Steam Property

The relative humidity of steam is always 100%.

Producer gas is what you get when you blow air through the coal (or coke) to raise its temperature after a run of blowing steam through, which produces water-gas. A limited amount of air is used so the coke only burns to carbon monoxide (flammable) so is a useful fuel gas (shame about the nitrogen!).

When the coke is white-hot the air is swapped for steam and water-gas is made. This is a mixture of hydrogen and carbon monoxide and has a much higher calorific value than producer gas, but the reaction absorbs heat so the coke cools, hence the need for the air blow when it reaches red heat.

Cheers........Codey

Hi Govind Rao,

As has been said the 'blow' to continue combustion and raise the bed temperature yields producer gas which has a low CV as there is a lot of CO2 present as well as the nitrogen. The steam injection phase produces 'blue water gas' which is generated without nitrogen and still contains CO2 in lesser quantities as some combustion occurs as the 'cracking' of the water occurs.

The important aspect of the steam injection phase ,as you enquire ,is to ensure that it is pure steam free from consdensate droplets as this serves only to cool the bed prematurely and therefore shorten what is the most useful part of the production cycle. A good separator is needed together with a PRV to induce aa good a dryness fraction as possible into the steam flow. Typically starting with 100psig steam dropped to 15 through the PRV and then put onto the tuyeres will give a good effect.

The blow to steam cycle is the critical aspect of the process and is usually carried out by an experienced operator. All late 19th century and early 20th C gas works worked on this principle as the second stage of going from coal to the final product called 'coke breeze' which was what remained in the retort at the end of the cycle.

The real skill in those days was juggling the whole process so that the mixture of gasses in the distribution network did not fall below 450BtU/ft3 as the this was the minimum declared calorific value considered acceptable to the customers.

Interestingly the consequences of excursions below this figure was the tendency for gas mantles to expire with a loud 'pop'. Guaranteed to really upset the burgers!

Good luck,

Massey.

Hello Massey

Interesting what you say about need for dry steam. Was the use of superheated steam ever tried? Possibly no advantage as the heat has to come from somewhere.

Cheers......Codey

Hi Codemaster,

I think you have got to set all this in the context of an actual gas works which because of the cyclic chain of events meant that steam was produced intermittently, as and when the 'full combustion process' was on the go and hence the stack gases were being passed to atmosphere. Therefore the steam drums on the gas works boilers were large in order to provide a reserve of steam during the other parts of the operating cycle; remember that the plant auxilliaries were steam driven also, gas boosters and ID fans.

So as you suggest the steam or heat energy had to be used carefully to get the best out of the process. I believe that both gas quality and quantity per charging of the retort varied considerably with the level of skill an operator brought to bear.

Therefore the presence of superheaters in the line-up would not be very cost effective and at 100 psig of limited benefit at best.

My family was involved in a municipal gas works in the 1920s and some of the stories were both hair raising and highly entertaining as most involved unexpected explosions not usually with fatal results but inevitably resulting in the demolition of tin huts and the shattering of office windows. It all seemed to be taken in good part with 'service as normal' being resumed in short order after most of these dramas.

Best wishes,

Massey.

As an interesting side note, I have noticed that established house fires, with their wood as a carbon source are well positioned to operate as partial water gas reactors...thus failing to quench the flames. True, the process is endothermic and so does perform some cooling, but far better if a totally inert gas was used.

I have thought of the use of a tanker truck of liquid nitrogen or compressed CO2 being used. I can see no flaw in it, save the cost and the difficulty of getting the tanker to the site. Water is in the street watermains = a lot less of a problem.

At airports they use a foam that coats as well as snuffs fires. Does that foam have a CO2 component? It seems to work very well, but I am sure it is costly, however, would we save $$ overall by saving structures by degree of combustion, or is a building a writeoff once the fires has gripped it, even if you snuff it faster?.

Hi Massey,

Thanks for the insight on the use of steam. Can we quantify the amount of steam to be injected so as to get the best out of the coal ? Is it possible that a certain amount of CO and H2 at the exit is consumed due to the presence of air from the blower?

Govind

Hi Govind Rao,

There will be many factors governing the quantity of steam used. Things such as the mass of the charge involved, its starting temperature and the degree of penetration into the mass. perhaps the manufacturer could assist. You are quite right when you say that any contiuing blower action during the steam injection phase will permit combustion of some of the blue water gas to occur.

I am given to understand that the operator would endeavour to shut down the blower as and when the bed temperature was at the optimum state and then put on the steam , therefore the atmoshere in the retort would rapidly become anoxic and combustion virtually ceasing whilst the steam was on. Clearly as this is an endothermc process it should not continue beyond the point at which the bed temperature falls below the reactive limit.

The plants of olden days featured a series of retorts connected to a common output line and sharing both a common steam generator and blower. The process therefore became a continuous one as the objective was to maintain the retorts at different stages of their regeneration and production cycles at any one time.

You do not say what level of instrumention you might be intending to use or what type of automation you would be using.

Best wishes,

Massey