A analysis of the possibilities of burning petcoke

        Pet-coke is an excellent fuel for boiler and termic fluid heaters because it is generally very cheap, burns cleanly and is difficult to burn with other combustion . When co-fired with coal (or most other FBC type fuels) up to about 80% by weight, pet-coke is very well behaved and typically does not cause any problems. It can even reduce furnace tube erosion, because much of the abrasive coal ash is displaced by less abrasive limestone reaction compounds. When fired by itself there are some potential issues that need to be addressed, but since that is not an issue here I won't go into them. 

           Pet-coke comes in two basic flavors: fluid or delayed. Fluid coke will have a very small particle size and very low volatile content. Delayed coke can vary tremendously in particle size and has a higher volatile content. Delayed coke can be further broken down into "shot" or "sponge" coke, which are differentiated only by their particle shape. Shot coke looks like BB's and sponge coke looks basically like crushed coal. 

         The main characteristics of coke that differentiate them from most coals is their low ash and volatile content (but relatively high reactivity to combustion) and relatively high sulphur content (1%-8%). 

         The low ash content is a benefit, because it means less inert material to be disposed of. Typically more than enough bed material is generated by the limestone and co-fired fuel. Although lack of ash for bed material could theoretically be a problem.

          The low volatile content results in somewhat more even combustion of the fuel in the fluidized bed, because it is actually the fuel particles that are burning rather than devolatilized gases. This results in somewhat improved sulphur capture performance (but don't forget that more sulphur is probably going in so you will still need more limestone). To my knowledge CO emissions have not been a problem (even at part load) with pet-coke. The low volatile content also seems to help reduce NOx emissions. Although not everyone has experienced a reduction in NOx emissions when firing pet-coke, most have. This is probably also, at least partly, due to the low volatile content of the pet-coke. 

            There are a couple of potential negatives that have to be evaluated. The first negative is the relatively high sulphur content. In most cases, co-firing pet-coke will displace a lower sulphur fuel with a higher sulphur fuel. This means that to maintain the same SO2 emissions levels, the limestone federate will have to be increased. In some cases, the existing limestone system will have sufficient capacity to co-fire the desired amount of limestone. In others, the system may be too small and require upgrading. At least one UNIT that I am aware of limits the amount of pet-coke that they fire based on the capacity of the limestone system. A corollary of the higher limestone federate is that the bottom and fly ash removal systems need to be able to get rid of the additional limestone. In most cases, there is more than enough capacity built into those systems to accommodate the switch to pet-coke. 

The other negative aspect of pet-coke firing is the presence of vanadium and nickel in the ash. Most environmental regulation agencies have some kind of regulations governing the amount of vanadium and nickel allowed in ash that is being disposed. I am sure that this varied by location. Typically the most important aspect is the leachable portion of the compounds, but total concentration may also be an issue. The good thing is that lots of testing has been done to determine the leachability of the vanadium and nickel and both have been found to be stably bound in the ash. The harder part related to this is getting a hold of the information and satisfying your local regulators that you aren't trashing the environment (actually its probably not that hard, but it does require some research and work). Another good thing is that when co-firing, it is very likely that the total concentration of vanadium and nickel in the ash will be well below any regulatory limits (this still requires a bit of work and research to verify). 

A new generation of industrial heater fules : PETCOKE

Introduction:

            Petroleum coke (often abbreviated pet coke or petcoke) is a carbonaceous solid delivered from oil refinery cooker units or other cracking processes. Coking processes that can be employed for making pet-coke include contact coking, fluid coking, flexi coking and delayed coking. Other coke has traditionally been delivered from coal.
                     This coke can either be fuel grade (high in sulphur and metals) or anode grade (low in sulphur and metals). The raw coke directly out of the cooker is often referred to as green coke. In this context, "green" means unprocessed. The further processing of green coke by calcining in a rotary kiln removes residual volatile hydrocarbons from the coke. The calcined petroleum coke can be further processed in an anode baking oven in order to produce anode coke of the desired shape and physical properties. The anodes are mainly used in the aluminium and steel industry.
                     Pet-coke is over 90 percent carbon and emits 5 to 10 percent more carbon dioxide (CO₂) than coal on a per-unit-of-energy basis when it is burned. As pet-coke has a higher energy content, pet-coke emits between 30 and 80 percent more CO₂ than coal per unit of weight. The difference between coal and coke in CO₂ production per unit energy produced depends upon the moisture in the coal (increases the CO₂ per unit energy – heat of combustion) and volatile hydrocarbon in coal and coke (decrease the CO₂ per unit energy). 

Types of petroleum coke:
             There are at least four basic types of petroleum coke, namely, needle coke, honeycomb coke, sponge coke and shot coke. Different types of petroleum coke have different micro structures due to differences in operating variables and nature of feed stock. Significant differences are also to be observed in the properties of the different types of coke, particularly ash and volatile matter contents.^[

                  Needle coke, also called acicular coke, is a highly crystalline petroleum coke used in the production of electrodes for the steel and aluminium industries and is particularly valuable because the electrodes must be replaced regularly. Needle coke is produced exclusively from either FCC decant oil or coal tar pitch.

                  Honeycomb coke is an intermediate coke, with ellipsoidal pores that are uniformly distributed. Compared to needle coke, honeycomb coke has a lower coefficient of thermal expansion and a lower electrical conductivity.

Fuel grade coke:

           Fuel grade coke is classified as either sponge coke or shot coke morphology. While oil refiners have been producing coke for well over 100 years, the mechanisms that cause sponge coke or shot coke to form are not well understood and cannot be accurately predicted. In general, lower temperatures and higher pressures promote sponge coke formation. Additionally, the amount of heptane insolubles present, and fraction of light components in the cooker feed contribute.

                 While its high heat and low ash content make it a decent fuel for power generation in coal fired boilers, petroleum coke is high in sulphur and low in volatile content, and this poses environmental (and technical) problems with its combustion. To meet current North American emissions standards, some form of sulphur capture is required, a common choice of sulphur recovering unit for burning petroleum coke is the SNOX Flue gas desulfurisation technology, which is based on the well-known WSA Process. Fluidized bed combustionis commonly used to burn petroleum coke. Gasification is increasingly used with this feed stock (often using gasifiers placed in the refineries themselves).

Calcined petroleum coke:

            Calcined petroleum coke (CPC) is the product from calcining petroleum coke. This coke is the product of the cooker unit in a crude oil refinery. The calcined petroleum coke is used to make anodes for the aluminium, steel and titanium smelting industry. The green coke must have sufficiently low metals content in order to be used as anode material. Green coke with this low metals content is referred to as anode grade coke. The green coke with too high metals content will not be calcined and is used for burning. This green coke is called fuel grade coke.

Desulfurization of petcoke:

           A high sulphur content in pet-coke reduces its market value and may prevent its use as fuel due to restrictions on sulphur oxides emissions for environmental reasons. Methods have thus been proposed to reduce or eliminate the sulphur content of pet-coke. Most of them involve the desorption of the inorganic sulphur present in the pores or surface of the coke, and the partition and removal of the organic sulphur attached to the aromatic carbon skeleton.
            Potential pet-coke desulfurization techniques can be classified as follows:

1. Solvent extraction
2. Chemical treatment
3. Thermal desulfurization
4. Desulfurization in an oxidizing atmosphere
5. Desulfurization in an atmosphere of sulfur-bearing gas
6. Desulfurization in an atmosphere of hydrocarbon gases
7. Hydrodesulfurization

As of 2011 there was no commercial process available to desulfurize petcoke.

Health Hazards:

         Petroleum coke is sometimes a source of fine dust, which can get through the filtering process of the human airway and lodge in the lungs. Once these small dust particles lodge in the lungs they can cause serious health problems.

             Petroleum coke can contain vanadium, a toxic metal, in sufficient quantities to poison people. Vanadium was found in the dust, collected in occupied dwellings near the petroleum coke stored next to the Detroit River. Vanadium is toxic in tiny quantities, 0.8 micro grams per cubic meter of air, according to the EPA.