LITERATURE SURVEY
CHAPTER 2 IJTERATURE SURVEY
The last stage is char buming. In this stage there is no visible flame with burning occurring on the particle. Any residual carbon is bumt out with the inorganics present in a molten state.
It is important to note that sulphur reduction occurs with sulphur reduced to sulphide.
Temperature and air supply govern the rate of the char burning stage. Unlike the devolatilisation stage, char buming is a relatively slow process and maybe the source of volatile inorganics. (Grace et aI., 1986)
Grace, as cited by Marklund (2002), reports that in this stage, mainly gas phase species react with the organics in the char to produce gases. The following reactions that take place inthe conversion of carboninchar:
C+Y2O2 - ) CO C+O2 - ) CO2
C+CO2 - ) 2CO C+H20 (g) -)CO
C+~Na2S04 -) CO2+Y2 Na2S C+~Na2S04 -)CO+~Na2S
(2-2) (2-3) (2-4) (2-5) (2-6) (2-7)
Reactions 2-4 and 2-5 are gasification reactions and char combustion will be dependent on these reactions when the oxygen concentration is low, as the oxygen available will be consumed by the combustion of gases surrounding the particle. (Marklund, 2002)
Finally, after the residual carbon is burnt off, the char particle collapses into a small droplet of smelt hence smelt fonnation. It consists of inorganic salts namely; sodium sulphide, sodium carbonate, and to a lesser degree sodium sulphite and sodium chloride.
2.3.3 The Sulphate-Sulphide cycle
Figure 2 - 5: The sulphate-sulphide cycle (Grace et aI., 1986)
The sulphate-sulphide cycle (Figure 2-5) permits two contradictOlY processes, namely;
carbon bum-up and sulphate reduction, to take place simultaneously. (Grace et aI., L986)
The main aspect of this cycle is that the direct oxidation of carbon during char combustion does not take place; instead oxygen reacts with sodium sulphide in an oxidation reaction to from sodium sulphate (Macek, 1999). Carbon reacts with the oxygen associated with sodium sulphate and reduces sodium sulphate to reform sodium sulphide. Carbon dioxide and carbon monoxide are also formed. Sodium sulphide then reacts with oxygen and the cycle continues (Grace, 1986).
Grace et a1. (1986) report that the relative rates of sulphide oxidation and sulphate reduction determines the reduction efficiency. For example, if the rate limiting step is the oxidation of sulphide by air, carbon will react with sulphate as soon as it is fonned and keeps the sulphate concentration low thus resulting in high reduction efficiency.
Macek (1999) points out that the desired products of char reactions for a recovery boiler is the fOlmation of sodiwn sulphide and sodium carbonate. Sodium sulphate is also fonned but is undesirable from a chemical recovery point of view as it leads to increased dead-load in the chemical recovery system. However, fonnation of sodiwn sulphate in an oxygen enviromnent is inevitable due to the sulphate-sulphide cycle.
2.4. Concerns with Chemical Recovery Boilers
Some recovery boilers are reaching the end of their service lifespan. The pulp and paper industry are aware of this and are faced with the decision of either renovating the old unit or building a new one.
The thought of having to build a new one is based on the EPA's MACT (maxirnwn achievable control technology) regulation, which requires more stringent and sophisticated pollution prevention equipment to be installed.
Consequently, such equipment is expensive but must be installed in older units in order to comply with environmental regulations. As a result costs escalate to such an extent that these costs are comparable to building a new recovery boiler, especially when downtime is considered.
In addition, the recovelY boiler constitutes a safety hazard. This is due to the catastrophic explosions that plague recovery boilers. These explosions are generally caused by cracked water tubes in the unit. As a result, smelt contact with water causes a rapid generation of steam and ultimately an explosion.
CHAP'TER2
2.5 Gasification 2.5.1 Introduction
I,ITERATIJRE SURVEY
From a technological point of view, gasification has the ability to revolutionise the chemical processing industry with a cleaner way of operating. At a time where enviromnental pollution is prevalent, gasification may provide the answer.
One of the major advantages of gasification is the ability to produce synthesis gas, which can be used to generate electricity, making indusnies less reliant on fossiJ fuel power. In addition, the ability to process a variety of potential feedstocks, such as sewage sludge and hazardous waste, makes it a very attractive and versatile option (Figure 2-6). FUlthermore, the financial aspects of constructing and operating a gasification process, is many times cheaper than constlUcting and operating an incineration process, making gasification an appealing alternative. Also, the great promise of the gasification process is that it may be able to provide a commercially viable recovelY process for the various types of sulphite pulping processes.
Potential Feeds Potential Products
Natural Gas Refinery Gas Residual Oil Orimulsion Petrolewn coke Sewage sludge Waste oils Biomass Black liquor Municipal solid waste Hazardous waste Coal
,
-~
::
'"~
(
---:::
~ '"
----+ Gasification
Combined ~
Plant
Cycle
Slag lor
~
construction materials or metals
Chemical
f+17
recovery
-.
Production~~
~~
Fischer- ~
~~
..
TropshSynthesis ~
Oxygen Nitrogen Argon Carbon dioxide Sulphur/Sulphuric Acid Steam
Hot Water
Electric Power
Hydrogen Carbon Monoxide Fertiliser (Arnmonia,urea, ammonium nitrate) Industrial Chemicals Methanol/ethanol Acetic Acid, acetic anhydride
Naphta Diesel Jet Fuel Wax.
Figure 2 - 6: Potential Feeds and Products (Phillips, 2002)
2.5.2 Definition of Gasification
The Gasification Technologies Council (OTC) has proposed the following defmition of gasification which was reported by Orr et al. (2000).
• "A process technology that is designed and operated for the purpose of producing synthesis gas (a commodity which can be used to produce fuels, chemicals interrnediated products, or power) through the chemical conversion of carbonaceous materials.
• A process that converts carbonaceous materials through a process involving partial oxidation of the feedstock in a reducing atmosphere in the presence of steam at temperatures sufficient to convert the feedstock to synthesis gas, to convert inorganic matter in the feedstock (when the feedstock is a solid or semi- solid) to a glassy solid material known as vitreous fiit or slag, and to convert halogens into the corresponding acid halides.
• A process that incorporates a modern, high-temperature pressurised gasifier (which produces a raw synthesis gas) with auxiliary gas and water treatment systems to produce a refined product synthesis gas, which when combusted produces emissions in full compliance with the Clean Air Act."
2.5.3 Motivation for Gasification Technology
In 1970's, the papermaking industry had to bare the unprecedented increases in oil prices.
This devastated not only the papermaking industry but all industries that were dependent on fossil fuel power. Many governing bodies decided that a need for alternative energy source was required so that a crisis like this could be averted in the future.
Many technological options were proposed but one with greatest promise was the use of gasification technology. Gasification is able to produce a synthesis gas that can be used to generate electricity for the plant via gas turbine technology, making the plant self sufficient.
This meant that;
• the energy/power requirements of the plant are satisfied "in-house".
• reduced dependence on fossil fuel energy for meeting power requirements.
An integrated combined cycle gasification system (lGCC) or gasifier combined cycle (GCC) cogeneration systems was born with the benefit of environmental friendly technology combining to produce electricity. The idea behind [GCe and Gce technology is the same.
2.5.4 Benefits of Gasification
• The technology is enviromnentally sound - "green technology"
;. Reduced SOx and NOxemissions.
, Reduced greenhouse gas emissions.