A gas-phase explosion in a storage tower with semichemical pulp at a paper mill has possibly been caused by combustion of a mixture of hydrogen and air. The hydrogen was formed by microorganisms in the pulp. Ignition may be due to electric sparks in connection with an electric field in the mist above the pulp.
A gas-phase explosion took place in a 1,300 m3 storage tower for semichemical pulp at a paper mill. The storage tower was 21 m high and equipped with an agitator at the bottom. By a pumping arrangement, the pulp was circulated from the bottom to the top through external pipes connected with the mill (Fig. 1).
On a given day the production was stopped at a time when the storage tower was loaded with 1,000 m3 pulp at a temperature of 60-65°C and a pH of 5.5-6.0. Simultaneous with the production stop, the circulation pump was stopped, whereas the stirrer at the bottom remained operational. After 24 hours of down time, mill production restarted and pulp was pumped from the tower to the mill and from the bottom to the top of the tower. About three hours after production start, a violent explosion took place in the tower, blowing out the cover over the manhole and deforming the top of the tower into a spherical shape. At the moment of explosion, the tower contained approximately 600 m3 of pulp, and recordings from the load cell showed momentarily full-scale deflection. Large amounts of steam mixed with droplets of pulp were spread over a wide area. There was a distinct smell of SO2 at the top of the tower and also the smell of H2S from samples of pulp from the bottom.
Due to the smell of SO2 and H2S at the time of explosion, it was decided to analyze pulp samples for H2S and CH4. Samples taken from the storage tower before and after the explosion were analyzed, but only traces of H2S were found. Microbiological examinations, however, showed that pulp samples from the paper mill were heavily contaminated by microorganisms. In order to check the possibility of gas formation by these organisms, a number of pulp samples from the paper mill were collected in 6L flasks equipped for gas collecting. One of the flasks was inoculated with pulp taken from the tower just before the explosion. All samples were incubated at 60°C. Within 24 hours, gas was produced in all flasks, about 300 mil in each. Gas chromatography and mass spectrometry showed about half of the gas to be H2, the other half being a mixture of CO2 and N2 in equal amounts. H2S and CH4 were not detected, but analysis taken few weeks later showed small amounts of H2S (50 ppm) in the gas phase.
This case illustrates the hazards connected with microbial gas formation under uncontrolled conditions. Thus, depending on circumstances, a storage tower for pulp may act as a fermentation tank if required precautions are not taken. Cellulose, sugars and residual pulping chemicals in the pulp may be utilized by microorganisms, the temperature of the pulp (60°C) being ideal for thermophile microorganisms having a temperature optimum between 55 and 75 °C. The extent of microbial growth will depend on the microbial contamination as well as type and condition of the pulp. Under anaerobic conditions, the microorganisms get their energy by fermentation, and H2 is one of the products that could be formed. Indeed H2 is produced under a wide variety of anaerobic conditions and is usually utilized by other microorganisms, yielding other products like CH4 and H2S.
The laboratory tests and analysis indicated that H2 had been the main combustible gas produced by microbial activity. In this case, it is not likely that the H2 produced in the pulp was utilized by methane-producing bacteria. The tests also indicated that only small amounts of H2S had been produced. The smell of H2S and SO2 previously referred to may be explained by the fact that these gases are smelled at very low concentrations.
During the plant stop, some of the gas may have been entrapped in the pulp (4 percent fiber content) until circulation started. By pumping the pulp to the paper machine, air was sucked into the tower and mixed with the gases. H2 and H2S, being combustible and explosive gases in mixtures with oxygen or air within wide limits, may have caused the explosion by ignition.
As ignition of the gas mixture could not be explained by open fire or electrical installation, it was assumed that electric sparks in connection with an electric field in the mist above the pulp had caused the explosion. It is well known that electric charges bound to a mist of tiny water drops accumulate in cargo tanks of oil tankers cleaned with water jets, and that electric sparks caused by these charges may ignite explosive mixtures in the tanks. Electric charges of the same density (-15 to -18 nC/m3) as found in crude oil tankers, have later been measured in the storage tower during normal production at the paper mill. The ignition energy for mixtures of air and H2 is only 1/20 of the energy required for ignition of vapor from crude oil.
- Electrical Equipment
- Process Equipment
- Pulp Storage Tank
The accident proves that under special circumstances, the potential hazard connected with unforeseen microbial gas formation and accumulation should not be neglected. Thus, to avoid new accidents at the paper mill described above, the tower has now been equipped with a high-capacity fan for dilution of possible gases with air. Precautions are also taken to reduce the microbial contamination.