For a mixed feedstock, the success of the gasification process depends on the reactor technology that carries out the desired objective with variable feed properties.

7.4.2.1 Combustion

For coal and biomass mixture combustion, in general three types of com­bustion systems can be identified: fixed bed combustion (i. e., stoker or grate combustion), fluidized bed combustion (bubbling and circulating), and pulverized fuel combustion (dust combustion). The rotary kiln outlined in Chapter 6 is mostly used for the hazardous waste.

A fixed bed combustor can handle both small — and medium-size coal and biomass particles. The grate-fired furnace and underfeed stokers can also handle shredded tires, plastic and polymer waste, and shredded MSW. In this type of reactor, drying, gasification, and charcoal combustion take place in the primary combustion chamber followed by incineration (usually in the separate combustion zone) of the gases produced after the addition of extra air [101]. Different types of grate furnace; fixed, moving, traveling, rotating, and vibrating are available with a maximum capacity of 20 MWe. Underfeed stokers are used in small — and medium-scale systems up to boiler capacity of 6 MWe [101].

The fixed bed boilers are normally used for parallel and indirect co-firing [1]. For a capacity of less than 20 MWe, they have low investment and operat­ing costs. They can be used for varying particle sizes (except fines) and for any kind of wood with moisture content up to 10-60 wt% and high ash content. Although fixed bed boilers can be used for a mixture of woods, they cannot be used for a blend of wood and straw or grass which have different combustion behaviors and ash melting point. For a nonhomogeneous mixture, an increase in temperature may cause ash melting and resulting corrosion [102].

The underfeed stoker has low investment cost for a small boiler (<6 MWe) [1] and low emissions at partial load operations due to good fuel dosing [101]. This kind of combustor is only suitable for fuels with low ash content and high ash melting point. It also has less flexibility with respect to particle size (<50 mm).

In fluidized bed combustion, the bed contains a mixture of inert mate­rial (about 90-98% silica sand and dolomite) and fuel (about 2-10% of coal and biomass) particles, and they are fluidized by flowing air that is passed through a perforated plate at the bottom of the bed. The bed can either be operated as a one pass-through bubbling fluidized bed or as a circulating fluidized bed where the unconverted solids are recycled back in the reactor. The bed material provides high thermal inertia and stability in the combus­tion process. Good mixing and heat transfer by materials give good tempera­ture control. The particle size for a bubbling fluidized bed (BFB) is generally less than 80 mm and that for circulating fluidized bed (CFB) is less than 40 mm [101, 103]. Low combustion temperature (800-900°C) prevents ash sinter­ing and resulting defluidization [101, 103]. The bed materials remain at the bottom of the reactor in BFB, whereas in CFB, the material is carried upwards with flue gas and separated in a hot cyclone or U-beam separator, and fed back into the combustion chamber [101]. In CFB, light soot (unburned car­bon) may leave cyclone along with ash. This contaminated ash may restrict its further use [104].

Fluidized bed combustors can be converted from coal into biomass/coal co-combustion with a very small investment, and they have large flexibil­ity in calorific value and moisture and ash contents of the feedstock. The combustion temperature in these reactors is low, and they emit low CO (<50 mg/Nm3) and NOx (<70 mg/MJ) which can be further reduced to 10 mg/MJ after using SCR [1]. The combustors can have efficiency up to 90% even with low-grade fuels. Sulfur can be removed by a direct injection of limestone in the bed. For a mixed feedstock, if a common feeder for the mixture is not possible, the installment of separate feeders for different feedstock may be expensive. Slagging and fouling on the combustor walls and tubes can occur when the feedstock has high alkali content. High alkaline and aluminum contents can also cause bed agglomeration. Similarly, high chlorine content can cause corrosion on heat transfer surfaces [1].

Fluidized bed combustors are only cost-effective for capacity >20 MWe for bubbling fluidized bed and >39 MWe for the circulating fluidized bed [1].

These types of combustors have a low flexibility for particle size and bed materials can be lost with ash. They also have a high dust load in flue gas. Low temperature makes fuel combustion incomplete and in CFB unburned carbon content can appear in the ash [1, 101, 103, 104].

Pulverized fuel or dust combustion systems are fed pneumatically with fuels such as sawdust and fine shavings [104]. Fuel quality needs to be rather constant with low moisture content (<20 wt%) and particle size of 10-20 mm. This type of combustor gives increased efficiency due to low oxygen excess and when appropriate burners are used, it gives high NOx reduction [1].