Hanam Canada Corporation

Pipeline Gas From Coal

Gas from coal is competitive with imports including LNG. Pipeline gas is produced in Beulah, ND, at a rate of 160 million standard cubic feet/d. There are 14 gasifiers each 13.5 feet diameter by 40 feet high hung from support steel in two rows.

Hanam has been engaged in a study of opportunities for expanding the Beulah coal gasification and coal derived pipeline gas plant. We obtained quotes for used gasifiers and newly fabricated ones based on the existing design. We assessed the market for additional coal derived gas for existing pipeline customers and completed separate market studies for byproducts: phenol, cresols, carbon dioxide and argon.

Gasifier system 

The first cylindrical coal gas producer in the US was built in 1880 by Charles Morgan in Worcester, Mass... By 1941 Morgan Construction had built and installed more than 9,000 gasifiers. Riley Stoker, now part of B&W, and Welman Thermal Systems offered gasifiers to industrial plants until the early 80’s. These were similar to DG’s gasifiers but most used air and operated at a lower pressure. Although the gasification technology is old, it is still the best available technology for the Dakota coal. DG’s coal gasification process and equipment design was licensed from Lurgi Mineraloeltechnik GMBH, Germany in 1973. South African Coal, Oil and Gas Corporation Limited (Sasol), hired as a consultant, tested Dakota coal and helped design the rotating ash grate. Lurgi sold its gasifier business to Sasol-Lurgi Technology Company, a subsidiary of Sasol.  

Coteau Properties mines and crushes the coal. They try to keep the lumps as large as possible under 2.5 inches. Coteau blends coal to maintain uniform heating value, moisture, ash, sulfur and sodium content. Variations in these parameters affect DG gas production. Coteau Properties and DG screen out the fines, less than about 3/8 inches and send them to the Antelope, Leland Olds, and Coal Creek power stations. The preferred size range for fixed bed gasifiers is 2 by 1 inch. Smaller size e.g. 5/8 by 3/8” reduces the gasification rate 40%. Riley Stoker reported a decrease of 75% in gas production rate per square foot of reactor area with the addition of 25% fines below 1/8 inch compared with a graded 1 ½ to ¾ inch fuel.

Coal Properties 

A uniform coal particle size avoids the problems of channeling at the gasifier wall. The sensitivity to size is related to fuel bed permeability. With increasingly smaller particles, bed pressure drop increases, weak points become susceptible to channeling; temperatures within the bed become more variable and off spec gas is produced.

The Hargrove grindability, a test of the amount of fines after a fixed time of grinding the coal, is relatively high, 68. The coal is easily broken down in the coal handling and gasifier feed system. Non uniform coal is more difficult to distribute evenly in the gasifier. Soft coal can break up as it drops into the gasifier and may more easily crumble.

When coal is dropped into the reactor, devolatilization occurs in the upper portion of the fuel bed. At high heating rates, a steep temperature gradient is produced throughout the large coal particles. Under these conditions the outer layer of the particle exists in a plastic or liquid state for a short period and then forms an outer shell with a deeper plastic layer underneath. The shell is strong enough to restrict further expansion of the coal particle.

The formation of this shell can be influenced by adjusting the ratio of steam to oxygen flow through the gasifier. A high ratio of steam to air reduces ash sintering or fusion. Coal with a low ash fusion temperature may require excessive steam to control slag formation. Some coals require pretreatment (oxidation) or mechanical charge agitation. The Freedom coal has a relatively low ash fusion temperature, 2,330 ^oF initial deformation under reducing conditions.  The range of ash fusion temperatures around this average, 1,940 to 2,700 ^oF is quite wide indicating that the coal is fairly variable.

A low swelling index of the coal, below 1.0, is desirable to reduce swelling and breakup of coal particles in the gasifier. With a low swelling coal, the temperature and heating rate in the upper fuel bed do not need to be as carefully controlled. A deeper higher capacity coal bed can be used. Bridging in the gasifier is avoided. Less mechanical stirring at the top 6 inches of the bed is required and its associated fines production is reduced.

Coal reactivity is a measure of the speed of reaction in which carbon is transformed to carbon monoxide in the gas stream. An advantage of the Freedom coal is its high reactivity.

The high moisture content of the coal means that the upper part of the gasifier bed is used for drying the coal. The capacity of the gasifiers could be increased if the coal were dried separately. But driers would break down the large pieces of coal required for optimum gasification.

The behavior of ash in the fuel bed and its separation and removal from the gasifier is an integral part of the process.  Formation of clinkers is one problem. Ash fusion temperatures provide one indication of the clinkering temperature. Other factors influencing clinkering are the quantity of ash, fuel size, bed permeability, gasification rate, and fuel reactivity.

The optimum point for a dry bed gasifier is controlled clinkering, the production of small granular pieces of a fused ash. Dakota coal has a relatively high 22% proportion of the natural flux calcium oxide. The ash produced is relatively fine and in the form of cemented particles rather than fused masses. It is easy to remove. Another advantage of the calcium is that it retains about 40% of the 0.6% sulfur content of the coal.

The two oxygen lines produce a max of 3,400mcf/h (3,640 tonO₂/d) at 510 psi header pressure. About 1,000 ton/d of the nitrogen is used on-site to make ammonia. There are two 25,000 HP main compressors. One is steam turbine driven, the other is electric driven. The plant runs at 3,260 mcfh very close to capacity. The economic calculations are based on purchasing additional oxygen from an on site cryogenic oxygen plant and purchasing power for the plant at $0.06/KWH. This assumption results in a very conservative cost for additional oxygen of $20/ton for the plant plus $19/ton for the power. There are several used oxygen plants available that could reduce the cost.