However, this is based on small scale conditions; information has been lacking to confirm whether this is true in large scale fires. Heat radiation from a gasoline flame will be partly blocked by soot particles, which reduces the emissive power from the flame surface, and soot production increases as the burning area increases due to incomplete combustion. As the flame from an ethanol fire is relatively free of smoke, this means that there will be a relatively smaller reduction of emissive power due to the presence of soot as the burning area increases. This implies that the heat radiation from an ethanol fuel fire will exceed that of a gasoline fire in larger scale, with the difference increasing as the scale increases. The consequence could be an increased risk for fire escalation and the need for improved fire protection systems to manage this risk. Knowledge about the true burning behavior of ethanol is necessary for effective fire protection system design and to make wise investment decisions.
Small scale data confirm the general experience that the heat flux from ethanol is much lower compared to gasoline.
As a first step of the WP5 work, a series of free-burning tests were performed to evaluate the measuring technique to be used in the large scale tests. Three tests were conducted using ethanol denatured with 3% gasoline (E97), a mixture of 85 % ethanol and 15% gasoline (E85), and a 50:50 mixture of ethanol and gasoline (E50). The pool was 1.6 m in diameter, corresponding to an area of 2,0 m2. The amount of fuel in each test was 200 L, providing a fuel depth of 100 mm. As expected, the results show that the heat flux from the ethanol fire (E97) is only about 50% of that generated by commercial gasoline.
Photos showing small scale (2 m2) fire tests using E97, E85 and E50 as fuel. The burning rate, heat flux, and flame height were measured. The scale in the photos correspond to about 1 m indicating that the flame height is about 2,5-3 m for E97, 3-4 m for E85 and 4-5 m for E50.
Diagram showing the heat flux at various distances from 2 m2 fires with various fuel mixtures of ethanol and gasoline.
Large scale free-burning tests shows that the heat exposure from ethanol fires is 2-3 times more than for gasoline!
Two large scale free burning tests were conducted, one using E97 and one using E85. The goal of the tests was to obtain full scale measurements on burning rate, flame height and heat radiation at various distances for the most common ethanol fuels handled and stored in large volumes. The tests were conducted in a circular pool with a diameter of 18 m, resulting in a fuel surface area of 254 m2. 20 000 L of fuel was used in each test.
The tests were performed at the end of August 2012 at a fire brigade training center in Borlänge, Sweden. The burning rate was measured in the center of the pool using an array of thermocouples. The heat radiation was measured at various distances up to 40 m from the rim of the fire in four directions from the pool center. The overall fire development and flame height were measured using several video cameras located in different positions. A thermal imaging camera was also used as a complement to the heat radiation measurements.
Both tests were made under similar weather conditions providing a good opportunity to compare the results from the two fuels. All measurement data have now been carefully evaluated and compared with test data and computer calculations of similar gasoline tests. The results confirm the suspicion that the heat flux from a large scale ethanol fire might exceed that generated by gasoline. In fact, the test results show that the heat exposure can be as much as 3 times higher for ethanol than for gasoline! In case of a larger fire area, the difference would likely increase even more! These results show that the risk for fire escalation is significantly higher in an ethanol tank fire situation compared to a similar gasoline fire. The increased heat exposure will also result in an increased heat exposure to firefighting personnel, making their work more difficult. Considering that ethanol is also a much more difficult fuel to extinguish, there is a strong need to develop effective methods by which to extinguish an ethanol tank fire in a safe and efficient manner.
A full report from the free-burning tests can be downloaded here.
A video camera was used to record the flame height. These images from the E97 test (left) and the E85 test (right) are taken 7:13 minutes from ignition.
Relative comparison of the heat flux from the E97 fire test compared to calculated and measured data for similar gasoline fires.
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