Failure Knowledge Database - 100 Selected Cases

Hydrogen Gas Explosion in Non-industrial Refuse Incineration Facility【July 6th, 1995

Isehara, Kanagawa, Japan】

Masahide Wakakura (Kanagawa Industrial and Technology Research Institute)
Mitsuo Kobayashi (Graduate School of New Frontier Sciences, University of Tokyo)
Masamitsu Tamura (Graduate School of New Frontier Sciences, University of Tokyo)

At around 10:00 on Thursday July 6th,

199 5, an explosion happened at an

incinerator of the envi ronmental s anitation union incin eration p lant of Hatano a nd
Isehara cities in Isehara City , Kanagawa. The in cinerator had a processi ng capacity of
90 ton/day. Three workers got burnt by high -temperature gas that spouted from theinspection door, and one of them died after ten days. The accident happened during theinspection and repair work of the inside of the furnace for fixing an abnormality in theash chute damper of the incinerator. In the repair work, the worker s injected water toremove some blockage. The water reacted with incinerated ash of aluminum and othermaterials to generate hydrogen. This hydrogen seemed to cause the explosion.
Although the damage to the facilities was no t very large, one inci nerator of the othertwo in the plant was ofthe sa me type as the incinerator thatincinerators of the three could not

exploded, s o tw o

be used until the cause of the acci dent was madeclear, and s ome part of materials t o be in cinerated had to be carried out t o anot her
incineration plant temporarily in the adjacent area.
The cause of the accident was the formation of hydrogen by a mechanism that no one
could have i magined. Furthermore, the cause of the hyd rogen formation was believedthat non-industrial waste containing a large amount of aluminum was carried into th e
incinerator. Although th ere ar e many kind s of r efuse which ar e not permitted t o betreated by burning in non-industrial refuse

incinerators, actually it is not possible toseparate all of the refuse by the insp ection at receiving. In this se nse, the accident wasinevitable, but a similar accident had also occurred in February 1983. After the court
trial that continued for a long ti

me, the ca uses of the accident and matters to beconsidered in op eration of incine rators we re reported in the jou rnal of theSociety in 1994. If thisWastereport had been gi ven sufficient consideration, and if i t h ad

been used f or w orkers’ education, this acci dent might have been avoid able. H owever,
the report

was about

the incinerated ashthat had accumulated

in an el ectricprecipitator, and it might be difficult to relate the two accid ents though the causative
agent in both cases was aluminum in the incinerated ash.

1

Failure Knowledge Database - 100 Selected Cases

1.

Event

At around 01:00, the ash chute damperof theNo.1 incinerator, which wa s acontinuous stoker typ e, showed an abnormality (R efer to F ig.1). The m oisture in t herefuse that is put into the hopper of the incinerator is r emoved in th e drying zone.
Then, the refuse is moved to the combustion zone. Hot air is supplied from below in thedrying zone and the combustion zone. The monitor inside of the incinerator showed theaccumulation of ash in the post-combustion pa rt of the incin erator. So, t he ash p usher
in the post-combustion lower secti on was operated manually , but the a sh pusher did
not work well and the workers gave up combustion and stopped the combustion furnace.
They st opped charge of refus e t o t he c ombustion furnac e and started the op eration
called “fire burial”, which finished at around 04:00.

After the day shift workers took over the operation, they started the inspection work.
The operators saw that the ash w

as filled in the post-combustion zone when they

opened the inspection door. They tried to remove the ash from the inspection door usinga shovel, but they could only remove the ash slightly becau se there was a solid layer,
which seemed to contain a clinker. A clinker is something like a volcanic rock, which isformed by

the solidification of some molt

en material such as high temperature

2

Failure Knowledge Database - 100 Selected Cases

aluminum with adhesion of the incinerated as h. The explosion occurred inside the ashchute, when the operators were trying to crush the clinker with a long prodding chiselwith injecting water int ermittently. Besides, water injection was not carried out when
this explosion occurred. Owing to exposuret o high-temperature gas and ash whichspouted out from the inspection door, the three workers received serious burns.
2.

CourseAt around 01:00 on July 6th, an abnor mality was found i n the ash chute damper .
Immediately, wo rkers c hecked the monitor mounted inside th e incinerator and foundan accumul ation of c ombustion as h. Th e ash pusher, whi ch is d esigned to p ush the
combustion ash into the ash p

it, was op erated, but it d id not op erate well. The

continuation of incineration was judged to be impossible. Charge of refuse was stopped
at around 01:45, and in order

to in spect the in side of the f urnace, p reparation of thefire burial work was started.
The work finished at around 04:00. The day shift crew took over the operation in thiscondition.
At around 08:20, they started an inspection and opened the incinerator. At 09:59, theexplosion occurred.
3. Ca use
The exp losion was estimated to ha ve occurred as follows: a combustible gas mainlycontaining hydrogen was formed in the ash chute and in the ash pusher, the gas mixed
with the air from the inspe ction door to fo rm a combustible gas-air mixture, and the
mixture was ignited by hot clinker or some other ignition sources.
The hydrogen was supposed to have been generated by a chemical reaction between
the water i njected into the ash chute and the hot alumin um contain ed in the clinker
and in the ash, and this hypothesis was confirmed by experiments. Although aluminum
is not allow ed to be put in the incinerator, a large amount of aluminum was containedin the non-industrial waste since the non-industrial waste was not correctly separated.
The water was injected into the furnace in order to help remove a blockage of the ashchute. A common reason why a bl ockage is produced in the ash chute is the generationof a clinker. The generation mechanism of a clinker is not sufficiently elucidated, but it
is reported from experience that a cl inker is easily formed when crushed material withhigh caloric value anda highcontent of aluminum is i ncinerated. The alumin um

content th e incombust ible resid ue of theincinerator was as much as 24%, and it

crushed mat erial incinerated in thiss caloric value was high. Th

3

ere isnoFailure Knowledge Database - 100 Selected Cases

information on what is crushed material containing a large amount of aluminum.
Blockage in the ash chute and insufficient operation of the ash pusher were indicatedas the ind irect caus es o f the accid ent. Th e bulk d ensity of the c ombustion ash at thetime of the accident was several times larger than the designed value of the ash pusher.
Although th e reason w hy the bulk density was large was not cl early describ ed, therefuse with a high aluminum content seemed to be one of the causes.
From th e a bove, the tr ue caus es of this accid ent were following two; combustion
processing of non-industrial waste contamin ated with crushed materia ls of metal s uch
as aluminum that should not be burned in an incinerator was conducted. Moreover, it
was known that a clinker was easily formed from the mat erial having a high caloric
value and a high aluminum content, and in addition, there was a report of an explosionthat was si milar to the accidentof this incinerator in the p receding year . If the

managers and engineers had coll ected the information and conducted the managementof the facilities with a sufficient safety consci ousness, it seems that the accident could
have been avoidable.
4.

Process of cause elucidation

From the conditions of the explosion, the possibility of a st eam explosion caused by a
hot clin ker and a ga s exp losion caused by gen eration of co mbustible gas w ere
considered. It was concl uded that it was not a

steam exp losion but a gas exp losion

based on the following reasons.
1) It was guessed that there was acombustio n exp losion because t here w ere

discoloration and peeling of anti-corrosive paint at the places that were damaged,
and the steel plate itself burned. Discoloration is not caused by steam explosions,because steam explosions cannot g enerate a te mperature high enoug h to cause
oxidation.
2) The exp losion occurred one and a half hours after the star t of the water inje ction,and the water injection was not executed at the time of the explosion.
3) Fro m th e site inves tigation, th e explosion seemed to have o ccurred in th e a shchute. This p osition is higher tha n the water injection position, and

it is also

higher than the cooling section of the incinerated ash. Therefore, the direct contactof the ash with the injected water is considered to be difficult.
Beyond on the hypothesis above, th e amount of gas gen erated inside the incinerator
and caused the gas explosion was estimated. From general information from literatureand other s ources, th e aluminum content in the ash was estimated. In the clinker , it
was 1 1.6%, and in theincinerated ash, it was 9.0%. This value is hi gher than t he

4

Failure Knowledge Database - 100 Selected Cases

reported value of 6 to 8%. It was confirmed through an experiment on the generation ofhydrogen gas by injecting purewater tothis incinerated ash that hydrog

en isgenerated when the ash coexists

with wate r of which p H value became 12 by thealkaline co mponent in the incin erated ash. C onsidering a wid e exp losion range ofhydrogen, there is a large possibility of the explosion by hydrogen gas.
5.

Immediate action
Among th e three incin erators operated by the p lant, two incine rators w ere stopped

until the investigation of the cause ended. One of the furn aces that were st opped was
the one where the accident happened, and the other one was the same t ype as the o ne
involved in the accident. The accident investigation committee was established the day
after the accident to investigate the cause and to study the prevention measures for theaccident. Besides, treatment of the waste that s hould be handled at the se incinerators
was entrusted to the w aste incinerators in the neighborin g area until the accident
cause was cleared.
6.

Countermeasure
From th e operation p erspective, in cineration treatment of crushed material with ahigh aluminum content was stopped, ni

trogen was introd

uced for p

urging a

combustible gas generated when bl ockage was rem oved, s ufficient cooling time a ndsafety confirmation steps were ex ecuted pr ior to the removing work, water inject ion
was stopped, etc..
At the faci lities, an industrial camera

and the rmometer for earl y d etection ofblockage w ere p repared, and th e capacity of the as h p usher was increa sed.
Furthermore, for management aspects, a work standard was prepared.
As administrative countermeasures, a warning of the separation of refuse was issued
using a citizen public relations magazine, and it emphasized the complete separation of
non-combustibles and combustibles.
7.

Knowledge
Accidents related t o treatment of non- industrial refus e occur rat her fr equently.

Although t he s eparation of c ombustibles and non-co mbustibles is car

ried out,

sometimes the separation may be insufficient and d angerous situations occur . A t thisplant, a large kettle w as found in the incine rator, or pesticides contaminated and apoisonous and bad smell gas was generated at the non-combustible dis posing facilitiesof the p lant. It may

be nec essary for th e persons concerned to5recognize t heFailure Knowledge Database - 100 Selected Cases

non-industrial waste disposing facilities as dangerous material facilities.
There was an earlier accident t hat wa s similar although not completely identicalwith this accident. If th e managers and engi neers had been consci ous t hat they we reoperating dangerous fa cilities, they could

have taken more effective measures toprevent acci dents by referring t o the earlier accident. A positive attitude of learni ng
from th e mi stakes of ot hers and establishment of th e syst ems f or collecting outside
information are important.
8.

Influence of failure

The huma n d amage from the ac cident consis ted of thr ee work ers h ospitalized forburns, and one wo rker died ten d ays later. Ph ysical damage included deformation of
the ash chute and damage to the humidifying cabin.
Damage to the administration oc curred beca use the waste treatment capacity of the
facilities decreased, and a request had to

be made to

the munici palities intheneighborhood to hand le the ex cess wastes. It seems that the p rocessing cost inc reased
and the quality of the refuse collection service lowere.
9.

On the side
Accidents at non-industrial refuse treatm ent facilities are unexpect edly frequent.

Some accident examples are describ ed in T able 1. In 1995, at treatment facilities in
Saitama, a dust explosion occurred as a resu lt of too mu ch paper refus e being carried
in. There are many accidents that can not be imagined at many places. A hydrogen gas
explosion that is similar to the accid ent described here also occurred around 1983, andthe cou rt tri al concernin g this accident co ntinued for 1 1 years. The p erson co ncerned
described that he could not open an y technical report about the causes of the accident
during the court trial period. If the caus e was announced earlier with sufficient PR,other accid ents might h ave been p revented. Although each accid ent ex ample is also
important, by grasping the co mmon o r “uppe r” conc ept of the accidents, it bec omes
possible to contrast th e knowl edge in the ex amples with an ind ividual case andtherefore failure information can be utilized more effectively.
Again, it is important that the cause of the failure is communicated even if it iscurrently just speculation. It is b etter not t o wait for the conclusion of the court t rial
and the formal accident investigation.

6

Failure Knowledge Database - 100 Selected Cases

generation place
Higashi-Kurume,
Tokyo
Asaka, Saitama

Kawasaki,Kanagawa

datetype of facilities
content of the accident
1981.1.14 Public,
non-industrial fire in the operation and restartrefuse incinerators
between an incinerator and anelectrostatic precipitator
1995.3.7
Public, non-industrial Paper powder in corrugatedrefuse incinerators
fiberboard boxes caused a dust
explosion in the refuse charge
division
1997.8.15
Private, A Gen eral Crushed, a nd not cut refus erefuse incinerator
stops in the hopper, cause a fire.

Table. 1 Accident Example of Non-industrial refuse
Reference・Kenji Yasuda, Hideo Tagota、Takashi Miyagawa and Yasuo Shimizu、” Hydrogen gas
explosion in non-industrial refuse inci

neration facility in Kanagawa Prefect”,

J.Japan Soc. for Safety Engineering, 36, 183-187(1997)
・Koh T akatsuki, “L esson of collected d ust bun ker exp losion” W aste Academic J ., 5,441-447 (1994)
・Health committee letter of the Isehara City, The No.10th, (1995)

7