Amendment to Master Agreement between KFx Plant II, LLC and Arch Coal, Inc.


    Exhibit
      10.80

    


    
      	
              ARCH
                COAL, INC.

            	 	 	
              C.
                HENRY BESTEN, JR.

              Senior
                Vice President

              Strategic
                Development

            
	 	 	 	 
	
              June
                29, 2006

            	 	
              VIA
                FACSIMILE AND OVERNIGHT MAIL

            
	 	 	 	 
	
              KFx,
                Inc.

              55
                Madison Street, Suite 500

              Denver,
                Colorado 80206-5810

              Attn:
                William G. Laughlin

            	 	
              KFx
                Plant II, LLC

              55
                Madison Street, Suite 500

              Denver,
                Colorado 80206-5810

              Attn:
                President

            	 


    

    


    Re:
      Master Agreement, dated as of October 5, 2005, by and between KFx Plant II,
      LLC
      and Arch Coal, Inc.

    


    Gentlemen:

    


    This
      letter will confirm our agreement with KFx Plant II, LLC, a Delaware limited
      liability company (“KFx Plant II”), and KFx, Inc., a Delaware corporation
      (“KFx”), to extend certain dates contained in the Master Agreement, dated as of
      October 5, 2005, by and between KFx Plant II and Arch Coal, Inc. (the
“Master Agreement”) and the Stock Purchase and Warrant Agreement, dated as of
      October 5, 2005, by and between KFx and Arch Coal, Inc. executed in
      connection therewith (the “Stock Purchase and Warrant Agreement”) in the manner
      set forth herein. Capitalized terms used herein but not otherwise defined shall
      have the meanings given those terms in the Master Agreement.

    


    By
      your acceptance of this letter, the parties agree to extend (i) the Operator
      Option Period by replacing the references to July 1, 2006 in Sections 2.1
      and 6.1(d) of the Master Agreement with October 1, 2006 and (ii) the term
      of the warrant granted by KFx to Arch pursuant to Section 2.1 of the Stock
      Purchase and Warrant Agreement by replacing (a) the reference to July 1,
      2006 in Section 3.1 of the Stock Purchase and Warrant Agreement and in each
      of
      Warrant Nos. 05-1, 05-2, 05-3 and 05-4 (collectively, the “Warrant
      Certificates”) with October 1, 2006 and (b) the references to
      August 1, 2006 in Section 3.1 of the Stock Purchase and Warrant Agreement
      and in each of the Warrant Certificates with November 1, 2006.

    


    If
      you are in agreement with the foregoing, please execute this letter in the
      space
      indicated below.

    


    Sincerely,

    


    


    /s/
      HENRY BESTEN

    C.
      Henry Besten

    Senior
      Vice President - Strategic Development

    


    cc: Steven
      F. Leer

    


    Accepted
      and agreed to this 29th day of June, 2006:

    


    
      	
              KFx,
                Inc.

            	 	
              KFx
                Plant II, LLC

            
	 	 	 
	
              By:
                /s/
                JAMES P. IMBLER

            	 	
              By:
                /s/
                JAMES P. IMBLER

            
	
              Name:
                  James
                P. Imbler

            	 	
              Name:
                  James
                P. Imbler

            
	
              Title:
                  SVP
                Business Development

            	 	
              Title:
                  SVP
                Business Development

            


    

    


    

      
        	 	
                A
                  Subsidiary of

              	 
	 	
                ARCH
                  COAL, INC.

              	 
	
                City
                  Place One, Suite 300

              	
                St.
                  Louis, Missouri 63141

              	
                (312)
                  994-2700