The air flow rates refer only to the period shown in the diagrams (Figures 15 and 16), since in test 5 variable speed drives were used for changing the speed of the fans. The air excess coefficient λ was calculated for the same period

**Table 2.** Test parameters

probes. Gas sampling was done with a probe placed near the furnace exit. Gas samples were

It should be noted that secondary air supply through the movable cross was not present in the first version of the demo furnace, which was examined in tests 1 and 2. The results from these tests stressed the need to introduce secondary air in the combustion zone, at the bale forehead, and the furnace with secondary air supply through the cross was examined in

continuously analyzed with two analyzers, collected every 5 seconds and stored on-line.

**Figure 6.** The appearance of the demo facility for burning large rolled straw bales

tion zone, in steady conditions, was quite stable (730-830o

**Char (%)**

**Table 1.** The proximate analysis of soya straw used in the tests

Test 1 was conducted with one bale of straw placed in the feeding channel. Only tempera‐ ture measurements were done, and the results showed that the temperature in the combus‐

period of time (40 minutes). It was noted that the amount of tertiary air did not contribute much to overall combustion conditions, and that in fact this air over-cooled the flue gases in

18.80 5.66 22.12 16.46 59.08 75.54 13686

**Volatile matter (%)**

**Fixed carbon (%)**

C, Figure 8) for a reasonable

**Net calorific value (kJ/kg)**

**Combustible matter (%)**

tests 3, 4 and 5.

62 Sustainable Energy - Recent Studies

the combustion zone.

**Ash (%)**

**Moisture (%)**

High level of CO concentration at the furnace top in test 2 urged the introduction of a small amount (approximately 10% of total air) of secondary air in the combustion zone, which would cool down the movable cross at the same time. It was also noted that tertiary air flow rate should be decreased, and therefore secondary air was introduced to the detriment of tertiary air. This change in design was examined in test 3, with two bales placed in the feeding channel.

The supply of the secondary air through the cross provided excellent conditions for combus‐ tion (Figure 11) – the concentration of CO was equal to zero for most of the time during the test. The air distribution (82% primary air, 12% secondary, 6% tertiary) was found to be well suited for maintaining steady conditions inside the furnace. On the other hand, the stability of the thermal output was found to depend largely on the active length of the bale immersed into the furnace.

0 20 40 60 80 100

Test 2 O2 concentration

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65

CO concentration

CO concentration [ppm]

Time [min]

1000 Temperature in the combustion zone

Test 3

0 20 40 60 80 100

Time [min]

0 20 40 60 80 100

Test 3 O2 concentration

Time [min]

Gas sampling probe choked

**Figure 11.** Test 3 – Temperature in the combustion zone vs. CO concentration

O2concentration [%]

**Figure 12.** Test 3 – O2 concentration at the furnace exit

T1 [oC]

O2concentration [%]

**Figure 10.** Test 2 – O2 concentration at the furnace exit

Gas sampling start

**Figure 7.** Schematic of the experimental demonstration unit for burning large rolled straw bales

**Figure 8.** Test 1 – Temperature in the combustion zone

**Figure 9.** Test 2 – Temperature in the combustion zone vs. CO concentration

Development of the Technology for Combustion of Large Bales Using Local Biomass http://dx.doi.org/10.5772/51095 65

**Figure 10.** Test 2 – O2 concentration at the furnace exit

**Figure 7.** Schematic of the experimental demonstration unit for burning large rolled straw bales

0

**Figure 9.** Test 2 – Temperature in the combustion zone vs. CO concentration

T1 [oC]

200

400

T1 [oC]

64 Sustainable Energy - Recent Studies

**Figure 8.** Test 1 – Temperature in the combustion zone

600

800

1000

0 10 20 30 40 50

Test 1 Temperature in the combustion zone

Time [min]

Test 2

Temperature in the combustion zone

Gas

sampling start

0 20 40 60 80 100

Time [min]

Gas sampling probe choked

80 <sup>0</sup>

CO concentration

CO concentration [ppm]

**Figure 11.** Test 3 – Temperature in the combustion zone vs. CO concentration

**Figure 12.** Test 3 – O2 concentration at the furnace exit

Therefore, it is of great importance to feed the bale uniformly in accordance with the com‐ bustion process, and to maintain this length as stable as possible, by moving the cross ac‐ cordingly. The temperature instabilities (from the minute 45 further on, Figure 11) during this test are a consequence of changes of this length. The only peak in CO concentration co‐ incided expectedly with low temperatures during this period. Nevertheless, this test proved that the adopted concept of the furnace provided good conditions for efficient combustion of soya straw bales, with O2 concentration ranging from 10-14% (Figure 12), and an optimal average value of λ.

ures 13 and 14, the temperature was in the desired range, and CO concentration was accept‐ able for most of the period (up to 350 ppm), the only rise in CO occurring at the time of the temperature downfall (minutes 100-110). It was spotted by visual inspection, through the in‐ spection openings, that the bale was not inside the furnace at the time of the downfall, due to the problems with manual bale feeding and cross positioning – the bale forehead re‐ mained inside the tube. This caused the flame to enter the tube at the time, which also occur‐

100 110 120 130 140

The aim of test 5 was to assess the influence of air flow rate control, with variable speed drives, on furnace performance. During a chosen period of 40 minutes (Figures 15 and 16), optimal air flow rates were obtained and bale feeding was kept stable. The concentration of CO was very low, with O2 concentration varying in the range of 10-15%. The temperature during this period was higher than the desired 850°C (which should not be exceeded in or‐ der to avoid ash melting), which will be taken into consideration in some of the conclusions.

100 110 120 130 140

Time [min]

Test 5 O2 concentration

Time [min]

Test 5

Temperature in the combustion zone

**Figure 15.** Test 5 – Temperature in the combustion zone vs. CO concentration

O2concentration [%]

**Figure 16.** Test 5 – O2 concentration at the furnace exit

T1 [oC]

CO concentration

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67

CO concentration [ppm]

red during test 5.

**Figure 13.** Test 4 – Temperature in the combustion zone vs. CO concentration

**Figure 14.** Test 4 – O2 concentration at the furnace exit

The principal aim of test 4 was to assess the possibility of longer furnace operation, with three bales placed inside the feeding channel. The bales prepared for this test were approxi‐ mately 1.2 m in diameter, and in order to secure stable manual feeding, the gaps between the channel wall and the bales were manually filled with more straw. Problems with feeding undersized bales caused instabilities in the first hour of the test. In the period shown in Fig‐ ures 13 and 14, the temperature was in the desired range, and CO concentration was accept‐ able for most of the period (up to 350 ppm), the only rise in CO occurring at the time of the temperature downfall (minutes 100-110). It was spotted by visual inspection, through the in‐ spection openings, that the bale was not inside the furnace at the time of the downfall, due to the problems with manual bale feeding and cross positioning – the bale forehead re‐ mained inside the tube. This caused the flame to enter the tube at the time, which also occur‐ red during test 5.

**Figure 15.** Test 5 – Temperature in the combustion zone vs. CO concentration

Therefore, it is of great importance to feed the bale uniformly in accordance with the com‐ bustion process, and to maintain this length as stable as possible, by moving the cross ac‐ cordingly. The temperature instabilities (from the minute 45 further on, Figure 11) during this test are a consequence of changes of this length. The only peak in CO concentration co‐ incided expectedly with low temperatures during this period. Nevertheless, this test proved that the adopted concept of the furnace provided good conditions for efficient combustion of soya straw bales, with O2 concentration ranging from 10-14% (Figure 12), and an optimal

60 80 100 120 140

Time [min]

60 80 100 120 140

Time [min]

The principal aim of test 4 was to assess the possibility of longer furnace operation, with three bales placed inside the feeding channel. The bales prepared for this test were approxi‐ mately 1.2 m in diameter, and in order to secure stable manual feeding, the gaps between the channel wall and the bales were manually filled with more straw. Problems with feeding undersized bales caused instabilities in the first hour of the test. In the period shown in Fig‐

Test 4 O2 concentration

Test 4

Temperature in the combustion zone

CO concentration

CO concentration [ppm]

**Figure 13.** Test 4 – Temperature in the combustion zone vs. CO concentration

O2concentration [%]

**Figure 14.** Test 4 – O2 concentration at the furnace exit

Gas sampling probe choked

Gas sampling probe choked

T1 [oC]

average value of λ.

66 Sustainable Energy - Recent Studies

The aim of test 5 was to assess the influence of air flow rate control, with variable speed drives, on furnace performance. During a chosen period of 40 minutes (Figures 15 and 16), optimal air flow rates were obtained and bale feeding was kept stable. The concentration of CO was very low, with O2 concentration varying in the range of 10-15%. The temperature during this period was higher than the desired 850°C (which should not be exceeded in or‐ der to avoid ash melting), which will be taken into consideration in some of the conclusions.

**Figure 16.** Test 5 – O2 concentration at the furnace exit
